Home batteries have moved from enthusiast curiosity to mainstream consideration in a few years, and the reason is not that batteries got dramatically cheaper. It is that two things changed around them: net metering arrangements began ending, and dynamic tariffs arrived. Both make stored electricity worth more than it used to be.
Whether one pays for you is a genuine question with a genuine answer, and it is arithmetic rather than opinion. Here is how to do the arithmetic honestly.
Three quite different reasons to buy one
Being clear which you are buying prevents most disappointment.
Self-consumption. Store your own solar generation for use after dark. Value depends on the gap between your import price and your export rate — large where net metering has ended, close to zero where it has not.
Tariff arbitrage. Buy electricity when it is cheap, use it when it is expensive. Value depends on the daily price spread, and crucially this works with no solar panels at all. In markets with volatile hourly pricing, this is increasingly the stronger case.
Backup power. Keep the house running during an outage. Value depends entirely on how often your supply fails and how much that costs or matters to you. This is insurance rather than investment, and it should be priced as such.
Most people buy for the first reason, and in several markets the second is now the better argument.
The arithmetic, step by step
You need four numbers, and three of them you can measure rather than estimate.
1. Usable capacity, not nominal
A battery sold as 10 kWh may make 9 kWh available, holding a reserve. Ask for the usable figure.
2. Round-trip efficiency
Energy in versus energy out, typically 85–95 per cent. The losses are real money: at 90 per cent, storing 10 kWh returns 9. Ask for this figure and be suspicious if it is not offered.
3. Your daily throughput
How many kilowatt-hours will actually pass through it per day? This is where most estimates go wrong. It is limited by whichever is smaller: your evening consumption, or your available surplus or cheap-rate window. A 15 kWh battery in a house that uses 6 kWh after dark cycles 6 kWh, not 15 — and you paid for 15.
4. Your value per kilowatt-hour cycled
For self-consumption: import price minus export rate. For arbitrage: expensive-hour price minus cheap-hour price. Both after all charges and taxes, not wholesale.
Then: daily throughput, times value per kilowatt-hour, times round-trip efficiency, times 365, gives annual benefit. Divide the installed cost by that to get simple payback in years. Compare with the warranty period and the cycle warranty.
Where the honest answer is usually no
Some situations do not add up, and it is better to know before spending.
Generous net metering still in place. If exported power earns the same as imported power, storing it achieves almost nothing financially. Wait until the arrangement changes.
Flat tariff with no time variation. No spread means no arbitrage. Get the tariff first; it costs nothing.
Low evening consumption. A household that uses very little after dark cannot cycle a battery enough to justify it.
Buying it for backup without checking. Many batteries cannot run the house in an outage at all — they require a grid connection to operate and have no island mode. If backup is the reason, verify that capability specifically and in writing.
Where it usually does add up
Net metering ended or ending, with solar already installed. The classic case, and the reason batteries are selling in the Netherlands.
Dynamic tariff with a large daily spread, even with no panels at all. Charge at 3am, discharge at 6pm, every single day, automatically.
An EV plus solar plus a dynamic tariff. The combination that makes the arithmetic comfortable, because you have both a large flexible load and a large price differential.
Genuinely unreliable supply. If outages are frequent and consequential — working from home, medical equipment, a business — the insurance value is real and does not need to be justified on payback.
Chemistry and safety, briefly
Most domestic batteries sold now use lithium iron phosphate — LFP — rather than the nickel-manganese-cobalt chemistry common in cars. For a home installation that is the preference, and for good reasons.
LFP tolerates being held at full charge better, which matters because a home battery spends a lot of its life sitting full. It offers more cycles, commonly several thousand rather than one or two thousand. And it is markedly more thermally stable, which is the safety argument: LFP is much less prone to thermal runaway, the self-sustaining overheating that makes a battery fire so difficult to extinguish.
The trade is energy density — LFP is heavier and bulkier for the same capacity. In a car that matters enormously. Bolted to a wall in a utility room, it does not.
Beyond chemistry, ask about certification and installation standards in your country, whether the unit needs a specific ambient temperature range, and whether your insurer wants notifying. Reputable installers handle all three as a matter of course; the answer to whether they have is a useful signal about the rest of their work.
Sizing without overspending
The commonest expensive mistake is buying too much capacity.
Start from your measured evening consumption — the kilowatt-hours you use between sunset and bedtime. For most households that is somewhere between 4 and 10 kWh, and that is the capacity that will actually cycle daily.
Then consider power output, which is separate and often overlooked. A battery with 10 kWh of capacity but only 3 kW of output cannot run an induction hob and a kettle simultaneously. Check the continuous output figure against your realistic peak.
And prefer modular systems where you can add capacity later, because you will understand your own pattern far better after a year with monitoring than you do now.
The questions to ask an installer
- Usable capacity and round-trip efficiency, in writing.
- Will it work with my existing inverter, or does it need replacing? Several brands only pair with their own.
- Can it provide backup during an outage, and what does that require?
- Cycle warranty and year warranty, and what capacity is guaranteed at the end.
- Can it charge from the grid, not only from solar? Arbitrage requires this and not all can.
- Can it be controlled by my own system, or only the manufacturer’s app? This matters for tariff automation.
- Indoor or outdoor rated, and where exactly will it go? Batteries are heavy and some need a specific environment.
What to do before deciding
- Reduce your standing load. Cheaper than storing electricity you did not need.
- Get onto a time-varying tariff if one is available. Free, and it creates the spread a battery monetises.
- Measure for a month: daily consumption, evening consumption specifically, and if you have solar, your actual self-consumption rate.
- Shift what you can for free — appliances into cheap hours, surplus into hot water.
- Then do the four-number arithmetic with your own figures.
- Size from measured evening consumption, choose modular, and check the output figure.
Step three is the one that changes decisions. Households frequently discover their evening consumption is half what they assumed, which means a smaller and considerably cheaper battery is the right answer.