Sizing your battery bank can be confusing and stressful, considering batteries are one of the most expensive investments in your system, second only to the panels. There are, however, many advantages to adding battery backup, such as the ability to maintain power to a separate sub-panel in the event of a grid outage, as well as other advanced programming options.

First, consider the power the system will consume each day. Typically, appliances provide this information in watts, which denotes the amount of electricity the load uses in one hour. Multiply this wattage by the number of hours the load will be running and you will arrive at the total watt-hours (Wh) it consumes each day. Divide this value by 1,000 to calculate the total kilowatt-hours (kWh) the load consumes. For example, a 60-W light bulb will consume 1.44 kWh if left on for 24 hours.

60 W x 24 hours = 1,440 Wh / 1,000 = 1.44 kWh

Once you determine the average daily load demand required, then you can decide on a nominal system voltage and battery type. The most common system voltages are 12, 24 and 48 V. Keep in mind that wiring your batteries in series (positive to negative) will double the output voltage, while wiring your batteries in parallel (positive to positive) will double the ampere-hour (Ah) capacity. To calculate the storage needed, use Ohm’s law (V x A = W) in conjunction with the data we have previously gathered to determine the exact battery bank capacity in Ah.

It is important to understand the characteristics of the battery you select. Batteries have different Ah ratings based on their charge/discharge rate. The most common charge rate is the C/20 rate, which indicates a complete charge/discharge over a 20-hour period. For example, OutBack Power EnergyCell 200RE batteries have a rated capacity of 176 Ah at the C/20 rate. This means that you can pull 176 A out of the battery over the span of 20 hours.

Another important factor to consider is the recommended depth of discharge (DoD) and the efficiency loss.

OutBack EnergyCell 200RE batteries have a recommended DoD of 50% and are 90 to 95% efficient depending on the DoD. This means that you can safely consume 88 A over the span of 20 hours, and you will be able to discharge 90 to 95% of the power with which you have charged.

Finally, decide the desired days of ancillary operation—how long your system can provide backup during an outage. Typically, use two days as a starting point and adjust accordingly based on application. Use this formula to determine the exact battery bank capacity needed:

[Average daily load in kWh / battery bank voltage = average daily load in Ah x desired days of ancillary operation] [5 kWh / 48 V = 104 Ah x 2 = 208 Ah]Using the OutBack EnergyCell 200RE battery, we calculate the following:

[Rated Ah capacity of battery / desired DoD % = actual Ah / inverter efficiency % = usable Ah x number of batteries in parallel = total battery bank capacity] [176 Ah x 0.5 = 88 Ah x 0.9 = 79 Ah x 3 = 237.6 Ah)With these results, we would need 12 OutBack EnergyCell 200RE batteries. Four of each battery in series to create a 48-V string, and three of these strings in parallel to reach 237.6 Ah.

**This installation tip was provided by Kelly Stone, technical support representative, OutBack Power**

Greg Smith says

Good article, but I don’t like mixing Ah with kWH. Ah is almost exclusively used for lead acid systems and kWH for lithium based batteries. I prefer kWH for lithium since the PV inverter energy is presented using the same term. If the solar generates 25 kWH a day, and you have a 20 kWH battery bank, a quick glance at the load list will tell you if you have enough solar left over to charge the batteries. Also, lithium does not have a 50% DOD limitation so the math works out a little differently.

chris parent says

I’m sorry- you’re saying it will take 12 batteries to to run a lightbulb?

Greg Smith says

Yes, if you want to run that 20W light bulb for 1422.75 hours.

Jay says

And of course they use a 20 hour discharge rate because where they live there are only 20 hours in a day.

Angelo says

Jeez! With an average of 4 peak hours of sunlight( the batteries won’t be discharging because there will be sunlight) you’ll be left with 20 hours of inadequate or no sunlight, thus the 20 hour discharge rate. This is common sense now ain’t it?

Greg Smith says

Jay, the discharge rate has nothing to do with how many hours in a day. C20 is a common discharge rate for lead acid. C10 is another one. C10 doesn’t mean there are 10 hours in a day; it measures the amount of current the system can provide and for how long. C rating changes with the Ah rating. Check out a lead acid battery spec sheet and you will see the relationship.

yazan harb says

what if you have annual load instead of daily load?

Greg Smith says

Divide by 365 😀

Martin Noel says

I nave a question

. As you can tell I am new to solar power research, but is it not possible to calculate your needs from your current electrical grid usage.? Just wondering?

Kelly Stone says

Hey Martin!

Sorry for the delayed response.

You can definitely calculate your usage requirements from your current grid usage! You want to make sure you’re sticking to the recommended 50% DoD practice so your batteries don’t fail prematurely. The harder you work your batteries, the less time they’ll last you. If you take your usage and double it, that should be an adequate battery bank size for your needs!

Cheers,

Kelly S.

Lucy says

Would the same formula apply to LiFePo4 batteries?

Greg Smith says

No, that is a lead acid calculation since that battery type typically does not like to be discharged lower than 50%. Lithium is much tougher, and most can be comfortably discharged to 90% DOD with no long-term effects.