What Is a Solar Forecast, and Can You Actually Trust One?

What "solar forecast" actually means

A weather forecast tells you if it'll rain. A solar forecast tells you something different, and more useful if you own panels: how many kWh your specific system is likely to generate, today and for the week ahead.

That distinction matters more than it sounds. "Sunny" and "good for solar" aren't quite the same thing — a hazy, high-cloud day can sometimes outperform a technically clear one, because diffuse light still reaches panels from every angle rather than in one direct beam. A proper solar forecast accounts for that. A weather app's little sun icon doesn't.

Why a forecast beats a spec sheet

Every solar panel comes with a manufacturer's rated output — the number on the box, measured under lab conditions nobody's roof ever sees. It's a ceiling, not a prediction. What you actually get on a Tuesday in November depends on cloud cover, panel angle, shading, temperature and where you live — none of which the box can tell you.

That's the actual job of a solar forecast: turn "my panels are rated at 400W" into "expect about 2.1 kWh tomorrow," personalised to your system and your weather, not a generic average for your postcode.

How accurate can a solar forecast actually be

Short answer: good enough to plan around, not perfect enough to bet the house on. We publish our own numbers rather than a marketing claim — real predicted-vs-actual accuracy from opted-in users, updated monthly, because a forecasting tool that won't show you its own track record isn't one you should trust.

The honest caveats: forecasts get less reliable the further out they run — tomorrow is more trustworthy than day 7 — and locations with fewer contributors take longer to build a reliable accuracy picture. Both of those are true of every forecasting tool, solar or otherwise. The difference is whether the tool tells you.

What makes a solar forecast actually useful, not just accurate

Accuracy on its own isn't the whole story. A forecast that's 95% accurate but only ever shows a national average isn't especially useful to you personally — a forecast that's personalised to your system's real best day, wherever you are, is worth more even at a slightly lower headline accuracy figure.

That's the whole mechanic behind Sun Hours: tell it the most your panels have ever generated in a day, and every forecast after that is scaled to your actual system, not a hypothetical one.

Free solar forecast apps: what to actually look for

We did the comparison properly elsewhere rather than repeat it here, but the short version: look for a forecast that asks about your actual system rather than just your location, shows its accuracy rather than just claiming it, and doesn't need an account or a subscription just to tell you tomorrow's number.

Try it on your own roof

Sun Hours is free, doesn't need an account, and gives you a 7-day solar forecast built around your system's own best day, not a generic postcode average. Enter your location, tell it your best day, and see what tomorrow actually looks like.