How many solar panels do you actually need?
Every honest solar estimate is three numbers multiplied together: how much electricity you use, how much sun your roof gets, and how much of the sunlight your equipment actually converts. This calculator asks for exactly those three and shows its working — no address forms, no "get a quote" wall, no phone number required.
Start with the bill. Your monthly kWh is printed on every electricity bill; if you'd rather not dig for it, switch to bill-amount mode and enter what you pay plus your per-kWh rate. A typical example: 900 kWh a month is 30 kWh a day. At 4.5 peak sun hours with 14% system losses, you need 30 ÷ (4.5 × 0.86) ≈ 7.75 kW of panels — about 20 × 400 W panels covering roughly 39 m² of roof.
Peak sun hours ≠ daylight hours. A 14-hour summer day might deliver only 5-6 peak sun hours, because mornings, evenings and clouds deliver weaker light. Use the region chips as a starting point, or look up your city's solar irradiance for precision. Getting this number right matters more than any other input.
The losses field is where most calculators quietly cheat. Panels are rated under lab conditions; real systems lose power to inverter conversion (~3-4%), wiring, dirt, mismatch and — counter-intuitively — heat, since panel output drops as temperature rises. The industry planning default is about 14% total losses. Skipping this makes a system look ~15% cheaper than the one that will actually cover your bill.
What this estimate can't see: your roof's direction and tilt (south-facing near-latitude tilt is ideal in the northern hemisphere), shading from trees or buildings, and local net-metering rules that decide what excess production is worth. Treat the result as a solid planning number for comparing installer quotes — a quote wildly below this size should raise questions about whether it will really offset your usage.