Solar power forecast: plan your day around your panels

A weather forecast answers "do I need a coat?". A solar power forecast answers a more profitable question: when is my electricity free today? If you own panels, that's not a nerd curiosity — it decides whether your dishwasher runs on sunshine or on the most expensive tariff of the day.

What a solar forecast actually predicts

Your panels' output is surprisingly predictable. It's the product of three things:

• Geometry — where the sun is, given your location, the date and the time. Known to the minute, years ahead.
• Your installation — peak power, orientation, tilt, and losses. Fixed properties you enter once.
• The sky — cloud cover and irradiance. This is the only genuinely uncertain part, and it's what weather models are for.

Multiply the three and you get a curve: expected watts for every hour of today and tomorrow. The deep dive into the maths is in how solar production forecasts work; the steefware Solar Forecast tool draws that curve for your roof, free, in your browser.

The one habit that pays: load shifting

In most of Europe the kilowatt-hour you export earns far less than the kilowatt-hour you import costs — often a factor of four or more. That asymmetry has a blunt consequence: electricity is worth the most when you use it the moment your roof makes it.

So the game is moving your movable loads — dishwasher, washing machine, dryer, boiler, EV, pool pump — into the production curve. The forecast is what makes that planning instead of guessing:

• Cloudy morning, clear afternoon? Delay-start the washing machine to 13:00.
• Tomorrow looks better than today? The EV can wait a night.
• Only a two-hour sunny window? That's the dryer's slot, not the boiler's.

None of this needs home-automation wizardry. Every appliance with a delay button plus one glance at the forecast at breakfast gets you most of the value.

Forecast for tomorrow, light for right now

A forecast plans the day; it can't see the cloud that just parked over your street. For the live picture I use an €8 LED strip wired to the smart meter: green means exporting right now, red means buying. Forecast in the morning, glance at the light before pressing any start button. Between the two, you've got the whole timeline covered — plan and reality.

Sizing before forecasting

No panels yet, or thinking about adding more? Forecasting starts with what fits: SolarFit works out how many panels your roof takes, with the geometry made explicit instead of hidden in an installer's quote.

Free, no accounts, no cookies — the forecast runs in your browser. Numbers about injection tariffs vary by country, region and contract; check yours before building a business case on a blog post.