How to Size a Battery Bank

How to Size a Battery Bank

Why Battery Sizing is Needed?

Knowing how to size a battery bank is one of the most important calculations in designing any viable off-grid power system for any off grid tiny house. I will use a long example in steps to explain the basic calculations needed to size a battery bank; and we will assume that deep-cycle lead acid (DCLA) batteries will be used in said battery bank.

First off, the DCLA battery bank should not be sized too small. If sized too small, the battery bank can overcharge often and reduce its lifespan; overcharging can result in a build up of gasses and cause the battery(-ies) to burst. A good charge controller can prevent overcharging.

Another issue with an under-sized battery bank is the risk of over-depletion from power usage. For DCLA batteries, there is a depletion threshold that should not be crossed. For example, most DCLA batteries have 50% max discharge threshold(aka Depth of Discharge(D.O.D)); depleting a battery beyond the rated D.O.D significantly shortens the batteries’ lifespan. At 50% discharge, the DCLA battery bank should be recharged.

On the other hand, if battery bank to sized too big, the battery bank is less likely to have passed the D.O.D threshold and will have a longer lifespan overall. However a larger battery bank risks being undercharged due to it being harder to fully charge; if a battery bank is undercharged for too long, the overall battery capacity will be affected.

When DCLA batteries are discharged, lead sulfate(PbSO4) forms around the battery’s anode and cathodes. If the DCLA batteries are left discharged for too long, the PbSOcrystallizes and shortens the life of the DCLA batteries. When the DCLA battery bank is fully charged without being overcharged, there is no more lead sulfate and the battery bank is in a neutral and safe state.

To prevent the problems associated with an under-sized or over-sized battery bank, it’s important to stick to an approximate daily usage of watts based on a chosen lifestyle. Then, the battery bank has to be appropriately sized to accommodate this lifestyle. These 2 related points are the basis of the sizing process.

I will break down the sizing process into steps and explain them. Finally, at the end, I will provide commentary on each of the steps in order to account for some real-world variables.

I.Determine the Approximate Daily Usage of Selected Household Items/Devices

First is to take all electrical items and determine their power ratings. Their power rating(or Watt usage) should be labeled in said electrical items by their respective manufacturer. Also, 100% efficiency of these items is assumed for ease of calculations. Here is a sample categorized list of electrical items/appliances that can be found in a tiny home with each of their power ratings along with a likely daily usage time for later calculations:

Lighting and Heating/Cooling:

  • 10 LUNO A19 Non-Dimmable LED light bulbs (3 in bathroom, 3 in bedroom, and 4 in kitchen/living room): 9 Watts/hour for each bulb. Daily usage is listed later in post.
  • 1 Frigidair Air conditioner (Model: FFRA0511R1): this model uses 450 Watts/hour; usage of 3 hours/day should be standard use during summer.

Kitchen Appliances:

  • 1 Danby 2-Door Apartment Refrigerator (Model: DPF073C1BSLDD): 343 kWh/year according to Energy Star. Assuming continuous use (=24 hrs/day), this spreads out to 39.2 Watts/hour.
  • 1 Westinghouse Black Microwave (Model: WCMH900B): uses 900 Watts/hour; estimated maximum usage is 30 minutes(=0.5 hr) per day.
  • 1 Mr. Coffee 4-Cup Switch Coffee Maker: rated at 600 Watts. One brewing for 4-cups of coffee takes up to 10 minutes (≈0.167 hr); one brewing of 4 cups is enough for one individual per day.
  • 1 SPT Countertop Dishwasher (Silver): Uses 424 Watt-hr per heavy wash load (for 100 min ≈ 1.667 hr); also uses 14 liters of water per heavy load wash according to the user manual’s Wash Cycle Table section. Power usage rate is roughly 254.4 Watts/hour. It’s assumed that heavy wash is once per day; in actuality, it maybe once per few days.

Laundry:

  • 1 Giantex Portable Compact Twin Tub Washing Machine and Spinner: This device is basically a combined washer and spinner that is portable. The former is used for washing; then the spinner is used for drying the same load. Washing uses 300 Watt per wash cycle load of 15 min(=0.25 hr). The spinner uses 110 Watt for one dry cycle of 5 min; however, because the washing tub is nearly twice the size of spinner/dryer tub, it may take 2 drying cycles to dry the entire wash load. Hence, the spinner uses 110 Watt for 10 min(≈0.167 hr) to dry the entire load.

