Does a retrofit solar battery pay off?

The most common error in the sums a seller shows you is counting the full price of electricity. A battery doesn't earn by "producing" energy — it earns by shifting it in time. What you actually gain is the gap between the price you'd pay for that kilowatt-hour in the evening and the price you'd get for selling it at midday. This walks through exactly the logic our upgrade advisor uses.

Where the saving comes from

A kilowatt-hour a battery shifts from midday surplus to evening use saves you the buying price and costs you the feed-in price you'd otherwise have received for exporting it. The margin earned is the gap between the two:

margin per kWh = buying price − feed-in price

Using Czech 2026 figures as a worked example, where we've verified them: buying runs 5.0–6.5 CZK/kWh (energy plus network charges, incl. VAT) and feed-in 1.5–3 CZK/kWh (fixed tariffs nearer the low end, spot exports nearer the high end). The margin therefore works out to roughly 2–5 CZK per shifted kilowatt-hour — and it's this margin, not the full price of electricity, that's the economic basis of the whole decision.

Your own country's numbers will differ, but the principle — buying costs more than exporting earns — holds almost everywhere.

How much can actually be shifted

This is the step seller math most often skips. Only as much energy can be shifted as is simultaneously available as surplus and simultaneously being bought back at another time of day — not the full annual production or the full annual import.

Sizing starts from the smaller of two daily averages (daily export vs. daily import), and even of that we only count 60–80 % — you never use a battery at a full 100 %: depth of discharge, an operating reserve, cycle losses, and above all the days when surplus simply isn't enough to fill it. On top of that comes a round-trip charging efficiency of around 90 % — on average 10 % of every kilowatt-hour stored is lost in the round trip.

Batteries are also sold in modules, not individual kWh — so sizing rounds to whole units (a SolaX module is 5.8 kWh), so the suggested size is something you can actually order, not a theoretical figure.

Price — and why it's always two line items, not one

We quote the price per kWh of capacity split into hardware and installation, never as a single figure. Someone who reads "battery at 7,000 CZK/kWh" and then gets a quote for 13,000 might think they're being overcharged — when the difference is wiring, inspection and the installer's margin.

• Hardware: 7,000–8,100 CZK/kWh. A verified sample of real prices from three Czech retailers for the SolaX T-BAT H 5.8 kWh module, incl. VAT (verified 2026-08-03 directly on product pages, not from search summaries).
• Installation, wiring, inspection and margin: 2,000–8,000 CZK/kWh. Derived as the gap between turnkey installation price and the verified hardware price — less certain than the hardware figure alone, because it's a derived remainder, not a direct price.

If you have a specific quote, you can override the price in your report with your own figure — the calculation then uses your exact number instead of our estimate.

When payback doesn't add up

Payback is assessed at both ends of the range, not just the favourable one — otherwise a case like "7–38 years" would pass as a recommendation on the strength of its optimistic end alone.

• Even the best case exceeds 12 years. Then a battery isn't recommended at all — with your balance it likely won't pay for itself.
• The upper end exceeds 15 years (the estimated battery lifetime). It can still be recommended, but with a clear caveat: under an unfavourable price trend the investment may never pay off.

A battery's lifetime is also set by cycle count, not years — one sitting unused doesn't age the same way as one discharged fully every day.

When a battery makes no sense at all

A battery earns by storing surplus. When there's barely any — you use most of what you produce directly — there's nothing to store, and the money makes more sense elsewhere: in more panels, if you're also importing a lot from the grid.

A battery also often requires a hybrid inverter. If you have an ordinary grid-tie one, add its replacement or an AC-coupled solution to the cost — the inverter data can't tell us this, so it's never folded into the estimate, only flagged.

What's always computed before the paid option

Shifting consumption to midday is free — running the water heater, dishwasher, washing machine or car charging when the system produces most earns the same margin as a battery, with zero investment. Recommending a battery before this would be selling, not advising — and we keep the same order.

What's left out of the estimate

• The Czech NZÚ subsidy (10,000 CZK/kWh of battery, capped at 140,000 CZK) — eligibility depends on configuration and past claims, so it isn't folded into the figure. With it, payback is shorter.
• Inflation, energy price growth or maintenance costs — this is a simple payback calculation.
• Seasonality. Capacity is sized from the ANNUAL average surplus. In winter such a battery will be oversized — surplus is strongly a summer affair.

Where Slunora helps

If you've connected an inverter, the upgrade advisor computes all of this from your actual balance after three months of measurement — including the cases where the right answer is "a battery doesn't pay off right now". Prices apply only to Czechia and its near border; outside that area the same logic is shown without figures.