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Solar Day Yield
A free solar panel calculator: estimate how many kWh your panels generate in one day for your location, date and weather, and how much of your home battery that charges. It works anywhere in the world and is tuned for typical UK conditions.
Generation today
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System size–
Peak output–
Daylight–
Sun height at noon–
Same date, 2025 actual–
Battery charge
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Into battery–
Ends the day at–
Left over to export–
Full charges from empty–
Battery over the next 7 days
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Starts at sunrise on day 1 with your starting charge. Each day runs to the next sunrise, so it includes the night. Tap a day to change its weather; picking a weather in the form sets every day.
Battery, your weather planBattery, 2025 actual weather (dashed)
| Day | Weather | Solar | House use | From grid | Exported | Battery at day end |
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When in the year it lasts 7 days
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Lasts the whole period
Runs out part way (paler = sooner)
Runs out on day 1
Your chosen date
Each column is a start date. The simulation uses your panels, battery, starting charge and house use. "Your plan" uses the day-by-day weather above; the other rows assume the same weather every day. "2025 actual" uses the real hour-by-hour sunlight from that year.
Your 2025 with real weather
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Generated
From the grid
House use
Runs hour by hour through 2025, starting 1 January at your starting charge, with your panels, battery and house use. Sunlight data: Open-Meteo.com (ERA5 reanalysis, CC BY 4.0), converted to your panel angle.
Same day, every weather
| Weather | Sun reaching panels | Generation | Into battery | Battery ends at |
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How it works. The sun's path is worked out for your latitude and date in 5-minute steps. Clear-sky sunlight uses an air-mass model, split into direct beam and sky light, then projected onto the panel's tilt and direction. Weather multiplies that clear-sky figure: sunny 0.9, partly cloudy 0.6, cloudy 0.3, overcast or rain 0.12. These are typical averages; a real day can swing well either side.
Bifacial panels add light reaching the back from the ground, the sky and the sun when it is behind them, scaled by the bifaciality; ground under the array is assumed partly shaded. Solar beyond the battery's solar input limit is lost, and charging and output are capped at its rated power. Battery losses use the round-trip efficiency, split evenly between charging and discharging. The house takes its daylight use first. Portable power station capacities are the rated figure; the usable amount at the plug is usually 10 to 15% lower. Times on the chart are solar time, where 12:00 is when the sun is highest; in the UK that is roughly 12:10 GMT, or 13:10 in summer time. Shading from trees, hills or buildings is not included.
Solar generation questions
How much electricity does a solar panel produce per day in the UK?
A 400 W panel facing south at about 35° produces roughly 2.5 kWh on a clear midsummer day, around 1.9 kWh on a clear day in March, and about 0.6 to 0.75 kWh on a clear day in late December. Cloudy days give roughly a third of the clear-day figure, and heavy overcast or rain closer to an eighth.
How much does cloud reduce solar panel output?
Panels still generate under cloud because they use diffuse sky light as well as direct sun. As a rough guide, a cloudy day gives about 30% of clear-sky output, partly cloudy about 60%, and thick overcast or rain about 10 to 15%.
How much of my home battery will solar panels charge in a day?
Take the day's generation, subtract what the house uses while the sun is up, and allow about 5% charging loss. For example, a 4 kWp system on a sunny June day makes about 25 kWh; after 3 kWh of daytime use it could put around 20 kWh into a battery, enough to fill a 10 kWh battery twice. On an overcast December day the same system may make only about 1 kWh.
What tilt and direction should solar panels face?
In the northern hemisphere panels produce the most energy over a year facing due south (180°), tilted at roughly the site's latitude minus a few degrees, which is about 30 to 40° across most of the UK. East or west-facing roofs typically produce around 15 to 20% less over a year.
How accurate is this solar calculator?
It uses a clear-sky model of the sun's position and strength for your exact location and date, so the clear-day figure is a good estimate. The weather settings are typical averages, so a real cloudy day can be noticeably better or worse. It does not account for shading from trees, hills or buildings.