Electronic Needs:

  • 1 iClever BoostCube 2-Port USB Wall Charger: 24 Watts/hour; used to charge USB-connected wireless devices like iPhone or Samsung Galaxy; 1 hour needed at most to charge iPhone and iPad devices.
  • 1 ZOZO Universal Laptop charger: This charger utilizes 45-90 Watts/hour depending on what Laptop brand is used. Hence, a mean value of 70 Watts/hour for 8 hours will be assumed. If one runs an online business, keeping the laptop charger plugged in for 8 hours isn’t unusual.
  • 1 Satellite Internet System: ~60 Watts/hour; this is as close as an average value I can get when it comes to the modem/hot spot being powered on. It is assumed that the dish will be on a fixed tripod mount instead of an automated mount (which would use additional power). The system will be on for 8 hours as this matches the laptop charger (hence internet) use. I will explain more about satellite internet options for tiny houses in a later blog post.

Note: There are probably more items, but these would make the bulk of energy consumption. Additionally, the above usage for some items assume maximum possible usage; this is to make sure the eventual battery bank is sized to accommodate the “worst case” in terms of usage. For example, the Frigidair Air conditioner is still accounted for even though it would only be used for summer. For another example, I assume a daily heavy wash load for the SPT Countertop Dishwasher even though the actual wash load maybe less heavy and only once per 2-3 days.

II.Using the Above Values, Determine Inverter Size:

Because the battery power is in DC, the inverter converts to AC power which is what nearly all appliances only accept. The inverter size has to be such that it can take a full load of all electrical appliances working at once. A smaller size could result in a bottleneck. The sizing is the sum of the power/wattage from all the devices being used at once (as shown below):

Electrical ApplianceQuantityPower Rating(W)Total(W)
Minimum Inverter Size**2897.6
LUNO A19 LED light bulbs (bathroom)3
927
LUNO A19 LED light bulbs (bedroom)3927
LUNO A19 LED light bulbs (living room)4936
Frigidair Air Conditioner1450450
Danby Apartment Refrigerator139.239.2
Westinghouse Black Microwave1900900
Mr. Coffee 4-Cup Switch Coffee Maker1600600
SPT Countertop Dishwasher1254.4254.4
Giantex Compact Twin Tub Washing Machine and Spinner (washing load)*1300300
Giantex Compact Twin Tub Washing Machine and Spinner (drying load)*1110110
iClever BoostCube Wall Charger12424
ZOZO Universal Laptop Charger17070
Satellite Internet System(Generic)16060

As shown above, the inverter size sum was calculated to be about 2897.6 Watts(≈3 kW). Purchasing an inverter with a higher size like 3.5 or even 4 kW should be considered in order to take into account a possible margin of error in case the actual power ratings of some appliances are a little higher than stated.

*Note #1: The washer and drying spinner on this device can run at the same time; hence, the two functions are listed separately for the inverter sizing.

**Note #2: In the above table, I only did the inverter sizing for standard power ratings. There is another inverter sizing that accounts for surge power ratings. This will be explained near the end of this post.

III.Calculate Daily Energy Usage from the Earlier List

The output of daily power usage is measured in Watt-hour(W-hr) or KiloWatt-hour(kW-hr). Watt-hour is an energy unit equivalent to 1 Watt of power expended over 1 hour.  Similarly, a KiloWatt-hour is equal to 1 KiloWatt used over 1 hour.

The daily power usage is calculated as follows:
Watt-hour(W-hr) = Total(W) x Daily Usage(hours)

KiloWatt-hour(kW-hr) = [Watt-hour(W-hr)] / 1000

The Total(W) for each of the items is found on the table from earlier. Also, the Daily Usage(hours) values for each item is found in the item list in the beginning. The daily power usage of each item (in Watt-hour) values are in the table below. Afterwards, the total daily use of power is the sum of each items’ individual daily energy use (W-hr and/or kW-hr).

