On-Grid System Sizing Calculator

Below are some guidelines to help you correctly size your on-grid solar system.  In the USA, the  average grid-tied home uses 20kwh/day while the average off-grid cabin/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

Where:

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 .75 for a grid-tied system 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 20kwh/day, and your average daily sun hours for your location is 5.5, and you plan to use 175 watt 24 volt Suntech panels , let’s do the math:

20kwh/5.5/.75 = 4.848 kw PV array (4,848 watts)
4848/175 (watts per panel) = 27.7 panels (round up to 30)
3 series strings of 10 panels = 30 modules

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 4848 DC watts we can convert back to AC watts by multiplying by the derating factor that we used earlier. So AC system watts = 4848 x .75 = 3636 AC watts. Thus, an inverter capable of handling 4000 watts would work. A great string sizing tool which contains specifications for different solar panels and inverters can be found at the Xantrex and SMA websites. Using the SMA sizing tool shows that a SB5000US grid tie inverter would easily handle 3 strings of 10 panels and up to 5000 AC watts.