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 Calculator

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..........


Back up Power Systems
  • - Configuration
  • - Components


1. Amp-hour rating for the battery you have chosen. (Reading at 20 hour rating)
2. Voltage rating for the battery you have chosen.
3. 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. (50% = 0.5)
4. Average winter time ambient temperature your battery bank will experience. (Not outside temperature)
5. Nominal voltage rating for the system you have chosen.
6. Enter your daily watt-hour requirement.
Use our Load Evaluation Calculator to find this number.
7. 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
7. This is the amount of amps-hours your system will need to store.
8-hidden. Divide line 3 by line 4.
8. Total battery capacity you will need.
9. Number of the strings wired in parallel.
10. Number of batteries wired in series.
11. Total number of batteries required.

Concorde AGM Batteries

Interstate Batteries

Solar-One Batteries

U.S. Battery
Rolls Surrette Batteries

Fullriver Battery

Trojan Battery Company

Universal Power Group
OutBack Power

Complete Battery Banks

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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