Electrical ApplianceTotal(W)Daily Usage(hours)Watt-Hours
Total Watt-Hours4737.17
LUNO A19 LED light bulbs (bathroom)27254
LUNO A19 LED light bulbs (bedroom)27381
LUNO A19 LED light bulbs (living room)365180
Frigidair Air Conditioner45031350
Danby Apartment Refrigerator39.224940.8
Westinghouse Black Microwave9000.5450
Mr. Coffee 4-Cup Switch Coffee Maker6000.167100
SPT Countertop Dishwasher254.41.667424
Giantex Compact Twin Tub Washing Machine and Spinner (for Washing)3000.2575
Giantex Compact Twin Tub Washing Machine and Spinner (for Drying)1100.16718.37
iClever BoostCube Wall Charger24124
ZOZO Universal Laptop Charger708560
Satellite Internet System(Generic)608480

From above, the approximate daily usage of 4737.17 W-hr is converted to 4.74 kW-hr.

IV.Decide Number of Autonomous Days

Autonomous days are the number of days worth of usable power stored in a battery bank without having to recharge. For example, if the chosen renewable energy source(wind, water, and/or hydro) was unavailable for a certain number of days (like due to poor natural conditions), the battery bank needs to provide that many days worth of power until the renewable energy source returns. The number of autonomous days assumes the maximum likely number of days where recharging is unavailable.

In this example, the battery bank will be configured to run autonomously for 3 days; meaning that the example battery bank can provide 3 days worth of power without recharging and without damaging the deep-cycle battery bank via over-depletion.

Because the issue of over-depletion is being accounted for only 3 days of continuous usage without recharge; that means that the depth of discharge(D.o.D) won’t even reach close to 50% in normal circumstances where each day’s worth of power usage is continually recharged by the renewable power system. This enables a long life for the battery bank.

Note: Later this post, I will bring up an important issue regarding autonomous days as additional commentary.

V.The Battery Bank Size is Determined

First, know that depth of discharge(DoD) is the percentage a battery capacity has been used up. The lower the repeated DoD of the battery bank, the more battery life, or cycles(recharge and discharge), the bank will have. The greater the repeated DoD, the less battery life, and cycles, there is for the battery bank. A common DoD range for lead acid batteries is about 40%-80%. A 50% DoD will be used as it is a common and maximum recommended DoD.

First is to take the daily watt usage and the days of autonomy and multiply them to get the 50% DoD of the battery bank.

4.74 kW-hr * (3 days) ≈ 14.22 kW-hr for 50% DoD

14.22 kW-hr * (100%/50%) = 28.44 kW-hr for the full watt capacity of the battery bank

To determine amp-hours, the previous value has to be divided by the battery system voltage(we will use 12 Volts here) to get total battery bank capacity in Amp-hrs.

28440 Watts-hour / 12 Volts = 2370 Amp-hours

Above, I used 12 volts for the battery bank as the assumed solar panels used here usually provide 12 volts. A higher voltage battery bank could be used if the system allows(like 24 & 36 volts), but there are legal restrictions that prohibit a battery bank from going over 48 Volts due to dangers associated with higher voltages.

VI.The Battery Wiring Configuration gets Determined: 

Before choosing the batteries needed to satisfy 2167 Amp-hours, you will need to know how different wiring configurations affect the overall voltage and storage capacity of the battery bank. There are only three wiring configurations which are series, parallel, and series-parallel hybrid.

Before explaining each of the wiring configurations using diagrams, all the DCLA batteries used here will be assumed to be of the same type (Flooded, AGM, or Gel), same brand, same voltage rating, and exact same amp-hr storage capacity for optimization and to prevent uneven charging and discharging. Hence, all batteries will be assumed to be FLA(flooded lead acid) batteries with 12 Volts rating and 200 Amp-hr (Ah) power storage capacity each.

Here are the 3 three battery bank wiring configurations diagrams below:

1.)Series: A diagram for lead acid batteries in series is shown belowBatteries in Series CircuitAny segment of the battery bank in series will have the sum of the voltages added up, but the Amp-hours(Ah) from both batteries will NOT add up to increase overall storage size. In the above series segment, the total voltage is 24 volts and the storage capacity is only 200 A-hr.

2.)Parallel: A diagram for the lead acid batteries in parallel is shown belowBatteries in Parallel CircuitUnlike in series where only the voltages add up; this time, only the charge capacity from each segment adds up. The result is that voltage of this section is still 12 Volts, but the total power storage capacity would be increased to 400 A-hr.

