Solar Panel Calculator

What this calculator does

Required array capacity = (daily load x offset share) / (peak sun hours x (1 – losses)). Panels are that capacity divided by the panel rating, rounded up to whole panels, which usually overshoots the requirement slightly, and the results show both the exact requirement and the as-built array so the rounding is visible. Annual production is then the as-built capacity times sun hours times (1 – losses) times 365, reported beside the load it offsets. The scenario section multiplies as-built watts by your cost per watt, values the self-consumed share of production at your rate, and divides for a simple payback: three multiplications whose every input you supplied, which is the correct epistemic status for payback arithmetic, a scenario to interrogate rather than a forecast to bank.

How to use it

• Get daily load from bills, plural. A year of bills divided by 365 beats any single month; solar sized on a summer bill starves winter, and sized on a winter bill wastes roof, because production and load both swing seasonally around the annual averages this method uses.
• Peak sun hours are a resource figure, not daylight hours. One peak sun hour is an hour’s worth of full-strength sun (1,000 W per square metre); a bright location’s 6.5 and a cloudy one’s 3.0 differ by more than a factor of two, which the size answer follows exactly. Use the regional guidance for a first pass and a proper resource tool for a purchase decision.
• Orientation and tilt ride inside the sun-hours figure. The standard guidance: in the northern hemisphere, face arrays south (true south), tilted near your latitude for annual production; east and west roofs produce roughly 15 to 20 percent less, and shading costs more than either. If your roof is compromised, lower the peak sun hours to reflect it rather than pretending the ideal figure applies.
• Losses are real and itemised in spirit. Inverter conversion, cable runs, dust and soiling, panel heating above rating conditions, and mismatch together land most systems in the 14 to 25 percent band; 20 is prefilled as a middle planning figure, and installers quote their own itemised version (the derate or performance ratio) in serious proposals.

How it is calculated

• Required kW = daily kWh to offset / (peak sun hours x (1 – losses/100)). Panels = ceiling of (required watts / panel watts). As-built kW = panels x panel watts / 1,000. Array area = panels x panel area.
• Annual production = as-built kW x peak sun hours x (1 – losses/100) x 365. Scenario: system cost = as-built watts x cost per watt; annual value = production x self-consumption share x rate; simple payback = cost / annual value.

Worked examples

The reference household. Load 30 kWh/day, 5.0 peak sun hours, 20 percent losses, full offset: required = 30 / (5 x 0.8) = 7.50 kW. In 400 W panels that is 18.75, so 19 panels, an as-built 7.60 kW covering 36.10 square metres at 1.9 per panel, producing 7.6 x 5 x 0.8 x 365 = 11,096 kWh a year against a 10,950 kWh load: the one-panel rounding shows up as roughly 1 percent of headroom, visible rather than buried.

The money scenario, labelled as one. At 2.80 per watt the 7,600 W array costs 21,280; with 80 percent self-consumption valued at 0.17, annual value is 11,096 x 0.8 x 0.17 = 1,509.06, and simple payback is 21,280 / 1,509.06 = 14.10 years. Change any assumption and the answer moves, which is the point of printing the assumptions on the form instead of inside a black box: quotes that promise drastically shorter paybacks are using different inputs, and now you can see which ones.

Reading your results

Read required capacity against your roof before your wallet: the area figure either fits your usable, unshaded, well-oriented roof planes or the design conversation changes (higher-wattage panels, partial offset, ground mount). Read annual production against annual load as a ratio, not a verdict: at these annual averages a 100 percent offset still imports on winter nights and exports on summer middays, and the bill outcome depends on netting and export rules the scenario simplifies into the self-consumption share. Read payback last and most sceptically, as a comparison tool between quotes on identical assumptions.

Limitations

• Annual averages hide the seasons. Production in December can run at half the annual-average rate in northern locations while loads peak; storage, grid netting rules and tariff structures decide what that costs, none of which a single average models.
• No shading or horizon analysis. A chimney’s shadow at 4pm is a production fact no form field captures; installers model it on site, and self-reported sun hours remain the user’s responsibility.
• The scenario ignores incentives, degradation and finance. Tax credits, feed-in rates that change, panel degradation around half a percent a year, loan interest, maintenance and inverter replacement inside the payback window all move real returns, in both directions.
• Off-grid sizing is a different discipline. This is grid-tied offset sizing; off-grid systems size storage and autonomy days first and array second, and using this page for one would underbuild exactly where it matters.

Frequently asked questions

How many solar panels do I need for my house?

Divide your daily use by your useful sun: the worked example’s 30 kWh a day under 5 peak sun hours with 20 percent losses needs 7.5 kW of array, which is 19 modern 400 W panels. The same house in a 3.5-hour climate needs about 10.7 kW (27 panels), and a 15 kWh household in a 6-hour desert needs about 3.1 kW (8 panels): the range across ordinary houses is that wide, which is why any answer that skips your load and your location is guessing in a confident voice.

What are peak sun hours?

A way of compressing a day’s total sunlight into equivalent hours of full-strength sun: if a site receives 5 kWh per square metre across a day, it had 5 peak sun hours, whatever the daylight length was. Panels are rated at that full strength (1,000 W per square metre), so multiplying capacity by peak sun hours, minus losses, gives expected daily energy directly. Daylight duration is the number people reach for and the wrong one; the resource figure, from climate data for your area, is what this calculator asks for.

Why does the calculator refuse to size from nameplate watts alone?

Because a watt of panel produces utterly different energy in different places and installations: the same 400 W panel yields around 2.3 kWh a day in a high-desert summer and 0.9 on a shaded northern roof in winter. Sizing from nameplate without load, resource and losses (the failure mode its brief forbids) produces arrays that are monuments to a brochure. The three extra inputs this page requires are the difference between arithmetic and estimation.

Is a 14-year payback worth it?

That is a household finance question wearing an engineering costume, and the honest answer is comparative: against panel warranted lives of 25 years and electricity prices that have historically risen, a simple payback near half the warranted life is a defensible scenario; against moving house in five years or borrowing at high rates, the same figure reads differently. The page’s discipline is to keep the scenario’s inputs on the table (cost per watt, rate, self-consumption) so the worth-it judgement is made on visible assumptions, by you, rather than smuggled into a green badge by the tool.

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Method: required capacity from daily load, user-supplied peak sun hours and system losses; production and payback shown as a labelled scenario on the user’s own cost, rate and self-consumption inputs. Last reviewed: 14 October 2026.