Solar panels look simple from the street: dark rectangles on a roof, quietly making power. The question every homeowner asks is just as simple: how much power would panels on my roof actually make? The honest answer depends on many things, but a rough estimate is still useful before you talk to any installer. This estimator gives you that rough number from only three inputs you can supply yourself: panel wattage, panel count, and peak sunshine hours, minus a percentage for system losses.
The maths is deliberately plain. Daily energy in kilowatt-hours is panel watts times panel count times peak sunshine hours, times one minus the loss percentage, divided by 1,000. Peak sunshine hours are not the hours of daylight; they are the equivalent hours of full-strength sun, a number that already accounts for weaker morning and evening light. A place with four peak sun hours gets the same energy as four hours of perfect midday sun, even though daylight lasts much longer.
Here is a worked example. Ten panels of 400 watts each make a 4 kilowatt system. With 4 peak sun hours a day and 14 percent system losses, the daily estimate is 4,000 times 4 times 0.86, divided by 1,000, which is 13.76 kilowatt-hours per day. That is about 412.8 kilowatt-hours a month and about 5,022 kilowatt-hours a year. Compare that with your yearly electricity use from your bills, and you get a first sense of how much of your use panels might cover, before any installer visits.
System losses deserve attention, because beginners often set them to zero and overestimate badly. The inverter that turns panel power into home power loses some energy, wiring loses a little more, dirt on panels loses a little, and heat loses more on hot days, because panels work less well when they are very hot. A default of 14 percent covers typical inverter, wiring, and soiling losses for a well-kept system. An old, dirty, or partly shaded system can lose far more, which is another reason this is only an estimate.
Roof direction and angle change real output more than any other single factor, and this tool does not model them. In the northern hemisphere, a south-facing roof at a moderate tilt usually performs best, while east or west roofs produce less and spread their output across morning or evening. A flat roof or a steep roof also changes the result. Shading is even more powerful: a chimney shadow across one panel can cut a whole string output for hours. None of that appears in this calculation, so treat the result as an upper guide, not a promise.
Seasons also matter, and a single yearly number hides them. Summer days give far more than winter days, because days are longer and the sun is higher, while winter output can fall to a third of summer output in cloudy northern places. If you use most electricity in winter for heating, panels will cover a smaller share then, even if the yearly total looks good. Look at your monthly bills, not just the yearly total, when you judge whether panels fit your pattern of use.
Use this estimator as a first filter, and say plainly what it is not. It is a rough estimate only, not a professional solar assessment and not a quote. It cannot see your roof, your shading, or your local weather, and no installer should size a system from it. If the rough number looks promising, the next step is a site survey from a qualified installer, who will measure your roof, check shading through the year, and give a real quote. This tool helps you decide whether that survey is worth booking, nothing more and nothing less.