Solar storage battery: usefulness, sizing and price

What is a battery for?

Your panels produce mostly in the middle of the day, while the home consumes mostly in the morning and evening. Without a battery, the day's surplus is exported to the grid (often at a tariff well below the purchase price), then you buy electricity back in the evening. The battery stores that surplus and releases it when you need it.

In practice, it raises the self-consumption rate: without storage you typically use ~40 % of your production on site; with a well-sized battery you often reach 60 to 80 %. Less energy bought from the grid means a lower bill — at the cost of an extra investment.

When does a battery make sense?

High consumption in the evening and at night

Yes — the most favourable case: you use the stored energy directly instead of buying it back from the grid.

Electric car charged at home, heat pump

Often yes — consumption is large and time-shifted, the battery absorbs the solar surplus.

Low consumption or tight budget

Often no — a battery alone struggles to pay for itself; better to first maximise direct self-consumption (shifting appliances to daytime).

Need for back-up during outages

A valid argument, but check that the inverter and battery actually handle back-up mode (not every configuration does).

Grant in Luxembourg

The Klimabonus grant for a storage battery can reach up to €2,250, which noticeably improves the payback calculation.

Sizing your battery (kWh)

A simple rule of thumb is a starting point: count about 1 to 1.5 kWh of usable capacity per kWp of panels installed. For a 6 kWp system, that means a battery of roughly 6 to 9 kWh usable. Oversizing makes no sense: a battery that is too big never fully charges and its extra cost does not pay off.

Then refine it to your evening consumption: the goal is to cover needs between sunset and the next morning, not to store several days of autonomy. On technology, favour lithium iron phosphate (LFP): safer, long lifespan and good resilience to daily cycling.

Common brands & models

Indicative prices, excluding installation and grants, to confirm by quote. Prices 'per kWh' refer to the module capacity.

Frequently asked questions

How long does a solar battery last?

An LFP battery is usually warrantied for 10 years or 6,000 to 10,000 charge/discharge cycles, well over a decade in home use (one cycle per day). Its capacity declines slowly over time; at the end of the warranty it often retains ~70-80 % of its initial capacity.

Is a battery worth it?

Let's be honest: a battery alone has a long payback, often around 10 to 14 years, sometimes close to its lifespan. The Klimabonus grant (up to €2,250 in Luxembourg), a wide gap between purchase price and export tariff, and high evening consumption all shorten that period. It is also justified by autonomy and comfort, not by the financial calculation alone.

AC or DC battery: what's the difference?

A DC battery (DC-coupled) connects on the panel side via a hybrid inverter: better efficiency, ideal for a new system. An AC battery (AC-coupled) has its own inverter and connects on the home side: simpler to add to an existing system, but with a slight efficiency loss from the extra conversions.

What is a 'virtual battery'?

It is a commercial offer from some suppliers: your exported surplus is 'credited' and you 'recover' it later on your bill, as if it had been stored. No physical battery is installed at your home. It can limit cost, but read the terms carefully (subscription, caps, taxes on the recovered energy) before comparing it with a real battery.