Off Grid Solar CalculationsSeptember 30, 2012

Off-Grid System Sizing Calculator

Below are some guidelines to help you correctly size your off-grid solar system. In the Caribbean, the average grid-tied home uses 20kwh/day while the average off-gird home uses 5kwh/day. Obviously, this indicates that the off-grid systems require a careful selection of appliances in order to function without degradation.

Basic PV system design:

The first step in sizing a solar system is to determine the expected daily kwatt-hour usage of the system. Basically this is the number of watts x the number of hours that each electrical device uses each day. Use this worksheet to calculate your system kwh/day requirements. For your convenience, you can also use an Excel/spreadsheet PV worksheet to size your solar system. Be sure to read the PV instructions worksheet. The Excel PV worksheet also contains information on selection of a backup generator and charge controllers. Once you have an estimated kwh/day number, the calculations are quite straight forward.

Kwatts of PV array required = kwh / daily sun hours / derating factor

kwh = daily electrical wattage requires daily sun hours = the average seasonal peak (noontime equivalent) sun hours in your location. See the NREL Redbook for the average sun hours at your location. See also the PVwatts website which includes the data from NREL and automatically calculates system solar production.

DC derating factor = a factor which accounts for real world inefficiencies which affect a battery based PV system (e.g., heat, humidity, bird droppings, wire losses, inverter losses, batter losses). We recommend using a derating factor of .67 to .77 which will result in a good safe estimate of the power that you will obtain from your system.

Likely, your system output will be higher but we believe it€™s better to estimate a lower system output and be pleasantly surprised when your system produces a higher than expected output rather than be disappointed by a lackluster performance.

PV sizing example (How many solar panels do we need?):

Now let€™s try out the above formula using an example.

Assuming your PV worksheet calculations show that your system will use 5kwh/day, and your average daily sun hours for your location is 5, and you plan to use 175 watt 24 volt Suntech panels , let€™s do the math:

5kwh/5.5/.67 = 1.36 kw PV array (1,360 DC watts)
1360/175 (watts per panel) = 7.77 panels (round up to 8)
4 series strings of 2 panels = 8 modules with each string producing 48 volts

Battery Bank Sizing:

A battery bank is sized to produce electricity when the solar modules don€™t produce optimum output. A good rule of thumb is to size the system to provide power for 3 to 5 days (days of autonomy). Using 3 days is a good number, while selecting 5 days may result in a battery bank that is quite expensive.

Continuing with our example:

We first need to convert watt-hours to amp-hours since that€™s how batteries are rated:

5000 watt hours per day / 48 (our system voltage) = 104.2 Amp hours required per day.

104.2 amp-hours x 3 (days of autonomy) = 312.6 Ah @48 volts

Solar charge controller sizing:

The size of a charge controller is based on the PV voltage array, desired system battery voltage and the short circuit current of the solar panels. Make sure that the solar charge controller that you use has enough capacity to handle the current from the PV array. A good rule of thumb is to take the short circuit current of the PV array and multiply it by 1.3. Using this factor the solar charge controller rating = Total short circuit current of PV array x 1.3. The Suntech 175 solar panels have a short circuit current of 5.4 amps, so since we have 4 strings of 2 panels each, the total current capacity required is 5.4 x 4 = 21.6 x 1.3 = 28.08 amps @48 volts. Using these numbers a good choice for charge controller would be a TriStar TS45 charge controller as it allows a system voltage of 48 volts and can handle up to 45 amps.

Inverter Sizing:

To find the correct inverters size we first determine the AC watts that will be provided by the Inverter. Using our example of 1360 DC watts we can convert back to AC watts by multiplying by the derating factor that we used earlier. So AC system watts = 1360 x .67 = total system watts = 911 AC watts. Thus, an inverter capable of handling 1000 watts @48 volts would work. A great choice for an off-grid inverter for our system would be an Outback FX2348ET since it handles 2300 watts @48 volts and would provide some upside potential if you decide to add more solar panels in the future.