solar PV systems size

How many solar panels do you need to meet your electric needs? Which should be the size of your solar PV system (for power generation)? These questions are particularly important for off-grid solar electric systems, to avoid power disruptions and to estimate the size of backup batteries.

To get the right size for a solar PV system you need to have two estimates: your electricity consumption and the amount of electricity a standard photovoltaic panel can produce in your region.

A typical home in Denver, Colorado, with a 600 kilowatt-hours month's electric bill, needs about 19 panels of 120 watts to supply its electricity needs (source: Vermont Department of Public Service)...

Power consumption calculation

There is no magic method or formula to determine the best size of your solar PV system, but the rules and steps to get it are rather simple.

First 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 calculate 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, on average… and add up all these watt-hours uses, but it's much more complicated.

Your Solar power production Calculation

Obviously, you still need another important calculation: the electricity you can produce with an individual type of PV photovoltaic panel in your region.

To get it you need to consider the average number of hours of sunlight in your site, the average weather conditions, and the wattage of the PV panel...

If you want to build a system with a 100 watt solar panel and you have full five hours sunlight on average, than the solar panel will produce about 500 watt hours/day (100 watts x 5 hours = 500 watt hours) of electricity. For 8 hours of full sunlight, a 200 watts solar panel would generate 1600 watt-hours of electricity, etc.

Comparing power consumption and power production

Once in possession of your home's power consumption and the power production of an individual solar panel in your region (in watt hours), you may easily estimate your PV system size (and the number of solar panels you need).

You just have to compare both values and make a final adjustment, to compensate battery losses and to avoid power outages: in off-grid systems, a solar photovoltaic system should be sized to generate around 110% of the needs.

Obviously, these type of calculations, only make sense for off-grid photovoltaic systems. If your home and your PV system is grid-tied you don't have to have a system to fully supply your needs. You may have a smaller (or a bigger) solar PV system... Only for off-grid cases these calculations are truly relevant.

See: Photovoltaic solutions, Photovoltaic technology and Solar and Wind Net Metering