Good battery sizing will keep batteries alive for many years. Save money and get the most out of your solar electric system.

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Battery Bank Sizing Worksheet

How much storage you would like your battery bank to provide? Base this on the number of days your system will have to provide power without solar input. How many cloudy days in a row are typical for your location? Do you have an alternate source of power—grid intertie or generator—during these days?

It's not good for batteries to be completely drained (ever), nor is it good for them to be consistently undercharged. It is best to size your battery bank so that it gets regularly charged to capacity.

If you are installing a system for a weekend home you may want to consider a larger battery bank, because your system will have all week to charge and store energy. Alternatively, if you are adding an array as a supplement to a generator-based system, your battery bank can be slightly undersized since the generator can be operated if needed for recharging.

When you sized your solar panel array, you were dealing with watts per hour that you need to produce. Now you will be dealing with amps per hour that you need to store. Remember watts = volts X amps. Now grab a calculator and do the math..........

1. Enter your daily amp-hour requirement.
(Divide watt-hours per day by 12, 24, or 48, depending on your system voltage)
AH/ Day ___________
2. Enter the maximum number of consecutive cloudy weather days expected in your area, or the number of days of autonomy you would like your system to support
(We use 3 - 5 days.)___________
3. Multiply the amp-hour requirement by the number of days. This is the amount of amps-hours your system will need to store.
AH ___________
4. Enter the depth of discharge for the battery you have chosen. This provides a safety factor so that you can avoid over-discharging your battery bank.
This number should not exceed 0.8. We use 50% maximum or 0.5 ___________
5. Divide line 3 by line 4.
AH ___________
6. Select the multiplier (at left) that corresponds to the average winter time ambient temperature your battery bank will experience.
__________
7. Multiply line 5 by line 6. This calculation ensures that your battery bank will have enough capacity to overcome cold weather effects. This number represents the total battery capacity you will need.
AH ___________
8. Enter the amp-hour rating for the battery you have chosen. Click specific battery page for AH rating on batteries—U.S. Battery, Surrette, Trojan, UPG, Concorde or Interstate.
___________
9. Divide the total battery capacity (#7) by the battery amp-hour rating (#8) and round off to the next highest number. This is the number of the batteries wired in parallel required.
___________
10. Divide the nominal system voltage (12V, 24V, or 48V) by the battery voltage and round off to the next highest number. This is the number of batteries wired in series.
___________
11. Multiply line 9 by line 10.
This is the total number of batteries required.___________
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Concorde AGM Batteries

Interstate Batteries		 Surrette / Rolls Batteries

Trojan Battery Company		 Universal Power Group

U.S. Battery
Ambient Temperature Multiplier
80F 26.7C 1.00
70F 21.2C 1.04
60F 15.6C 1.11
50F 10.0C 1.19
40F 4.4C 1.30
30F -1.1C 1.40
20F -6.7C 1.59
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