3.)Series-Parallel Hybrid: Below is a diagram that shows a hybrid of series and parallel circuitsBatteries in Series-Parallel CircuitThe series-parallel is most common as it has the best of both worlds. From the above circuits, each series branch adds up to 24 volts each with the 200 amp-hr for each branch. When the branches are brought into parallel, 24 volts stays the same, but the battery capacity adds up with a total of 400 amp-hr. Overall, the above diagram showcases a battery bank of 24 volts and 400 amp-hours.

When finally putting together the battery bank; all three configurations need to be kept on mind in order to optimize both the battery bank capacity and voltage to the system requirements.

VII.Additional Commentary

While all the steps have been done, there is more that can be done at each of these steps in order to do a sizing that closer to real life sizing. Here are some additional considerations when sizing for a real battery bank.

Real life Sizing Considerations for each Step

Under step “I”: For the items list, I mostly chose appliances that can be considered both necessary and the bulk of energy usage. I mainly excluded:

  • common low-wattage items like electric shavers, dryers, and electronic clocks which can be added at one’s discretion.
  • high-wattage luxury items like a TV set which is not essential to living.
  • and high-wattage items that can be replaced with alternatives like electric stoves and vacuum cleaners.

For real-life battery bank sizing, two important things that weren’t included in the item list (but should have been) are the power consumption values of the charge controller (~5 watts/hr) and inverter (~10 watts/hr in sleep mode). Check the device manuals for the actual values.

Next, because renewable power can generate a limited amount of watts at once, one should consider using non-electric alternatives to replace high wattage electrical items that have necessary functions. For example, electric stoves can be replaced with propane and wood burning stoves for cooking. Choosing non-electrical alternatives depend on whether one can provide proper power to electrical alternatives or tolerate possible inconveniences that come with non-electrical alternatives.

Additionally, one needs to consider the impact of the seasons on the item list throughout the year and as well as on renewable power generation. For example, a solar array would be able to generate enough power to support the high-watt Frigidair air conditioner during the summer. Then in winter, even though the air conditioner is no longer used until next summer, the foggy winter days reduce solar power and can then impact how one rations his/her item and energy usage.

Lastly, as I stated in the beginning, I only used the items’ advertised power ratings for all sizing calculations. Before doing any real-life sizing, one should first measure the actual power rating of each listed item as the actual power ratings can deviate from the advertised power ratings for any number of reasons.

Under step “II”: Besides the inverter sizing for the standard (aka continuous) power ratings of devices, there is also an inverter sizing for the “surge power” of devices. A device’s surge power rating is the power used by a device during its start up; also, surge values are usually higher because more power is needed for starting-up than running.

The total surge power is the addition of all the surge ratings of the devices with surge capacity. For ease of calculation and as a rule of thumb the inverter’s surging size would be about 2-3 times of the inverter sizing for the continuous power ratings. Using the table from Step II as reference, the surge-accounted inverter size would be ~6000-7000 Watts (or 6-7 kW).

Under step “IV” (no comments for step “III”): the days of autonomy accounted for was set to 3 days. However, if continuous power usage exceeds even more than 1 day (due to recharging being unavailable), the battery bank will end up being continuously undercharged. This is because the renewable power source is usually scaled to recharge the battery bank for only a little over 1 day’s worth of power usage. Scaling the renewable power source to account for more than 1 day’s worth of power would be both costly and excessive for most days.

In that case, the deeply depleted battery bank would have to be manually recharged to full capacity; or the battery bank would be damaged from being undercharged for too long. This can be done with something like a propane/natural gas/diesel AC power generator.

Lastly, one could consider setting a larger autonomous number of days like 5 days. However, this will lead to a larger battery bank. Also, like before, this battery bank will also have to be manually charged if the power depletion exceeds the daily combined power usage and recharge rate.

Under step”V”:  Another potentially important aspect of battery sizing is taking into account the inverter’s inefficiency, when DC power is converted to AC via inverter, there is a slight drop in overall power during the conversion process. If an inverter only has 85% efficiency, added power would be needed to compensate for the loss as calculated:

1000 Watts/(efficiency/100%) = 1000 Watts/(0.85) ≈1176.5 Watts

From above, if 1000 watts of power was needed for an AC appliance, then 1176.5 Watts would need to be drawn from the battery bank as the 15% conversion loss (aka 85% efficiency) of 1176.5 Watts would result in the needed 1000 Watts (=1176.5 Watts * 0.85).

