Off-Grid PV Calculator Help

Daily Electricity Usage

Enter the power (in watts) used by each device you want to run off-grid in the table. There are two tables to input AC loads and DC loads separately. Along with the power, also enter the number of hours per day and days per week each device is in use. If you are unsure of how much power a device uses, it can often be found on the device’s label. There are also many tables available online that include the power consumption rates of various household appliances and devices.

Minimum Inverter Size

The inverter is sized based on the AC loads that will be connected to it. The inverter should be large enough to supply power to all the loads that may be run simultaneously. This calculator assumes that all of the loads input in the table will be turned on at the same time. If this is not true, a smaller inverter may be used.

Average Peak Sun Hours Per Day

This value is the average daily solar irradiation for your geographic location. Average daily solar insolation is also known as Peak Sun Hours, which is the number of hours per day that your location receives 1 kWh/m2 of solar irradiance. An excellent source for finding the average daily solar irradiation for your region is

A PV system should be designed based on your electricity usage patterns. If your system will be designed for seasonal use, use the irradiation values from only those months in which you will need electricity. If you need an uninterrupted supply of electricity from an off-grid system year round, use the December irradiation value in the system sizing calculator. Designing based upon the least sunniest month will ensure that the batteries will be sufficiently charged during the rest of the year.

Nominal System Voltage

Standard system sizes are 12V, 24V, and 48V. Note that this value must be at least as large as the voltage of the PV panel chosen. If a 12V system voltage is chosen along with a 24V PV panel, the calculator will display “Error” when calculating the number of panels required.

Battery Efficiency

A value of 0.8 is common, but if most loads coincide with daytime charging, the efficiency will be closer to 0.9.

Battery Reserve

This is the number of days of that the system can operate during periods of cloudy weather.

Battery Depth of Discharge

When using standard deep cycle lead acid batteries, the battery lifetime is shortened with a deeper level of discharge. With a higher depth of discharge, fewer batteries are needed, but they will need to be replaced sooner. With a lower depth of discharge, more batteries are needed, but they will last longer. Note that typical car batteries do not work for solar applications because they are not designed for deep cycle use.

Battery Voltage and Capacity

These will depend on the battery chosen for the system. Standard battery voltage are 6V and 12V and can be combined in series to increase voltage or in parallel to increase total capacity.