Servicing · Guide
How much electricity should my solar panels generate?
How much electricity do solar panels generate? In most of the UK, an unshaded south-facing system makes roughly 850 to 1,000 kWh a year for every kWp of panels. About 70% of that arrives between April and September, so a quiet December isn’t a fault on its own.
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Per year
How much electricity do solar panels generate in a year?
Here’s what’s normal through the year, what affects output, how to check yours, and when a low number means a fault. Sizing a new system? See how many solar panels do I need?
Panels are rated in kilowatts peak (kWp): the most power the system can generate in ideal conditions, as the Energy Saving Trust puts it. What you actually get is energy in kilowatt hours (kWh), the units on your bill.
MCS publishes the figures behind every certified installer’s performance estimate. For an unshaded south-facing roof at 35 degrees, they run from about 830 kWh per kWp a year around Glasgow and Inverness (less in the islands) to about 1,130 on the south coast, with much of the country between 850 and 1,000.
A simple rule of thumb, used by our FIT check for a south-facing roof, is kWp × 850: about 3,400 kWh a year for a 4 kWp system.
Real systems often make a little less. Government figures for feed-in tariff systems, shaded and east- or west-facing roofs included, work out at about 800 to 840 kWh per kW in the last two years.
Month by month
Month by month: summer against winter.
The European Commission’s PVGIS model gives these monthly averages per kWp for an unshaded south-facing roof near Manchester:
- May, June and July: about 110 kWh a month each, or 3.5 to 3.7 kWh a day.
- April and August: about 100 kWh.
- March and September: about 80 kWh.
- February and October: about 45 to 55 kWh.
- November to January: about 27 to 37 kWh, under 1 kWh a day in December and January.
Multiply by your kWp. April to September make about 70% of the year’s total, and November to January only about a tenth. December makes between a sixth and a third of what June does, with Scotland at the low end. Do solar panels work in winter? explains why.
Bright and cloudy
Bright days and cloudy days.
Averages hide big daily swings. In PVGIS’s hourly figures for that roof over 2014 to 2023, each kWp made:
- about 6 kWh on a bright June day, and around 1.5 kWh on a dull one;
- about 2 kWh on a bright December day, while about half of December days made under 0.5 kWh.
So a 4 kWp system might make about 25 kWh on a bright June day and under 2 kWh on a grey December one.
Panels still work under cloud: the Energy Saving Trust says cells don’t need direct sunlight, though stronger sunshine means more electricity. A dull June day usually still beats an average December one.
What affects it
What affects your output.
- Direction. In the MCS figures, an east- or west-facing roof makes about a fifth less than a south-facing one. The Energy Saving Trust says 15 to 20% less. See best direction and angle for solar panels for our figures.
- Pitch. Anywhere from about 30 to 45 degrees makes little difference. Panels lying flat make about 15% less.
- Shading. MCS’s installation guide warns that even a small amount of shade on part of an array can have a very significant impact on its output.
- Dirt. The Energy Saving Trust says UK rain cleans panels tilted at 15 degrees or more. Shallow roofs, dusty areas and bird mess are different. See solar panel cleaning.
- Heat. Panels lose a little efficiency as they warm up: about 0.5% per degree above their test temperature, in MCS’s example.
- Age. SMA puts panel output loss at around 0.5% a year. See how long do solar panels last?
- Inverter size. Some inverters are smaller than the panels’ total rating. SMA explains that panels often don’t reach their full rating, and a larger array makes more in the mornings and evenings. On the brightest middays the inverter holds at its maximum and your app’s graph flattens off: that’s clipping, and it’s by design.
Checking your figures
How to check your own figures.
- Find your expected figure. Your original quote or handover pack should include an MCS performance estimate. MCS says it’s guidance for the first year only, because sunlight varies by place and by year. No paperwork? Use kWp × 850 for a south-facing roof, or about 700 for east or west, as our FIT check does.
- Read the generation meter. If you have one, it counts every unit your panels have made since it was fitted. Read it on the same date each month and write it down. Feed-in tariff meter readings shows how.
- Look at the inverter or app. Most show today’s generation and a running total. On a bright day the graph should rise and fall in a smooth curve.
- Use your FIT statements. FIT payments are based on measured generation, so your statements show what your system made. Our FIT check compares your payments with what a working system your size should earn.
Compare a month with the same month last year, not last month, and allow for weather. For that Manchester roof, PVGIS shows the sunniest June between 2014 and 2023 making about half as much again as the dullest, but the best and worst years only about 12% apart. So a whole year tells you more than one month.
Fault or weather
When low output means a fault, not the weather.
Signs it’s more than the weather:
- Nothing on a bright day, or a generation meter that hasn’t moved. See solar panels not working?
- A red or amber light, or an error code on the inverter.
- Output roughly halved on bright days. On a system with two strings of panels, one failed string can look like this.
- One panel low in panel-level monitoring. It can point to a failed optimiser or panel.
- Output that stops at the same time every day, which can mean new shading.
- A year more than about 10% down on the last, when the weather wasn’t unusually dull.
These usually aren’t faults: an app showing offline while the inverter runs normally, a flat top on the brightest middays, and low winter figures that match last winter’s. For an app that has gone offline, see solar monitoring offline?
Our yearly service checks generation against your performance estimate. If something’s wrong, our engineers test each circuit to find the loss, on systems fitted by any installer, across England, Wales and southern Scotland. You get a clear written quote before any work starts. See solar panel servicing.
Sources: Energy Saving Trust; MCS irradiance datasets and MCS 032:2025; PVGIS 5.3 (European Commission JRC); DESNZ feed-in tariff load factor analysis; SMA technical guidance.
Questions
The things people ask before they ring.
Not answered here? Call 0800 156 0191, Monday to Friday, 9:00am–5:00pm.
How much electricity does a 4 kW solar system generate per day?
Averaged over a year, about 9 to 12 kWh a day on an unshaded south-facing roof. PVGIS figures put a June average at about 15 to 18 kWh a day and a December average at about 2.5 to 5 kWh, lowest in Scotland.
How much power does a 400 W solar panel produce in the UK?
Roughly 330 to 450 kWh a year on an unshaded south-facing roof, going by the MCS figures, or about 340 kWh at 850 per kWp. That’s around 1 kWh a day on average: about 1.5 kWh a day in June and under 0.5 kWh in December.
Do solar panels work at night?
No. Panels need daylight, so generation stops at dusk and the inverter switches itself off until morning. To use daytime solar in the evening you need a battery, which the Energy Saving Trust notes can store surplus electricity for later. See solar battery storage.
Why does my 4 kWp system never show 4 kW?
Because kWp is a rating under ideal conditions. SMA notes panels often don’t reach it in real life, because of lower light, heat, dirt and the sun’s angle. Some inverters are also smaller than the panels, which caps output at the inverter’s maximum. A peak below the rating is normal; one much lower than on a similar day last year is worth checking.
Ready when you are
Numbers don’t look right?
We’ll call you back to talk it through, and you get a clear written quote before any work starts. Mon–Fri 9:00am–5:00pm.