How this calculator works
A solar panel is rated at its output under standard test conditions: 1,000 watts per square metre of sunlight at 25 degrees Celsius. Real roofs rarely see that, so the usual design method counts peak sun hours, the daily average of hours at that standard irradiance. Vancouver averages about 3.0 peak sun hours over the year and Regina about 4.2, which is why the same array produces 40 percent more in Saskatchewan. The daily energy you need is your annual use divided by 365, multiplied by the share you want to cover. Dividing by the peak sun hours and by a system derate of about 0.8 gives the array size. The derate covers the inverter, wiring, soiling, snow and the fact that panels get hot and lose output. The panel count is rounded up, so the array is slightly larger than the exact target, and the roof area is the panel area times the count, before access gaps between rows.
Formula
target energy (kWh per year) = annual consumption x offset share. array size (kW) = target / (peak sun hours x 365 x derate). panels = round up(array kW x 1000 / panel watts). actual array = panels x panel watts / 1000. generation (kWh per year) = actual array x peak sun hours x 365 x derate. savings = generation x electricity price. emissions avoided = generation x 0.12 kg CO2e per kWh. payback = installed cost / annual savings.
Worked example
A house using 10,000 kWh a year, covering all of it at 3.8 peak sun hours with an 80 percent derate, needs 10,000 / (3.8 x 365 x 0.8) = 9.01 kW. With 400 W panels that is 23 panels, an actual array of 9.2 kW and an area of 23 x 1.8 = 41.4 m2. Generation is 9.2 x 3.8 x 365 x 0.8 = 10,208 kWh a year, worth $1,531 at $0.15 per kWh and avoiding about 1,225 kg CO2e. At $2,000 per kW the installed cost is $18,400, a simple payback of 12 years.
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Frequently asked questions
Should I cover all of my consumption?
Not necessarily. Without storage, surplus power exported to the grid may earn less than you pay for power, and a smaller array often has a better payback. Many households cover 60 to 80 percent of annual use. Set the offset to model that.
Why use peak sun hours rather than the panel rating?
The panel rating is a laboratory condition. Peak sun hours fold the real weather, latitude and season into one number that you can multiply by the array size and the days in a year. It is the standard first estimate in both the PVWatts model and Natural Resources Canada guidance.
What does the derate cover?
Inverter losses, wiring, soiling, snow, shading and the temperature coefficient, which reduces output as panels warm. A clean, well-ventilated array with a modern inverter might reach 0.85; an older or partly shaded roof can be nearer 0.7.
How long do the panels last?
Most panels carry a 25 to 30 year performance warranty and still produce about 80 percent of their rated output at the end. The payback period here is financial and ignores inverter replacement, which usually happens once in that time.
Sources
- Natural Resources Canada: solar photovoltaic energy and photovoltaic potential in Canada
- National Renewable Energy Laboratory: PVWatts calculator and performance model
- Environment and Climate Change Canada: emission factors and reference values for greenhouse gas quantification (national inventory edition, updated annually)