Is solar worth it? Sizing a system from your own usage
7 min read · Updated 2026-10-12 · BuildCalc
The question "is solar worth it" has no universal answer, because it depends on your electricity price, how much sun you get and how much roof you have. What can be worked out precisely is how big a system you need to cover your usage, and what it would generate. Those two numbers turn an open-ended question into a comparison.
Start with consumption, in kilowatt-hours
Your bill states how much electricity you use, usually as a monthly or annual figure in kilowatt-hours. Divide annual use by three hundred and sixty-five to get a daily average. That daily figure is the target: the array should generate roughly that much over a year to offset the bill.
Beware of averages. Consumption varies through the year in most homes, higher in winter for lighting and heating and in summer for cooling, while solar output runs the opposite way in many climates, peaking in summer. Offsetting a full year does not mean offsetting every month, which is where export tariffs and battery storage enter the picture.
Turning daily use into panels
Two figures convert consumption into system size. Peak sun hours is how many hours a day the sun is strong enough for a panel to produce its rated output, typically four to six in temperate regions. The performance ratio accounts for inverter losses, wiring, heat, dirt and shading, and about eighty per cent is a fair working value for a good installation.
Divide daily consumption by peak sun hours and again by the performance ratio to get the system size in kilowatts, then divide by the wattage of a single panel to get the count. A four-kilowatt array needs a good eighty to a hundred square feet of unshaded, reasonably south-facing roof, so the physical space is as much a constraint as the arithmetic.
Making the judgement
Compare the annual generation figure with your tariff to see the gross saving, then against the installed cost to get a payback period. High electricity prices, good sun and a suitable roof produce the shortest paybacks; low prices, poor orientation and heavy shading can push them out beyond the useful life of the equipment.
Shade deserves particular respect. A single shadow across an array can cut the output of the whole string on some systems, and no amount of good orientation compensates. Get a site survey before committing, since a professional will measure the shading through the year and the condition of the roof, which must be sound before panels go on top of it.
Run your own numbers
Free calculators that answer the “how much” version of this question. They run entirely in your browser — nothing you type is uploaded or stored.
Common questions
How many solar panels do I need?
Enough to match your annual consumption. Divide daily use by peak sun hours and the performance ratio to get the kilowatts, then by panel wattage to get the count.
Does shading matter?
Enormously. Shade on part of a string can cut output across the whole array, so an unshaded roof is worth more than perfect orientation.
BuildCalc provides general information and planning estimates only, not professional construction, engineering or electrical advice. Material quantities are illustrative and depend on your own project, products and local codes. See our full disclaimer.