If the above math is then applied to the battery bank’s capacity(amp-hr), the entire battery bank can be further sized to take into account inverter efficiency. This calculation is as follows:

(2370 Amp-hours)/0.85 ≈ 2788 Amp-hours

With this this, the new battery bank size that accounts for both autonomous days and inverter inefficiency is 2788 Amp-hours. The reason the old battery bank sizing capacity can simply be divided by 0.85(=85% efficiency) is because all the computations from the beginning were scalar.

In the appliances list, you may have noticed that I haven’t used any DC devices. If there were any DC-powered devices in the appliances list, those DC devices can be directly connected to the battery bank if their voltages matched(like a 12V DC device for a 12V battery bank). This direct connection would bypass the inverter’s inefficiency. However, if there is a DC voltage difference between the device(s) and battery bank, then a voltage converter would be needed. In that case, the conversion efficiency of the voltage converter would need to be accounted for; a good voltage converter would have an efficiency of 90% or higher.

In total, the chosen inverter should account for total continuous power, surge power, and the inverter efficiency (which should be specified with the inverter product).

Under step “VI”: In the production of DCLA batteries, there is a common trend; DCLA batteries are higher capacity for lower voltages and high voltage for lower capacities. Here are some FLA(flooded lead acid) battery models from these brands:

  • Trojan (model:  SCS225):  12 volts and 130 amp-hr.
  • Rolls Surrette (model:  6 CS 25P):  6 volts and 820 amp-hr.
  • Trojan (model:  L16RE-2V):  2 volts and 1100 amp-hr.

As shown above, the voltage and power capacity (amp-hr) are inversely related. Also as stated before, one should use multiples of only one battery model in any of the three wiring configurations. Even if multiple battery types within a wiring configuration theoretically yield the right voltage and amp-hr; the charging and discharging can end up uneven due to the battery models’ different attributes. This can lead to long-term damage to some batteries within the battery bank.

Next, when choosing the best battery model, one should aim to keep the number of batteries as few as possible (for ease of maintenance) while making sure the wiring satisfies both the voltage and amp-hr needs. The dimensions of the batteries should also be taken into account when building the storage space needed to hold the battery bank.

Lastly, when making the storage space, the battery bank should be kept at a stable temperature(like at room temp.); and the storage space needs some venting to release any gasses coming from the lead acid batteries. The buildup of these flammable gases can lead to an explosion.

Stationary VS Mobile Tiny Houses

One last and very important point I would like to make is that this entire battery bank sizing process was more suited for tiny homes built upon land than tiny homes on wheels. With a property, there would be more needed space to install the larger off-grid power apparatus needed to support multiple comfort appliances. This would be much harder for mobile tiny homes because:

  • limited space on tiny house roofs to install solar panels (wind and hydro power is completely unviable)
  • and the limited available space inside a tiny house to store a large battery bank

For mobile tiny homes, a smaller solar energy apparatus limits the amount of comfort appliances that can be powered. Additionally, the limited battery bank size also restricts power usage as well as limit possible autonomy days; in order to avoid the 50% depth of discharge limit of most battery banks.

In the case of mobile tiny homes, the best option might be to use an electrical system similar to an RV’s where the battery is manually charged at road stops like RV stations and parks. Then, the solar panels on the roof can be used for supplemental power; as well as power critical low-load items like lighting, laptop chargers, and the internet (for communication needs).

Ending Note

By now you should know how to size a battery bank, just remember that sizing a battery bank comes after:

  • determining the daily usage since it cannot exceed the daily generated renewable power
  • accounting for conditions that affect the implementation of a renewable power system (like if the tiny house is on land or on wheels).
After determining the needed battery bank size for the off-grid power system, the next part is choosing the right charge controller and inverter components; because there are electrical specifications that these devices need to adhere to in order to be compatible with the off-grid power system. The next two posts will cover how to choose for these two critical devices.

If you liked this post, be sure to subscribe for email updates(if you already haven’t) for new post updates. Also leave a comment below regarding your thoughts, experiences, and questions regarding how to size a battery bank for off-grid systems.

