How the solar panel calculator works
This tool estimates how many solar panels you need, how large the system should be, and roughly how much roof space it will take up. It works from two inputs: how much electricity you use and how much sun your location gets. Everything else - panel count, system size, yearly output - is derived from those two numbers.
You can enter your usage as a monthly electricity bill (with your rate per kWh) or as a monthly kWh figure directly, whichever you have on hand. The calculator converts monthly usage into a daily average, then works out how large a system is needed to cover it.
The formula in plain English
The core calculation runs in four steps:
- Daily kWh = Monthly kWh ÷ 30
- Effective sun hours = Peak sun hours × (1 − Shading %)
- System size (kW) = Daily kWh ÷ (Effective sun hours × Performance ratio)
- Panels needed = round up (System size in watts ÷ Panel wattage)
As an inline expression, the key step looks like this: System size (kW) = Daily kWh ÷ (Peak sun hours × 0.8). Panel count, roof area and yearly output all follow from that one number.
Understanding peak sun hours
Peak sun hours are not the same as daylight hours. One peak sun hour equals one hour of sunlight at an intensity of 1,000 watts per square meter, the standard used to rate solar panels. A location might get 10 hours of daylight but only 5 peak sun hours, because early morning and late afternoon light is weaker than midday sun.
Peak sun hours vary a lot by geography and season. Desert regions in the southwestern US or parts of Pakistan can average 6 to 7 peak sun hours a day, while cloudier places such as the UK or Northern Europe often average 2.5 to 4. The calculator's built-in dataset covers major cities across the US, UK, Australia, India, Pakistan, Europe and Africa, or you can enter a manual figure if you already know your local value.
Why the performance ratio matters
A panel's wattage rating (say, 400 W) is measured under ideal lab conditions, and real systems never quite reach that figure in the field. That's why the calculator applies a performance ratio, defaulted to 0.8, to account for:
- Inverter conversion losses (turning DC power into usable AC)
- Wiring and connection resistance
- Heat - panels lose some efficiency as they warm up
- Dust, pollen and general soiling on the panel surface
- Minor mismatches between panels wired in a string
A performance ratio of 0.75 to 0.85 is typical for a well-installed residential system. Nudge it lower in a very hot or dusty climate, or slightly higher for a newer, well-ventilated install in a mild climate.
How shading hurts your output
Shading is entered as a percentage and directly reduces effective sun hours. Even partial shading on part of an array - from a tree, chimney or neighboring roofline - can cut output more than the shaded area alone suggests, since one shaded panel can drag down the output of an entire string. If your roof has any shading for part of the day, it's worth being conservative with this input rather than assuming a fully clear roof.
Reading your bill for kWh usage
If you only have a bill total in dollars, pounds, rupees or euros, divide it by your rate per kWh to estimate usage - most bills list both the total charge and the rate, or the total kWh consumed for the period directly. Averaging several months of bills, ideally including a summer and a winter month, gives a more reliable figure than relying on a single month.
Typical system sizes by country
| Country/region | Typical monthly use | Common system size |
|---|---|---|
| United States | 800-1,200 kWh | 6-10 kW |
| United Kingdom | 250-400 kWh | 3-5 kW |
| Australia | 400-700 kWh | 5-8 kW |
| India | 200-500 kWh | 3-6 kW |
| Pakistan | 300-600 kWh | 3-7 kW |
These ranges shift with household size, climate, and whether a home uses electric heating, cooling or an EV charger. Costs and incentive schemes also vary a great deal by country and even by state or province, so treat this table as a starting point rather than a quote.
Checking your roof area
Once you know your panel count, multiply it by the panel's physical footprint - about 2 square meters (21.5 square feet) for a 400 W panel, scaling roughly with wattage. Compare that figure against your usable roof area, keeping in mind you'll lose some space to vents, roof valleys, chimneys and the setbacks required along roof edges by most local codes.
What to do next
Treat this output as a planning estimate, not a final design. A local installer will carry out a proper site survey, check your roof's structural capacity, and quote based on real shading analysis and current local pricing. Use this calculator to walk into that conversation with a realistic sense of system size and expected output.