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Solar Panel Calculator

Calculators · Added

Enter what you use and the peak sun hours where you are, and this gives you the array size, the panel count, the roof area and the annual kWh. It reports energy and nothing else — no payback figure, because that would need four numbers this page cannot know.

Straight off a bill

Annual daily average for the site — not hours of daylight

14% is the PVWatts default

Adds a month-by-month breakdown. Negative for the southern hemisphere.

How to use the solar panel calculator

  1. 1Enter your electricity use from a bill, and say whether that figure is daily, monthly or annual.
  2. 2Set the peak sun hours for your location — the selector lists typical figures by region.
  3. 3Adjust the panel rating and area if you have a specific panel in mind.
  4. 4Enter your usable roof area to see whether the array actually fits.

Examples

A household covering its use

Input
900 kWh a month, 4.5 peak sun hours, 450 W panels
Result
About 7.65 kWp — 17 panels, roughly 34 m² of roof, around 10,800 kWh a year

Constrained by the roof

Input
The same target with 25 m² available
Result
12 panels fit, covering about 71% of use — before allowing for setbacks and walkways

About the solar panel calculator

Sizing to 100% is not the same as never buying electricity

An array sized to match a year's consumption generates far more than the building uses in June and far less in December. Unless there is storage or an export arrangement that credits the surplus at the same rate it charges for imports, the annual figures balancing does not mean the bills do.

Which is why the monthly breakdown is worth looking at. Above about 45° of latitude the seasonal swing is severe — December can be under a third of June — and a system that covers the year on paper may cover only a fraction of the winter.

Usable roof area is less than roof area

The area figure here is panel area, and a real layout needs more. Fire setbacks around the edges, walkways for maintenance, and clearance around vents, chimneys and rooflights all take space, and panels have to be arranged in whole rows.

As a rule of thumb, allowing about a third more roof than the bare panel area is a reasonable first estimate for a domestic pitched roof. On a flat roof with tilted frames, the row spacing needed to stop one row shading the next takes considerably more than that.

Frequently asked questions

What are peak sun hours?
Not hours of daylight. It is the day's total solar energy on a square metre expressed as the number of hours that energy would take to arrive at the standard test intensity of 1,000 W/m². A site getting 4.5 kWh/m² a day has 4.5 peak sun hours, whether that arrived over eight winter hours or fourteen summer ones. Panels are rated at that same intensity, which is what makes the multiplication meaningful.
Why is the default loss figure 14%?
Because that is what NREL's PVWatts model uses as its total system loss, covering soiling, shading, wiring resistance, inverter conversion, mismatch between panels and light-induced degradation. It does not include temperature, which costs a further 5 to 10% in a hot climate — cells lose roughly a third of a per cent of output per degree above 25 °C. That is why the figure is an input rather than a constant.
Why is there no payback or savings figure?
Because it would need an installed cost, a tariff, an export rate and a subsidy regime, all local, all changing, and all assumed twenty-five years forward. A number built on guesses at four unknowns is a financial claim this page cannot stand behind. The annual kWh figure is the one a real quote gets checked against, and that is what it gives you.
What can this not account for?
Shading and orientation, which are the two things that most often make a real array underperform its paper size. A single chimney shadow crossing one panel can pull down a whole string, and roof pitch and compass direction move annual yield by tens of per cent. Neither is expressible in the fields above, and only a site survey settles them.

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