Size your solar PV system

From what you use to the kWp, in one step

The size you need is your yearly use divided by the yield of your roof, that is how many kWh one kWp makes at your place in a year. An example: 3,200 kWh a year in central Europe, a south facing pitch at 30 degrees, a real yield around 1,120 kWh/kWp, works out at 2.9 kWp, which with 440 W modules becomes six panels, 2.64 kWp on just under thirteen square metres. If you already hold a quote, type its size and the tool tells you how far it sits from that figure, which is the real question when a salesman offers you six kWp.

The yield of your roof is a calculation, not a lookup table

The starting figures (950 kWh/kWp in the UK, around 1,120 in central Europe, 1,420 around the Mediterranean, 1,750 in desert climates) are long term averages for a system at the best pitch facing south, system losses included. The correction for your own orientation, though, is worked out every time by simulating the position of the sun every half hour for 365 days: declination, air mass, the split between direct and diffuse light with the Erbs correlation, transposition onto the tilted plane with the HDKR model and the reflection loss at oblique incidence. What comes out matches the handbooks: a flat roof gives about 10% less than the best case, east or west at 30 degrees 15% less, a south facing wall 32% less, north at 30 degrees a third less. That is also why the tool tells you which pitch would be the best one where you live.

Self consumption is the number that decides everything

A kWh you use while you are making it is worth what you pay on the bill, often 0.25 or 0.30 euro. The same kWh sold because nobody was home is worth a fraction of that. This is why the result depends more on when you use energy than on how many panels you fit. Without an hourly load profile self consumption cannot be measured, only estimated: the curve used here is calibrated on published measurements, so an average home uses on site about 48% of the output when the system is small compared with its needs, 32% when output matches use and 19% when it makes twice as much. A battery moves the daytime leftovers to the evening and is worth about 210 full cycles a year, not 365: in winter there is often nothing left to store.

Is the quote inflated?

The classic sign is a system far larger than your use, sold with the promise that you will resell the rest anyway. Look at three things: the price per kWp installed, which the tool shows and which is worth comparing across two or three quotes; how much energy ends up on the grid and at what price it is paid; and whether the roof space you have really takes that many panels. A system larger than needed only adds up if a heat pump, an electric car or a battery is coming next, because daytime use grows and the surplus turns back into self consumption.

How the payback is worked out

The tool builds the cash flow year by year: the price of the quote goes out, the avoided bill plus the energy sold minus the yearly costs come in every year, and so do the instalments of the tax credit for as long as they last. Panels lose about 0.5% of their output a year and the sums allow for it. No rise in the price of energy is assumed, so if prices climb the payback will be faster than what you read. On the tax credit the only honest thing is to ask you: rates and duration change from country to country and from year to year, and in most schemes you only get the money if you owe enough tax. Put at least the inverter replacement in the yearly costs, since it lands around the twelfth year.

Declared limits

This is an estimate with a realistic margin of 10 to 15%, not a design: a designer measures the shadows hour by hour and checks the strings, the load the roof can take and the power limit of your grid connection. The model covers the northern hemisphere. Shading here is a percentage you choose, not a simulation of the obstacles around you. Energy communities, which change the value of exported energy, and net metering schemes, which are being withdrawn in several countries, are not taken into account. Nothing you type leaves your browser. For the exact figure at your address use PVGIS, the free service of the Joint Research Centre of the European Commission, and for the tax side ask whoever files your return.