Working out how many PV panels you need to cover your electricity requirements is not a simple matter. Here's an example walkthrough so you can get a handle on your own calculations.

Step 1 – List your plug loads

Walking around our imaginary house, we list every appliance, gadget and other plug load that uses electricity. Next to them, we write down the power use and the estimated number of hours they're switched on in summer and winter.

Our table looks a bit like this (though a lot longer):

Overall, let's say we have a summer usage of 25 kWh per day and winter usage of 35 kWh per day.

Step 2 – Figure out insolation values

This bit's more difficult. Most of the resources for calculating insolation values are horribly complicated. The two easiest we've found are at NREL (National Renewable Energy Laboratory) and Solmetric.

NREL: http://www.nrel.gov/gis/solar.html
Solmetric: http://www.solmetric.com/annualinsolation-us.html

The Solmetric one's very cool because it has a simple lookup tool (by state and weather station) to get an insolation map for the whole year; the NREL one shows more detailed maps for each period – essential if you want to separate the two seasonal figures for a daily peak calculation.

For our example, we'll do a simple calculation for the coldest month (which we'll say is February) and the hottest (August). Our insolation values are 4.5 for February and 6.0 for August.

Step 3a – Work out the most expensive period

Our summer months calculation comes out at 25 kWh/day divided by insolation of 6.0, which is 4.17 kWh. The winter months give 35 kWh/day divided by insolation of 4.5, which is 7.78 kWh – much higher.

Therefore, if we're trying to cover our peak daily usage, we'll need enough PV panels to generate 7.78 kWh to cover our 35kWh/day requirement in winter.

Step 3b – Net-zero calculation

If we're doing a net-zero calculation, we can average the two periods: we'll need to generate (25+35)/2 = 30 kWh per day with an insolation value of (4.5+6)/2 = 5.25 throughout the year.

An easier way to do a real-world calculation for this figure is to take your annual electricity bill (or, ideally, several of them for previous years) and work out how many kWh you used on average every day over all 365 days of the year. You'll still need to look up and work with insolation values but the electricity usage figures should be pretty accurate.

Step 4 – Work out how many PV panels you need

This is quite complex because each individual circumstance is different: your location, the angle of the roof (assuming roof-mounted panels), the efficiency rating of the chosen panels, the azimuth and facing... there are tons of factors.

However, for a quick guesstimate, we'll just use our energy requirement and insolation figures.

Our daily usage figure tells us we need 35 kWh per day at a rate of 4.5 kWh per square meter per day (the insolation value). That means we need an output of 7.78 kWh. Assuming each PV panel outputs 200W, that's