Image Attributions (You may skip this):
Click to see all image & diagram attributions for this post
  1. Header Image: Derived image created from “Goombagarin battery bank for solar array” by sridgway via Flickr
  2. Series Circuit Diagram Image: Original diagram made by and for this website. You may use if you give proper attribution as explained in this site’s “Content Reuse & Attribution Policy” from the bottom.
  3. Parallel Circuit Diagram Image: Same as image #2
  4. Series-Parallel Hybrid Circuit Diagram Image: Same as image #2
  5. Facebook/Google+ Post Header Image: Same header image from #1
  6. Pinterest Hidden Image(only visible with Pinterest Browser Button): I own all rights to this derivative/composite image.
How to Size a Battery Bank. The right #battery bank size is essential because undersizing or oversizing the #batteries will damage and shorten their lifespan. Click through to learn the complete process to properly #sizing your #offgrid #batterybank.

9 Responses

  1. Louis janke says:

    How does this work with seasonal variation? We use a cabin in summer for three months on and off and other 9 months have no power use.

    • Bobby Kundu says:

      Battery bank sizing will work with seasonal variation as long as you account for the amount of (renewable) power you can collect and use during your preferred time of the year. However, this means you may need to resize the battery bank if you ever decide to change the time of the year for using your battery bank; since the amount of renewable power you can collect and use can vary with the time of the year.

      With your battery bank being idle for 9-month periods, it is strongly recommended that you keep your (lead-acid) battery bank fully charged at the start of the 9-month period since they will slowly discharge during their shelf life. The shelf life discharge rate (depending on the battery type and brand) is roughly 3-6% of the battery bank’s capacity per month . This means you will HAVE to do maintenance recharge of the batteries at least 1-3 times during the 9-month period to compensate for the missing charge. Simply ignoring this results in a permanently lowered battery bank capacity. If maintained properly, lead-acid batteries can last 5-10 years (depending on whether the lead-acid battery type is flooded, sealed, or AGM).

      If your battery bank consist of lithium-batteries, their shelf life discharge rate is a slightly lower 2-3% per month. Similarly, being left idle and undercharged can lead to reduced lifespan. Maintenance charging would then need to be 1-2 times per 9-month period. However, during any kind of charging, you especially need to make sure lithium-ion batteries are NOT being overcharged since these can (without internal protection circuitry) burn up and explode if overcharged. The main advantage of lithium-ion batteries is that they can recharge much quicker for faster reuse than lead acid batteries; however, this leads to having a maximum lifespan of roughly 2-3 years even with proper maintenance. Since you use your battery bank periodically, lead acid batteries would be a better option for their longer overall lifespan.

      Lastly, you will want to keep your battery bank stored at a stable room temperature at all times of the year for maximum lifespan. When your battery bank is use, also make sure there is ventilation in the battery storage space since gases can be released during use. All lead-acid batteries will release small amounts of gases during use; however, there are few types lithium-ion batteries that can also release minuscule amounts of gases as well.

      I hope I have answered your question at all angles. Be sure to let me know if you have any more questions. Finally, I hope you will continue to find my blog posts and pages helpful to you.

  2. Whit says:

    This is a hard subject to take on, you have the base for a good article. Several things should be a part of a description of sizing battery banks. It should be made clear the distinction between watts used at a single point in time, watt hours, and watts over time.

    Inverter sizing should be relative to the number of items you want to use at one time, NOT the total possible. Simple rules like you don’t run the microwave, coffee maker, and dish washer at the same time. Anyone would reduce your inverter size to a more reasonable level. This is particularly important when talking about a 12 volt system! 12 volt inverters should be kept at a reasonable size. Even a 2000 watt 12 volt inverter can draw 2000 x 1.1(inverter efficiency)÷12= 183 amps. This would require a 4/0 copper wire to connect!

    It would be important to also relate battery bank to amperage draw. Batteries are typically rated at a discharge of 1/20th of their capacity per hour. If you draw more than that, they will effectively be a lower capacity battery bank.

    Seasonal use is also important. Items such as running an air conditioner, often come with more or less available energy/sunlight. Something like an air conditioner will often work well with the available energy profile during long sunny days of summer.

