Solar Energy Calculator
Energy output of a solar panel system
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What the array produces
Twenty 350 W panels, five peak sun hours a day and 85% system efficiency give 29.75 kWh a day, which is 892.5 kWh a month and roughly 10,859 kWh a year.
That is a mid-size residential array: about 7 kW of nameplate capacity, sized to cover a house that uses 25–30 kWh a day in a sunny climate.
The formula
Daily kWh = panels × watts per panel ÷ 1000 × peak sun hours × efficiency.
For the defaults: 20 × 350 = 7,000 W, or 7 kW. 7 kW × 5 h = 35 kWh at standard test conditions. The 85% factor — inverter losses, wiring, heat derating and soiling — brings it to 29.75 kWh.
Worked example
Daily = 20 × 350 W × 5 h × 0.85 = 29,750 Wh = 29.75 kWh
Monthly = 29.75 × 30 = 892.5 kWh
Annual = 29.75 × 365 = 10,859 kWh
At a US average of about 16 cents per kWh, that annual output is roughly $1,740 of electricity — before any net-metering or time-of-use adjustment.
Why peak sun hours are not daylight hours
A day with 14 hours between sunrise and sunset rarely delivers more than five or six “peak sun hours”, because the figure is irradiance summed and divided by 1,000 W/m². A panel in the morning, at sunset or behind cloud produces a fraction of its rating. Desert locations reach 6–7; northern Europe and the UK average 2.5–3.5.
What moves the real number
- Shading. A single shaded cell can cut a whole series string, so a chimney shadow can cost more than its area suggests.
- Azimuth and tilt. A roof facing away from the equator in the northern hemisphere loses 15–30% over a year.
- Temperature. Panels lose about 0.35% of output per °C above 25 °C, so a hot roof runs below its rating at midday.
- Inverter clipping and standby. A slightly undersized inverter is normal and efficient, but overnight draw and conversion losses are real.
What it cannot tell you
This is a generation estimate, not a financial model. Battery round-trip losses of 10–20%, export tariffs and the difference between self-consuming a kWh and exporting one all sit outside it.
Frequently Asked Questions
Why 85% efficiency rather than 100%?
Real arrays lose energy before the meter: inverter conversion costs 2–5%, wiring 1–2%, soiling 2–4% and heat a few points more. 85% is a reasonable all-in working figure for a clean residential system.
How many kWh does a 350 W panel make a day?
About 1.5 kWh in the defaults here — 0.35 kW × 5 peak sun hours × 0.85. In a high-irradiance desert it can reach 2.1 kWh; in a cloudy northern city, under 1 kWh.
Is 892.5 kWh a month enough to run a house?
It covers roughly a third of typical US household use (about 10,500 kWh a year) and a large share of a low-consumption home. The default array is about 7 kW, a common residential size.
Should I use panel wattage or inverter wattage?
Panel wattage. The array can be oversized relative to the inverter — a DC/AC ratio of 1.1–1.3 is standard and only clips during the brightest hours — so sizing on the panels reflects the energy available.
How much does tilt matter?
At latitude tilt the annual loss is minimal. Flat mounting loses 10–15% in mid-latitudes from a poor winter angle; steeper than latitude favours winter production at some cost in summer.
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