How much does every kWh passing through the battery cost

The most common way to compare two storage systems is to divide the price by the capacity. It gives a figure in euros per kilowatt-hour, it's easy to calculate, and it has one big flaw: it tells you what it costs to buy, not what it costs to use.

Two products can have the same cost per kWh and a usable lifespan that differs by a factor of five. The first looks like the better deal for the whole length of the buying decision, and stops looking that way the day after the warranty expires.

The full sum

The figure that reconciles price, lifespan and losses is the cost of stored energy: what it costs to pass one kilowatt-hour through that battery, counting its entire usable life.

cost of stored energy = price / (capacity × battery life cycles × efficiency)

The four ingredients, one by one:

• the price is the manufacturer's official list price, not today's price in a shop. A week-long deal shouldn't shake up a ranking;
• the capacity is the usable one, i.e. the energy that actually comes out, not the number printed on the label;
• the battery life cycles are the first limit to run out among cycles, years and guaranteed energy throughput — the calculation is explained in How long does a battery storage system really last;
• the efficiency is the fraction you get back: what goes in, minus the conversion losses.

The result is compared against one thing, the figure on your electricity bill: the price of a kWh from the grid. If storing a kWh costs more than buying it, that battery isn't saving you energy. It might have other reasons to exist — staying powered through a power cut, having electricity where the grid doesn't reach — but that isn't one of them.

What happens when you apply it to a whole catalogue

As of 23 September 2026, of the 88 products tracked, 61 have an official list price and therefore a calculable cost of stored energy. The range runs from 9.3 cents to €2.07 per kilowatt-hour: a factor of twenty-two between the two ends of the same catalogue.

The benchmark to compare them against, for a typical household, comes from Eurostat, second half of 2025, taxes included: an EU average of 28.96 c€/kWh, 38.69 in Germany, 29.66 in Italy, 24.35 in Portugal.

Put together, the two figures say this:

Eleven products out of 61 sit below the European average grid price, and ten of the eleven are balcony battery storage systems. On most portable power stations, storing a kilowatt-hour costs two to four times what it costs to buy one.

The reason isn't the price: it's the lifespan, i.e. the warranty. Balcony battery storage has ten years and portables have two to six, and that ratio goes straight into the denominator of the formula. A power station is a bad deal not because it costs too much: it's a bad deal because the manufacturer stands behind it for too short a time.

The assumptions, which need reading alongside the figure

The sum is done with what manufacturers declare, and they declare little. Three fall-backs, all printed next to the figure on the spec sheet:

Nominal capacity instead of usable capacity. Of the 88 products tracked, two declare how much energy actually comes out. On the other 86 the sum runs on the nameplate capacity, which is larger: the real cost is therefore higher than what you read.

No losses. Round-trip efficiency is declared by two products out of 88. Where it's missing, the sum behaves as if nothing were lost between socket and socket — which never happens. Here too, the true figure is higher.

The missing warranty. Where the manufacturer doesn't declare the years, no short limit cuts the cycles, and the product comes out with a better cost of stored energy than one whose warranty is actually declared. It's a flaw we've seen on real products, which is why the figure carries the warning: staying silent isn't a virtue.

Three fall-backs that all pull in the same direction, and one fact that pulls the other way: the list price is higher than what you'd actually pay. The figure isn't a truth: it's a comparison made with the same rules for everyone.

What the sum doesn't include

It doesn't include installation, it doesn't include the cost of money, it doesn't include standby consumption. It's the cost of the product, not of the project: if the question is "does it pay for itself?", the answer depends on the country you're in, and the tool built for that gives it.

Standby consumption is worth pausing on. It's the current the device draws to stay on while doing nothing, and it's declared by five products out of 88. On one of those five it's twenty watts. Twenty watts for a full year is 175 kWh, or about fifty euros a year at average European prices — on a device that stays plugged in all year round, that's a cost item that doesn't appear in the cost of stored energy but does appear on your bill.

How to use this figure

• Between two products in the same family, it's the fairest comparison you can make given the declared data.
• Between different families, it says something true and blunt: portable and balcony don't compete on the same ground, and choosing them on the cost of energy means choosing balcony.
• Against your country's grid price, it tells you whether that battery is actually saving you money or buying you something else — autonomy, quiet, a socket in the middle of a field. Those are legitimate reasons. They aren't savings.

You'll find the figure on every spec sheet, under "The sum," together with the ingredients that produced it and the assumptions it had to make. If you want to redo it with your own numbers, the formula is the one at the top of this page.

Sources

• Eurostat, Electricity prices for household consumers, consumption band 2,500–4,999 kWh, taxes included, second half of 2025 — Source 1
• Y. Preger et al., Degradation of Commercial Lithium-Ion Cells as a Function of Chemistry and Cycling Conditions, J. Electrochem. Soc. 167 (2020) 120532 — on the fact that cycles aren't kWh — Source 2
• HTW Berlin / aquu, Stromspeicher-Inspektion 2026, for the energy caps in residential warranties — Source 3
• The costs of stored energy are calculated by us across the 88 products tracked on 23 September 2026, using manufacturers' official list prices and the warranty limits declared on each spec sheet. A calculated value has no source: it has a formula, and it's written above.