Cloudy, night and winter: what to expect from production

A grey day, drizzle, and you looking out the window with the app open: the number is on the floor. First reaction: has something broken? Second: have I been sold a lemon?

Neither. But the question deserves a complete answer rather than a generic sales line, so here are the three big ones: night, clouds and winter.

At night the panels don't generate. Nothing, zero. There's no "moonlight" technology and no small print that changes this: the panel converts sunlight into electricity, and at night there is none.

So what happens in your house at ten at night? The same as always: you draw from the grid, as you do now. The difference is in the monthly balance: during the day you generated, part of it you used directly, part was exported and credited to you, and that credit offsets some of what you consume at night. The system doesn't disconnect you from the grid: it changes your account with it.

Want to use your own energy at night? That exists and it's called batteries: you store the midday surplus and use it later. It's a separate decision, with its own numbers to weigh, and we've given it a full article.

Here the answer is less binary, and more interesting: on a cloudy day the system keeps generating, but less. The diffuse light coming through the clouds is light too, and the panels use it. How much less depends on the type of cloud: a high veil is not the same as a closed storm sky.

What matters is the shift in mental scale: a system isn't judged by days, it's judged by the year. When your installation was sized, the cloudy days in your area were already in the calculation: they're in the solar statistics used to work it out. Grey days aren't a surprise that ruins the plan. They're part of the plan.

A pleasant side effect of monitoring: you'll start looking at the sky differently. Everyone goes through the phase of opening the app on a cloudy day "just to see how much it's doing". It's normal. It passes.

Winter combines three things: shorter days, a lower sun and, depending on the region, more cloud. Result: a winter month generates visibly less than a summer one. The gap between the best and worst month of the year is large, and seeing it for the first time in the app is alarming if nobody warned you.

So here's the warning: the annual generation curve is belly-shaped. High in summer, low in winter. Your June will be lean. Everyone's is lean. It isn't a fault; it's astronomy.

Two clarifications that usually come as pleasant surprises:

• Cold isn't the enemy. Heat lowers a panel's efficiency slightly; cold temperatures make it more efficient. A freezing, clear midday can show excellent output. For fewer hours, that's the catch.
• The year balances out. The generous months offset the lean ones, and the credit scheme with the utility helps that balancing. The calculation that matters is the annual one.

• Ask for the estimate month by month, not just the annual figure: that way your first winter doesn't catch you off guard
• Always compare the same month: June against June, never June against January
• Grey day with a low number: normal, don't touch anything
• Night with zero generation: normal, that's how it works
• What does deserve attention: a large deviation against the same month last year, with similar weather (there's a whole article on that)

The one-line version: your system lives by day, runs at full tilt in summer and at half throttle in winter, and the economics are made on the yearly average. Which is exactly how it was calculated.