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MasterMath

Solar Panel Calculator

How many solar panels you need given your annual consumption and the sunshine where you live, with the roof area and the estimated saving.

Currency and number format for

Panels needed

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Panels needed—
Capacity to install—
Roof area—
Estimated generation—
Share of your consumption—
Estimated saving—
CO₂ avoided a year—

How this was worked out

    Indicative result. This is indicative sizing. A real installation needs a shading study, the orientation and pitch of the roof, and the local rules on self-consumption and grid export.

    The formula

    capacity (kWp) = daily consumption ÷ (peak sun hours × system efficiency)

    Where it comes from

    Peak sun hours are not hours of daylight: they are the equivalent hours of radiation at full intensity. Northern Britain sits near 2.5, southern Spain near 5.5. The 80 % efficiency figure covers losses in the inverter, the cabling, the heat and the dirt.

    How to work it out by hand

    1. Divide your annual consumption by 365 for the daily figure
    2. Divide by the peak sun hours where you live
    3. Divide by the system efficiency, around 80 %
    4. Divide the resulting capacity by the power of each panel

    What is worth knowing

    Oversizing does not always pay. Generate far more than you use at the time and the surplus goes to the grid, paid at a good deal less than you buy at. What usually pays is covering the daytime load and looking separately at whether a battery is worth it, because it nearly doubles the cost of the installation.

    Frequently asked questions

    How many solar panels do I need?

    It depends on your consumption and your latitude. For 3,500 kWh a year at 4.5 peak sun hours with 450 W panels, about 6 panels.

    What are peak sun hours?

    The equivalent hours of radiation at full intensity. They run from about 2.5 in northern Britain to 5.5 in southern Spain.

    How much roof does each panel take?

    A 450 W panel takes about 2.1 m². Six panels need around 13 m² of clear roof.

    Is a battery worth adding?

    They nearly double the cost of the installation. They tend to pay only where outages are frequent or where consumption is mostly at night.