Self-consumption: why it matters more than how much you produce

Most owners watch one number: how many kilowatt-hours the system produced. That's understandable, and it's misleading. The money isn't in the kilowatt-hours produced — it's in the ones not bought.

Two identical systems on two neighbouring roofs can produce exactly the same and save one household twice what they save the other. The difference is when the electricity gets used.

Two numbers that get confused

They sound alike and measure almost opposite things.

Self-consumption — how much of what you produce you use at home:

self-consumption % = (production − exports) / production

Self-sufficiency — how much of what you consume the system covers:

self-sufficiency % = (production − exports) / (production − exports + imports)

A small array on a large house has high self-consumption (all of it gets used) and low self-sufficiency (it isn't enough). A large array on an efficient house is exactly the reverse. Neither number is "better" on its own — only the pair says anything.

Why it decides the money

A kilowatt-hour has two different values depending on where it goes.

When you use it at home, you avoid the full purchase price from the grid — including network charges and taxes.

When you send it to the grid, you get the export price, which is substantially lower and varies a lot by contract.

The gap between those two is the entire point of managing self-consumption. Moving one kilowatt-hour from export to household use earns roughly that gap — and it earns it without producing a single extra kilowatt-hour.

A worked example from the Czech market in 2026, where we've verified the figures: buying costs roughly 5–6.5 CZK/kWh including network charges and VAT, while exports earn approximately 1.5–3 CZK/kWh. So a kilowatt-hour kept at home is worth on the order of two to four times one exported. Your numbers will differ by country, supplier and contract — these are ranges, not a single figure, because a single figure doesn't exist. What holds almost everywhere is the direction: buying costs noticeably more than exporting earns.

What's typical

A household with no load management at all sits around 30 % self-consumption. That's a rule of thumb, not a measured average — treat it as the line below which it's worth thinking, not as a target.

Which is why our diagnostics flags self-consumption below 30 %. It's filed as an opportunity, not a problem — nothing is broken, there's simply money on the table. And it only reports after three months of measured balance, so it isn't a conclusion drawn from one sunny week.

How to work it out yourself

You don't need a service for this. Most inverters keep cumulative counters since installation — total production, total exported, total imported. Put them into the formulas above and you have both numbers for the whole operating life.

What those counters can't tell you, and it's honest to say so:

• They're averages over the entire operating life. Last year's heat pump or electric car dissolves into them, and recently improved habits don't show.
• They may not start at the same moment. An inverter replacement or metering fitted later at the connection point shifts them relative to each other, and a single reading won't reveal that.
• They say nothing about the shape of the day — how much surplus arrives at once at midday. Which is exactly what matters for sizing a battery.
• Seasonality disappears. Surplus is heavily a summer affair; an annual average hides that.

What to do about it — in order of cost

This ordering matters more than the individual measures. It's sorted by cost per unit saved, not by how attractive each option sounds:

1. Shift consumption to midday. Free. Dishwasher, washing machine, dryer, car charging moved to when the system produces most. Most appliances have a delayed start and it costs nothing. Always start here.
2. Heat water from surplus. The hot water tank is the largest battery already in your house — it just stores heat instead of electricity. A surplus controller can fill it with what would otherwise go to the grid. (We quote no price: the retailers that carry these controllers keep price lists behind a trade login and the manufacturer doesn't publish one. Until we can verify it from a primary source, no figure beats an invented one.)
3. A battery. It works, but it's the most expensive of the first three steps and carries hard constraints: it often requires a hybrid inverter (otherwise you also pay to replace yours, or for an AC-coupled solution), its life is measured in cycles rather than years, and capacity sized from an annual average surplus is oversized for winter. The full calculation, including when payback doesn't add up, is in does a solar battery pay off.
4. More panels. These usually lower self-consumption — they produce more at times when there's already surplus. They make sense in a different situation: when you use everything and still buy on top.

Recommending a battery before free load shifting would be selling, not advising. We keep to that order even where the reverse would earn us more.

Working out which situation is yours

What decides is the ratio of exports to imports, not absolute production:

And winter

The ratio flips across the year. In summer most output goes to the grid at a low export price; in winter you use most of the little you make at home, displacing expensive imports. A December kilowatt-hour is therefore worth more to your wallet than a July one, even though there are far fewer of them — see solar output in winter.

Where Slunora helps

If you've connected an inverter, we compute both numbers from the connection-point balance continuously, and after three months we turn them into a recommendation — including the cases where the right answer is "do nothing".

And where an estimate would rest on data that's too thin, we don't make one. Splitting "produced" from "used at home" requires metering at the connection point; without it the field stays empty rather than filled with a guess.