ASK THE EXPERTS: Calculating Solar Capacity


What should be the power rating of a solar-electric array if I wish to use a 1 kW load all day?

Arvind Agarwal • via

It’s important to be realistic about the load. Unless this is a remote telecommunications site or something similar, it’s doubtful that you have a 1-kilowatt load that’s always running. Most loads vary by the time of day and other factors.

The first step in sizing a solar-electric system is to do a load analysis. In an on-grid situation, this can come from your utility bill. Look for the average kilowatt-hour (kWh) usage per billing period.

If you don’t have a useful utility billing to work with, you either need to measure the loads, or do some estimating. For multiple loads in the system, I recommend a detailed spreadsheet or list showing each load with its watts and predicted hours of use. A watt meter can determine an appliance’s true power consumption. With this information, you can calculate the watt-hour energy usage of each device, and add them up to get a daily kWh number.

Next, determine the solar energy available at your site. Find the “daily peak sun-hours” number for your area. It’s generally easy to find this number via print and online resources (see Peak sun-hours refers to the solar energy available. It’s based on your local weather, but doesn’t take into account the specifics of your individual site, such as shading from trees or buildings. If you intend to supply this load with an off-grid PV system year-round, you will need to choose the worst case for sun-hours, which is usually in the winter.

Once you have the sun-hours figure, you may need to modify it because of shading at the installation site. You can quantify the shading with a tool such as the Solar Pathfinder and then factor it into your calculations.

Next, you need to derate the solar-electric module’s STC power rating to reflect what it will actually produce. PV module specifications are from unrealistic laboratory conditions (at 1,000 watts per square meter at a 25°C cell temperature). In the real world, they don’t see these conditions very often—especially that low of a temperature. When modules get hot, their voltage drops and performance suffers. So it’s necessary to apply a derate factor, which is in the general range of 0.60 for off-grid battery-based systems and 0.75 for grid-tied batteryless systems.

A 1 kW continuous load would require 24 kilowatt-hours per day (1 kW × 24 hours). If your location has 4 peak sun-hours, 15% shading, and you want a batteryless grid-tied system, the numbers might look something like this:

24 kWh per day ÷ 4 peak sun-hours per day ÷ 0.85 shading ÷ 0.75 derate = 9.4 kW PV array.

Or, if you want to start with a specific capacity of PV array, you can work the formula the other way. Working the calculations from PV capacity to load, you might see:

9.4 kW PV array × 4 peak sun-hours × 0.85 shading × 0.75 derate = 24 kWh per day

Taking into account all of the factors—and they will be different in each case—will help you get realistic information about what a solar-electric array will do at your site.

Ian Woofenden • Home Power senior editor

Comments (1)

akinyedani's picture

I have a 250W module with Voc = 37.4V, Vmp = 30.7V. Is it a good idea to assume that this module can charge 24V battery (2 units of 12V batteries in series)? Also, for designing the charging voltage, which one should I use between Voc and Vmp?

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