Number of Solar Electric Modules: sizing a PV System

How many solar modules do you need to meet your electric needs?

This question is important for off-grid electric systems; and it may also be important for on-grid systems, whenever the utility or electric provider do not purchase the excess electric generation (law may not require the utility to purchase the excess generation, but many agreements do involve that purchase).

Power consumption calculation

There is no magic method or formula to determine the number of solar electric modules you need, but the estimations are rather simple.

First of all you need to know your average monthly power consumption and convert it into watt hours (do not forget: volts times amps equal watts).

Your electric bills are the best way to determine your wattage needs. You may also determine your power consumption taking into account the wattage rating for each appliance in your home and the amount of time that each appliance runs each day… and add up all these watt-hours uses, but it will be more complicated.

Your power production Calculation

Your electric power production depends on the number of hours of sunlight (and on the number of watts of the panel). If you have a 100 watt solar panel and around full five hours sunlight on average, than your system will produce around 500 watt hours/day (100 watts x 5 hours = 500 watt hours). For 8 hours of full sunlight and a 200 watts solar panel, you will have 1600 watt-hours of electricity, etc.

Comparing power consumption and power production

In possession of your power consumption and the power production of an individual solar panel in your region (in watt hours), you may easily estimate the number of solar modules you need, or whether a particular solar panel or set of solar panels are enough to your needs. You just have to compare the two values.

To have a rough idea on how many panels you will need, take into account a typical home in Denver, Colorado, with a 600 kilowatt-hours monthly electric bill. According with Vermont Department of Public Service, that average home will need to use 19 panels of 120 peak watts, to provide its electrical needs.

Off grid situations, with battery storage: 10% plus

In off-grids systems, to compensate battery losses, and to avoid power shortages and damages in your batteries, you should consider around 10 percent plus of electric generation.