Enter your UK postcode and get today's peak sun hours plus a full hour-by-hour solar irradiance curve — free, instant, no account needed. Then take it further with a personalised kWh forecast in the Sun Hours app.
Why "how much will my solar panels generate today?" is the right question
Most solar apps answer the wrong question. They show you last month's export, last year's carbon saved, or a spec sheet summary from your installer. Useful data — but not what you actually want to know on a Tuesday morning when you're deciding whether to put the washing on now or wait until noon.
The question that matters is: what will my panels generate today, and when?
That's exactly what the Sun Hours calculator on this site answers — and it does it in about five seconds, for any location in the UK or worldwide.
What peak sun hours actually mean for your panels
"Peak sun hours" is the metric that matters most for any solar panel system, but it's also one of the most misunderstood numbers in home energy.
It's not the number of daylight hours. It's not the number of hours the sun is "out." It's the total solar energy your location receives in a day, expressed as the equivalent number of hours at full standard irradiance (1,000 W/m² — the intensity used to rate solar panels in lab tests).
Here's why that matters practically: a 4 kW solar panel system in a location with 4 peak sun hours will generate roughly 16 kWh on an average day at that level. The same system with 2 peak sun hours — a grey November day, or a location further north — will produce roughly half that.
The calculator at the top of this page fetches live data from Open-Meteo's weather API and calculates today's peak sun hours for wherever you enter. It's the same underlying data the Sun Hours app uses.
Why the hour-by-hour curve changes everything
The daily total matters. But the shape of the day is what actually lets you make decisions.
If you know your panels peak between 10am and 2pm on a typical sunny day, you can:
- Run high-draw appliances (dishwasher, washing machine, tumble dryer) during that window — using your own free electricity rather than drawing from the grid
- Time your EV charging to coincide with peak generation rather than overnight
- Decide whether to run the immersion heater now or wait for tomorrow's forecast
- Spot when a weather front is rolling in and act before your free energy window closes
The chart above shows what a good solar day looks like in the southwest of England — peak irradiance around 850 W/m² just after 1pm, with a meaningful window of strong generation from about 10am to 4pm. The curve looks similar across the UK in pattern, though the peak height varies significantly by region and season.
UK regions: what the calculator shows you by location
Enter different postcodes and you'll see genuinely different numbers. The UK has more solar variation than most people realise:
| Region | Example postcode | Avg. annual peak sun hrs/day |
|---|---|---|
| Cornwall / Devon | TR1, EX1 | 3.2 – 3.6 hrs |
| London / South East | SW1, RH1 | 2.9 – 3.3 hrs |
| Bristol / South Wales | BS1, CF10 | 2.8 – 3.1 hrs |
| Birmingham / Midlands | B1, CV1 | 2.6 – 2.9 hrs |
| Leeds / Yorkshire | LS1, HG1 | 2.5 – 2.8 hrs |
| Manchester / NW England | M1, PR1 | 2.4 – 2.7 hrs |
| Edinburgh / Central Scotland | EH1, FK1 | 2.2 – 2.5 hrs |
| Inverness / NW Scotland | IV1 | 2.0 – 2.3 hrs |
These are annual averages — in summer, every region can hit 5+ peak sun hours on a clear day. In winter, even Cornwall can drop below 1.0. The calculator shows you today's actual figure, not a long-run average.
From irradiance to kWh: bridging the gap
The web calculator tells you solar irradiance — the energy arriving at the ground, in W/m². That's universal: it's the same for every solar panel regardless of size, brand, or efficiency.
To get from irradiance to kWh generated by your specific system, you need one more piece of information: how your panels actually perform. And that varies enormously by:
- System size (2 kWp vs 6 kWp makes a large difference)
- Roof orientation (south-facing vs east-west split)
- Shading (a chimney casting a shadow at 3pm costs more than you'd think)
- Panel degradation (a 10-year-old system may produce 10–15% less than new)
- Inverter efficiency
The Sun Hours app bypasses all of this complexity with a single question: what's the most kWh your system has ever generated in one day? That "best day" number already bakes in every one of those factors — it's the fingerprint of your specific installation. The app then uses that alongside the irradiance forecast to tell you exactly what to expect, hour by hour, in kWh rather than W/m².
How to use the calculator on this page
- Enter your UK postcode — or any city or town name, or a lat/lon coordinate pair for locations outside the UK
- Hit Calculate — the chart loads in a few seconds using live weather data
- Read the peak sun hours figure for today at your location
- Scan the hourly chart to see when the generation window opens and peaks
- Use that to plan your day — or download the Sun Hours app to convert those irradiance figures into personalised kWh predictions for your roof
The calculator is free, requires no account, and stores nothing. Your postcode never leaves your browser.
The question your inverter app can't answer
Every inverter brand — SolarEdge, SMA, Enphase, GivEnergy, Solis — has its own monitoring app. They're useful for reviewing what happened. What they don't do is tell you what will happen tomorrow, or whether to run the dryer at 11am or 2pm.
Sun Hours is a forecast tool, not a history viewer. It's designed for the morning question: how much free energy do I have today, and when?
Try the calculator on the homepage or download the app to go deeper with a personalised 7-day kWh forecast.
Try the free Solar Panel Output Calculator
Enter your postcode at sunhours.app to see today's peak sun hours and hour-by-hour irradiance curve — then download the app to get a personalised kWh forecast based on your actual panels.
Get it on Google Play →