The evening decides the battery size, not the roof.

Self-consumption is the share of the solar yield used in the building, directly or via the battery. Self-sufficiency is the share of the building’s consumption covered by solar power. The same kilowatt-hours give two different percentages, because they are divided by different things.

A battery raises both by moving surplus from the middle of the day to the evening and night. Its value lies in the difference between the price of imported electricity and the payment for exported electricity, minus the battery’s losses.

Annual consumption says little about the battery. What matters is when the consumption happens: a house with an electric car charged overnight and one with a heat pump running around the clock have different evenings. Hourly meter readings from the grid operator for the past year show this directly. Without them, a standard profile will have to do, with more uncertainty.

The yield is calculated for the roof hour by hour, with the panels’ orientation and tilt. Set the two series against each other: the surplus in the middle of the day is what the battery can be charged with, and the shortfall in the evening and night is what it can supply.

Simulate a year hour by hour for several sizes and work out what each step adds in money per year. The first kilowatt-hours are used almost every day with solar surplus. The last ones are used only when both the surplus and the evening consumption are large.

The curve flattens, and the knee is where an extra kWh starts to add clearly less than the first ones. Beyond the knee, the payback period gets longer with every module, even if self-consumption keeps rising a little.

The season moves the knee. In Stockholm, a south-facing panel yields barely a tenth as much in December as in June according to PVGIS; in Madrid, two thirds. A battery sized for the summer surplus therefore sits unused more in the north than in the south.

With hourly prices, the battery can do more than move the building’s own solar power. It can charge from the grid when the price is low and supply when it is high, and save solar power for the most expensive hours instead of using it straight away. A plan for the next day or two decides when.

That changes the sizing. A battery that also works against the price is used on more days of the year, even in winter when the solar surplus is small, and the knee can move towards a larger size. The benefit comes from the price differences over the day and varies between years and bidding zones. Charging from the grid only pays when the expensive price is higher than the cheap one divided by the efficiency.

If the battery is to keep essential loads running during a power cut, it is also sized for power and duration: essential load in kW times the number of hours gives kWh. 1.5 kW for 8 hours needs 12 kWh usable, plus a margin because the battery is seldom full when the power goes.

Backup requires an inverter with a backup output or the ability to run in island mode, and an installation built for it with a changeover switch or gateway as the manufacturer specifies. Also check the maximum power of the battery and inverter against the loads that have to start.

A high-voltage battery has to match the inverter’s battery input in voltage and current, and the manufacturer has to have approved the combination. The number of modules is set by the voltage window: there is a minimum and a maximum number that fit.

Charge and discharge power are limited by both the battery and the inverter. A large battery behind a small inverter is not emptied faster than the inverter allows, and a short winter evening with a high load can then require imported electricity even though the battery still has energy left.

Sweden: the tax reduction for green technology covers 50 % of the labour and material cost of systems for storing self-generated electricity, up to SEK 50,000 per person and year for green technology in total. The battery has to be connected to PV panels that are already in place or installed at the same time, and the reduction is applied on the invoice.

Germany: supply and installation of solar panels, essential components and batteries that store the solar power carry 0 % VAT (§ 12 (3) UStG) when the system is on or near homes or certain public-benefit buildings. The condition is deemed met when the system is no more than 30 kWp according to the Marktstammdatenregister.

Spain: with simplified compensation (compensación simplificada, RD 244/2019), surplus is valued hour by hour, but the compensation over a billing period of at most one month may not exceed the value of the electricity bought. Surplus beyond that earns nothing, and that is where a battery is useful.

Italy: the renovation deduction (Bonus Casa) for 2026 expenditure is 36 %, or 50 % for your own main residence, on at most €96,000 per dwelling according to the Agenzia delle Entrate. ENEA describes the bonus as covering measures for energy saving and renewable energy. Check in the authorities’ guidance that the battery is covered and which documents are required.