    Also, just basic things that need to be changed,a toaster is a high wattage use item! You need to change the labels in the charts from “Kilowatt hours” to “watt hours”. It’s a simple mistake; but you might also want to switch a few places where you say watts but mean watt hours. I too do this way too often, but it confuses people who are learning.

    • Bobby Kundu says:

      Battery bank sizing is indeed a hard subject and process to explain in its entirety. Hence, I just explained as much of the process as I could without being excessive so readers can do further research on their own. As for all your listed concerns:

      First off, I did make the distinction between watts and watts-hour. It’s already known that watt is a unit of power. In “Step III”, I explained that watt-hour is a unit of energy equivalent to 1 watt of power is used over 1 hour.

      As for your thoughts on inverter sizing, I partially disagree with your assessment. The reason I did the inverter sizing to account for ALL items is because one should consider the “worst case” scenario whenever possible. Basically, if its possible to find an inverter that can handle all chosen items being used at once (with regards to continuous and surge power), one should get it. However, if one is UNABLE to find or afford an inverter that can handle all chosen items at once; then it would make sense to set item usage rules to accommodate the inverter.

      As for your mention of 4/0 copper wiring, this post is about how to size a battery bank and not about physically setting it up. Explaining the wiring thickness is beyond’s the scope of this post.

      Your statement on batteries having a maximum tolerance of roughly drawing 1/20th of battery capacity per hour is something I’ll further look into. However, accounting for a max. depth of discharge of 50% (or less) during a battery bank sizing theoretically deals with this issue (even without accounting for autonomous days). If 1/2 the capacity of a battery bank was used over 24 hours (before recharging), then the draw is roughly 1/48th of the battery bank capacity per hour. This much less the 1/20th you mentioned.

      As for seasonal use, I just added a mention within the real-life commentary for “Step I”. However, anyone who delves deeply enough into off-grid power generation and battery sizing would notice the impact of seasons without me pointing it out.

      Finally, I did look around the charts and I noticed the errors you mentioned. I changed mislabeled “Kilowatt-hours” to “Watt-hour” on the charts as well as correct any other inconsistencies.

      Thanks to your long critique, I managed to improve my post. In the future, I hope will come to read and like my other blog posts.

  3. Thabang says:

    Wooow…well explained…now I know what to look for when establishing an off grid system for my little farm.

  4. Larry says:

    You’ve laid it all out here Bobby. We were just watching Tiny Houses this evening and I don’t believe I’ve ever seen them mention back-up power. Most of the houses there are going to be grid connected – but this makes enormous sense, providing the greatest flexibility. Is it common to run a DC circuit from the battery bank just for, say, the fridge? It is more efficient than going through the inverter, but do you not need a whole protection system (fuses, breakers) then for that system too? This is a great resource Bobby. Thanks for putting it together and checking the math!

    • Bobby Kundu says:

      I am glad you think so highly of my post. It only makes sense to use an off-grid power system with a tiny house given that tiny houses have a lower energy footprint than conventional homes; which means it’s easier to completely power it up with renewable energy.

      Unlike what many think, the true starting point of any off grid power system is the battery bank and circuitry; and not the solar panels, wind turbines, and water wheels. Without a properly sized battery bank and circuitry into the tiny house, any of the latter won’t become of relevance.

  5. Tony Hargreaves says:

    You have presented one of the most detailed posts covering the steps required for sizing a battery bank.
    My interests lie in this area as I perform repairs and maintenance on a farm property that has several buildings that are set up without mains power. We depend on rainwater, solar panels and diesel generators.

    Your excellent descriptions and suggestions of components – and the way they are connected is outstanding.
    Although you give battery information based on wet cell lead batteries, do they perform as well as the gel type (low maintenance)?

    • Bobby Kundu says:

      Wet cell lead acid batteries should perform just as well as gel types batteries if they are being compared with the same nominal voltage. However, gel types don’t last as long as wet cells due to it being maintenance free. The only kind of maintenance that can be done on gel types is to do slight overcharging to keep the gel cell from having hardened lead sulfate on cathode and anode. In the wet cell, however, the distilled water can be periodically added to extend the life of the wet cell battery because is used and evaporated during the charging and discharging cycles of the wet cell.

      The only reason I used wet cells in the post is because they’re common and to keep everything standard as I explain my content.

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