Home batteries cut electricity costs to €0.05/kWh

Home batteries: Paying for electricity at €0.05 all day

Buying home batteries to charge at night and use during the day is no longer just for eco-conscious homeowners with solar panels. It’s a matter of tariff arithmetic. A user who stores 10.8 kWh in lithium iron phosphate (LiFePO4) claims to pay €0.052 per kilowatt-hour 24 hours a day, without solar panels, by plugging the system into the grid during off-peak hours. He says his bills never exceed €18 a month for about 400 kWh of consumption. Previously, he paid between €60 and €100.

The calculation is simple: if the average pool price was €0.133/kWh and he pays €0.05, the monthly saving is around €30. With an outlay of €5,300 for the whole set—an EcoFlow DELTA Pro with two accessory batteries, one registered as a UPS to deduct VAT—the amortization stretches between ten and twenty years, depending on whom you ask.

The problem is that nobody has seen one of these batteries die of old age. And that’s where the debate begins.

How long do LiFePO4 batteries really last?

The manufacturer promises 20 years. The buyer, admitting he has no idea, grants them 50% to 70% credibility and estimates they will last ten or twelve years. Ten is enough for him: he recovers the investment and is satisfied. He says LiFePO4 technology is safer than traditional lithium-ion, with less energy density but more robustness.

The discrepancy is significant. A calculation circulating in the discussion argues that current batteries last at least 20 years and return 200% of the investment; with discharges not exceeding 30% and charges not surpassing 95%, the figure rises to 300%. The response is skeptical: every X years they must be replaced because they degrade, and they are not cheap.

There is one measured data point. Charging losses in equipment only a few months old with slow charging below 1,000 Wh do not exceed 1%. If charged above 2,000 Wh, losses climb to 7% or 10%. Logic suggests it will worsen over time, but nobody has verified this yet.

Real cost: €5,300 versus €1,200

Here comes the punchline. An alternative calculation dismantles the price of the compact unit: modular LiFePO4 batteries at €100 per kWh, a 3 kW charger-inverter for €300, and panels at €200 per kW result in a 5 kWh installation for about €1,200. The same package in a “plug-and-play” format costs €5,300. According to this view, the difference is what you pay for not knowing how to assemble it yourself.

Cell prices have fallen below €60 per kWh in some markets, and a 280 Ah cell is around 1,000 W. With these numbers, the argument that batteries are expensive holds up less and less. What remains expensive is the packaging.

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Fridge, freezer, and startup peak

The realistic use is not powering the entire house. It is sustaining the essentials. A no-frost fridge consumes nearly 1.4 kWh daily, more than a standard one, and its startup peak slightly exceeds 1,000 W. To avoid shortening the battery's life, it should be sized double: 2.8 kWh just for the fridge.

The repeated advice is to buy a consumption meter for €7 to €10 and spend a week measuring before deciding anything. The experience of someone who started wanting to back up two chest freezers and two fridges, ending up with a larger system, is the best warning: the purchase grows on its own.

Reusing electric car batteries

A battery that no longer provides peak power to drive a car uphill is sufficient for a house, which does not have those peaks. This idea is being applied with Deye or Ingeteam inverters for 12S configurations or higher voltages, although some batteries require reverse engineering. The obstacle is not technical.

Blackout as a stress test

When the power went out, routers plugged into a socket stopped working even though fiber remained active. UPS systems kept the equipment running. A home battery did the same for the freezer storing meat from the slaughter. That is the other business of these devices: not savings, but continuity.

Charging laptops: USB-C yes, but with nuances

Modern solar generators antiestéticature USB-C outputs of 20 V and up to 5 A that charge laptops without a transformer, provided the device has a USB-C power input and the voltage matches. The laptop regulates the amperes it requests. Not any USB works: only the one for power input.

What is still unknown

No one has had a home battery running daily cycles for twenty years. Calculations are theoretical, and practical results won’t be known until years pass. The uncertainty is not whether the technology works, but how long it takes to stop working and at what price it is replaced.

With these differentials, anyone with a roof and a peak tariff should be installing batteries. Most continue paying their bills. No number closes the gap between calculation and decision.