How to size battery banks

How to size battery banks, let's take 10kwh using per day as example.

The hardest part about shopping for batteries is understanding how to size the battery bank properly for your system.

The size of a battery bank is based on how much power you'll use on a daily basis,measured in kilowatt hours per day. lf you're not sure how much power you'll be using, our off-grid sizing calculator will help get you on the right track:

lf you're using 10kWh per day, you need a battery bank capable of storing that much power after accounting for battery efficiency, discharge depth and a few other things.

if you don't have enough battery capacity, you'll run out of power and end up using a lot of generator fuel. On the other hand, buying too much battery capacity can be a waste of money and you might need to add more solar or a bigger generator to be able to fully recharge your oversized battery bank.

Another reason it's crucial to dial on on your battery capacity is because it can be difficult to expand a battery bank. With lead-acid batteries, adding new batteries to an old bank causes the new batteries take on the capacity and other characteristics of the existing bank meaning you are affectively aging your new lead-acid batteries when installing them with old ones.

If you are expanding within the first year, this might not be an issue, but you really can'texpand an old lead-acid battery bank.

Once you know how much power you'll be using, use this method for calculating battery capacity:

1. Start with your daily usage (for example, 10kWh per day)

2. Add in the battery in efficiency factor (80% for lead-acid or 95% for Lithium)

Lead-acid:10kWhx1.2 inefficiency

Lithium: 10kWh×1.05 inefficiency

3. Account for the discharge depth (typically 50% for lead-acid and 80% for Lithium)

Lead-acid:10kWh×1.2×2for50%DOD

Lithium:10kWh×1.05×1.25 for 80%DOD

4. Add in another inefficiency factor for the inverter and charge controller (5-10% will work for most equipment)

Lead-acid:10kWhx1.2×2×1.05 inefficiency

Lithium: 10kWh×1.05×1.25×1.05 inefficiency

5. Account for the impact of temperature, which can reduce battery capacity.

Lead-acid:10kWh×1.2×2×1.05×1.1l temperature multiplier,based on 60°Fambient temp. See spec sheet for other temperature multipliers.

Lithium: 10kWh×1.05×1.25×1.05×1.05 temperature multiplier,based on 60°F ambient temp. See spec sheet for other temperature multipliers.

This gives us an idea of how much battery capacity (in kilowatt hours) is required based on our usage,the equipment,battery DOD and temperature.Let's look at both examples:

Lead-acid:10kWhx1.2×2×1.05×1.11=28kWh lead-acid capacity

Lithium: 10kWh×1.05×1.25×1.05×1.05=14.47kWh Lithium capacity

These numbers are the minimum capacityof your battery bank, because the formula is based on supplying one day's worth of power. Your solar array (and backup generator)will need to be designed to recharge the batteries every day.

The Lithium battery bank has less overall capacity because it can handle deeper discharges and has a higher efficiency.

Most batteries are rated in amp hours. To find a suitable battery bank, divide the overall capacity (from the previous step) by the voltage:

Lead-acid:28kWh to Ah conversion:
28kWh/48volts=583 amp hours
28kWh/24volts=1,166 amp hours
28kWh/12volts=2,333 amp hours


Lithium 14.47kWh to Ah conversion:
14.47kWh/48volts=301.46 amp hours
14.47kWh/24volts=602.92 amp hours
14.47kWh/12volts=1,205.83 amp hours

Generally speaking, a higher voltage(24 or48 volts) is used for bigger systems where you have more battery capacity and larger inverters. This is because the higher voltage is more efficient, uses smaller wire, and allows you to install more solar power on each charge controller. Most bigger inverters are going to be either rated for 24 or 48 volts.

System voltage is based on a few factors, but battery capacity (in kilowatt hours) is the best place to start designing an off-grid system. Start by understanding how much battery capacity you need on a daily basis, then size the other parts of the system (like your panels and inverter) to meet that demand.

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Jim is the creator of InkPV. An good solar installer,  he is passionate about all aspects of solar projects for the home and commercial.