Solar Array Sizing Calculator
How many panels it takes to cover your bill
Turn your annual electricity consumption into an array size, a panel count and a roof area — then see how much of your bill each option actually offsets, and where adding panels stops helping.
The numbers
- Consumption used in this calculation
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- Effective yield after orientation and shading
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- Value of the generation at today's rate
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- Roof area you said you have
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- Largest system that roof supports
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Show the working
How this is calculated
Every assumption is listed so you can disagree with it. If a figure here is wrong, tell us and we will fix it.
The model
Sizing an array is the payback calculation run backwards. Instead of asking what a given system produces, it asks what system produces a given amount:
effective yield = specific yield × orientation factor system kW = (annual kWh × offset target) / effective yield panel count = ceil(system kW × 1000 / panel watts)
The orientation factor is the fraction of a perfect south-facing, unshaded installation you actually get. East or west facing costs you roughly 13%; afternoon shade from a single tree can cost far more than people expect, because shading one cell in a string drags the whole string down unless the array uses optimisers or microinverters.
Why offsetting 100% is often the wrong target
Sizing to wipe out the bill entirely is the default request, and it is frequently a poor deal. The last few panels are the ones whose output you are least likely to use on site, so under any billing arrangement that pays less for exports than it charges for imports — which is now most of them — they earn a fraction of what the first panels earn while costing the same.
Try 70% or 80% in the offset field and compare the numbers against the payback calculator. A smaller array very often has the better return, even though it leaves a bill behind.
Roof area
The area figure counts panel glass only. Real installations need setbacks from roof edges and ridges for fire access, space around vents and chimneys, and room between rows. Budget roughly 25 to 35% more roof than the panel area shown here, and remember that only the well-oriented planes of the roof count at all.
Panel defaults are a current mainstream residential module: 430 W at about 21.5 sq ft. Higher wattage panels get you the same kW in less area, which matters only if the roof is the binding constraint rather than the budget.
What this leaves out
No seasonal matching — a system sized to your annual total will overproduce in summer and underproduce in winter, and how that is settled depends entirely on your utility's billing rules. No inverter clipping, no snow cover, no future consumption changes. If you are about to buy an electric car or a heat pump, add their consumption before sizing rather than after.
Sources
The constants and default values above come from these public sources. Every one is free to read and none of them are ours — check the figures against them.
- NREL PVWatts Calculator Model your actual roof pitch, azimuth and shading instead of using our regional yield estimate.
- EIA — Residential Electricity Consumption Typical household annual kWh, for sanity-checking the consumption figure you enter.
- DOE — Planning a Home Solar Electric System Roof suitability, setback requirements and how to read an installer quote.
- NREL — Solar Resource Maps The irradiance data the regional specific-yield presets are drawn from.
Built by Vano Gelashvili
Vano Gelashvili is a software developer who builds the calculators on this site. He is not an HVAC engineer, and says so on every page: what he brings is the arithmetic, worked openly against published data.