This question comes up a lot so I thought I would take a couple of minutes to explain How and Why.

The how is really a simple formula. The first step and most crucial is to determine how many watt hours you will need in a 24 hour period. You then multiply that number by 1.5 which is a fudge factor to account for charge/discharge efficiency, power factor, and inverter efficiency. If no inverter is involved then use 1.3. This will give you your adjusted watt hours per day. So for example let’s say you will use 3000 or 3 Kwh per day. 1.5 x 3000 wh = 4500 or 4.5 Kwh per day.

The second step is to determine how many days of reserve capacity are required. The minimum is 2.5 days and up to 14 days. Then multiply this by 2 (for 50% depth of discharge as you never ever want to discharge more than 50%). So let’s select the minimum of 2.5 days. 2.5 days x 2 = 5 days.

Third step is to take the number of days determined from step two, and multiply by the adjusted daily watt hour usage. So in this example 4500 wh x 5 = 22,500 wh or 22.5 Kwh.

The last step is to dived the watt hour figure in step three by the battery system voltage. So now we must decide on a battery voltage of 12, 12, 36, 48, 60 and so on with multiples of 12 volts. For consumer grade applications the highest voltage (limited by equipment available to the public) is around 48 to 60 volts. Commercial applications can go as high as 500 volts and higher with special exemptions by limiting access to only qualified personnel. The voltage you choose is restricted mainly by the charge controller current capacity vs the solar panel wattage. Basically you want to use as high of voltage as you can afford to minimize power losses on the wring, and to keep the wiring as small as possible to minimize cost. For example the largest charge controller current available today to consumers is 80 amps. These controllers can be used on 12, 24, 36, and 48 volt systems. So a typical MPPT charge controller of 80 amps will have solar panel wattage limitations. For example at:

* 12 Volts max panel wattage = 1000 watts

* 24 Volts = 2000 watts

* 36 Volts = 3000 watts

* 48 volts = 4000 watts

So for this example let’s assume we live in Kansas City which has a winter Sun Hour Insolation of 3.3 Sun Hours. So from another calculation (found elsewhere) we know we have to have minimum solar panel wattage of 1400 watts. So in this example the minimum battery voltage we can use is 24 volts. Now we have all the information we need to determine the battery Amp Hour Capacity needed. The formula is Watt Hour Capacity / Battery Voltage so using the numbers from previous steps AH = 22,500 / 24 volts = 938 Amp Hours.

Fourth step is to select a battery. If possible we want to only have one single string of batteries wired in series to obtain the voltage needed. We only want to use true deep cycle batteries made for renewable energy. This limits manufactures, and you will not find them at Walmart. A very good manufacture with the best warranty is Rolls-Surrette. They have a very good selection tool you can use. Check the RE battery option, input Desired AH, input +/- Percentage (10 to 15%), and 20 Hour Rate. Then you will see your choices to the right. Select a battery with enough AH capacity to construct with 1 string if possible. In this case the Rolls S1380 is the right choice. It is a 2 volt @ 1050 AH battery so you would need 12 of them wired in series to make 24 volts. The battery carries a 7 year warranty, with 2 years free replacement and last 5 prorated. You can expect 5 years of life out of it with excellent care.

Finally you might be asking why do I need so may batteries and reserve capacity. Well the answer is batteries only have so many cycles (discharge/charge) in them and the number of cycles depends on the depth of discharge. Generically the Cycles vs Depth of Discharges (DOD) look like this:

* 50% = 200 cycles

* 40% = 500 cycles

* 30% = 1000 cycles

* 20% = 2000 cycles

* 10% = 4000 cycles

Hope this helps.

SK