Sizing a Home Battery Without Guessing

Ten kilowatt-hours has become the default home battery size for no better reason than that it is a round number. Some households would extract more value from five; others waste half their solar surplus with fifteen. The right answer comes from your own load profile, and getting it requires data you probably already have.

What a battery is actually for

Three distinct jobs, often confused:

Self-consumption shifting. Store midday solar surplus, discharge it in the evening. This is the main financial driver in most homes.

Tariff arbitrage. Charge from the grid when energy is cheap, discharge when it is expensive. Valuable only where a time-of-use or dynamic tariff creates a spread above the round-trip cost.

Backup. Keep essential circuits alive during an outage. This requires specific hardware — a transfer switch or a hybrid inverter with backup capability — and not every battery installation provides it. Ask explicitly.

Size for the job you actually want. A backup-focused sizing exercise looks nothing like a self-consumption one.

Step one: get your evening load

Most utilities will provide interval data — usually half-hourly — through an online portal. Export a full year if you can, a winter month and a summer month if you cannot.

What you need is the energy consumed between the moment solar production ends and the moment it resumes. In practice, sum consumption from roughly 17:00 to 08:00 and take the median across the year. For a typical European household without electric heating this lands between 5 and 9 kWh. With a heat pump it can exceed 25 kWh in winter, which changes the conversation entirely.

The median matters more than the mean. Sizing to cover the worst evening of the year means buying capacity that sits idle for 360 days.

Step two: check the surplus is there to fill it

A battery cannot store energy the array does not produce. Take a summer day's production profile and subtract the daytime load. If your 6 kW array in June produces 30 kWh and your household consumes 8 kWh during daylight, the surplus is 22 kWh — plenty. In December the same array might produce 6 kWh with 5 kWh consumed during daylight, leaving 1 kWh of surplus and a battery that never fills.

This seasonal asymmetry is the single most underappreciated fact about home storage. A battery in a temperate climate earns most of its money between March and October and does very little in midwinter, exactly when consumption peaks.

Step three: convert to usable capacity

Datasheets quote nominal capacity; what you can actually use is smaller.

• Depth of discharge. LFP chemistry typically permits 90 to 100 percent; older NMC systems often reserve more.
• Round-trip efficiency. 88 to 94 percent for a modern AC-coupled system, a point or two better DC-coupled.
• Reserve for backup. If you want backup, some capacity stays permanently held.

A "10 kWh" battery with 95 percent depth of discharge and 90 percent round-trip efficiency delivers about 8.6 kWh to the house. Size against that number, not the badge.

Step four: sanity-check the power rating

Capacity is kilowatt-hours; power is kilowatts. A battery rated at 3.6 kW continuous cannot run a 7 kW electric shower and an induction hob simultaneously, whatever its capacity. If backup matters, list your simultaneous essential loads and compare against the continuous and peak inverter ratings — and note that motor loads such as pumps and compressors draw several times their running current at startup.

Chemistry, briefly

Lithium iron phosphate now dominates home storage and deserves to. It tolerates full discharge, holds up over 6,000 cycles at 80 percent retention, does not use cobalt, and has a thermal runaway threshold well above the nickel-manganese-cobalt alternative. NMC packs more energy into less space and mass, which matters in vehicles and rarely matters on a garage wall.

Whatever the chemistry, read the warranty as an engineer would. It will specify a term in years, a throughput in megawatt-hours or a cycle count, and a retained capacity percentage — and it expires at whichever comes first. A ten-year warranty with a 24 MWh throughput cap on a 10 kWh battery permits roughly 240 full cycles per year; heavy tariff arbitrage can exceed that and quietly void the term.

A worked example

A household consumes 4,200 kWh per year, with 6.5 kWh median evening load. The array is 6.5 kW producing 6,800 kWh annually. Daytime surplus averages 14 kWh from April to September and 2 kWh from November to February.

Sizing at 6.5 kWh usable — roughly a 7.5 kWh nominal LFP unit — covers the median evening, fills reliably for seven months of the year, and cycles close to once per day in season. Estimated shifted energy: about 1,700 kWh per year. At a 0.26 €/kWh spread that is €440 annually against perhaps €6,000 installed, giving a fourteen-year simple payback against a ten-year warranty.

Doubling to 15 kWh nominal would shift perhaps 2,100 kWh, not 3,400, because the surplus runs out. The extra 7.5 kWh earns €100 per year for €4,000 of hardware. The marginal kilowatt-hour is where oversizing gets punished.

Installation details worth insisting on

• Location. Batteries dislike heat. A garage that reaches 40 °C in summer will age the cells faster than an interior utility room. Some jurisdictions restrict installation inside habitable spaces or under escape routes.
• Ventilation and clearance. Follow the manufacturer's clearances exactly; they are fire-safety derived, not cosmetic.
• Isolation and labelling. Emergency responders need to find and isolate a DC energy source they cannot switch off remotely.
• Monitoring. Insist on data you can actually see. A battery whose state of charge you cannot inspect is a battery whose faults you will discover late.

Frequently asked questions

Will a battery let me go off-grid? Almost certainly not in a temperate climate. Covering December from stored summer solar would require capacity in the hundreds of kilowatt-hours. Off-grid living means a much larger array, a generator, and a different lifestyle.

Can I add a battery later? Yes, but AC-coupling a battery to an existing string inverter costs more and loses a couple of points of efficiency compared with a hybrid inverter installed from the start. If storage is likely within five years, buy the hybrid inverter now.

Do batteries need maintenance? Firmware updates and an annual check of connections and ventilation. The cells themselves are sealed.

What happens at end of life? Capacity fades rather than failing outright; a pack at 70 percent retention still works, just for less. Recycling routes for LFP exist and are improving, and reputable manufacturers operate take-back schemes.

The short version

Pull your interval data, find the median evening load, check the seasonal surplus, divide by depth of discharge and efficiency, and buy that size. It will usually be smaller than the quote, and it will earn more per euro than the round number would have.