Battery sizing: the 5/10/15 kWh decision tree

This guide exists because the question behind it — battery sizing: the 5/10/15 kwh decision tree — is one of the most searched, most mis-sold topics in home energy in our three markets. What follows is the version we would want to read before spending five figures: the mechanism, the real numbers from our benchmarks, the mistakes we see in actual quotes, and a short list of actions you can finish this week. No products are pushed in this guide; brands appear only where the data requires them.

What you need to know first

A home battery is not a product, it is a strategy with hardware attached. The hardware stores kilowatt-hours; the strategy decides when they are bought, when they are used and when they are sold back. Everything that matters in ownership — payback, warranty stress, even fire safety — flows from that combination. In Belgium the strategy is shaped by the capacity tariff; in the Netherlands by the end of salderen; in Germany by §14a and supplier choice. The same box on the wall behaves like three different investments depending on which rulebook it sits under.

The numbers that matter

The numbers first. Installed residential storage in Belgium and the Netherlands runs €470–€780 per kWh all-in. A correctly sized system shifts self-consumption from roughly 32% to 60–80%, worth €800–€1,100 a year on a dynamic contract against today’s prices. Payback concentrates between six and nine years. The single most expensive mistake is oversizing: our quote benchmark shows one in four batteries sold at least 30% too large for the household’s evening consumption — dead capital that never pays back.

How this plays out in Belgium

In Belgium, three levers decide the outcome. The capacity tariff rewards peak-shaving — a battery or a smart charger earns a second income here that German and Dutch hardware economics do not include. The 6% renovation VAT rate applies to homes older than ten years and must be claimed correctly on the invoice. And Fluvius registration is mandatory, currently backlogged — file early, keep the confirmation, and operate only within the regime your registration states.

How this plays out in the Netherlands and Germany

In the Netherlands, the end of salderen reframes everything: exported solar is worth a feed-in rate instead of the retail rate, so self-consumption and storage carry the case. Watch supplier feed-in charges — they differ by hundreds of euros a year. In Germany, the bar is documentation: certification, §14a conformity for controllable loads, and a service network that answers in German. Price leads in Belgium, structure leads in the Netherlands, proof leads in Germany.

The mistakes we see in quotes

The five mistakes we see most in reader quotes for this topic: equipment specified by stock rather than by the household’s load profile; exclusions hidden below the fold (scaffolding, inspection, registration); monitoring turned into a subscription the buyer never asked for; warranty summaries that quote years but hide throughput caps; and sizing justified by ‘future-proofing’ — the phrase that sells the upgrade and strands the capital.

A second pattern deserves its own paragraph: the upgrade suggestion that arrives at the kitchen table rather than in the quote. A larger battery, a faster charger, a premium module line — offered verbally, framed as a favour, and rarely documented. If it is not in the written quote with a written reason, it does not exist. And if the written reason is ‘future-proofing’, ask which future, priced how.

Your action list

Your action list. One: gather your actual consumption data — twelve months, by quarter-hour if your meter allows. Two: collect three quotes and normalise them line by line. Three: ask the warranty question in writing before signing, not after. Four: register with the grid operator early; the backlog is real. Five: re-run the maths after the first energy bill arrives — the first winter tells you whether the sizing was honest.

A worked example

A worked example from our benchmark set. A semi-detached home in Leuven, 3,800 kWh a year, 4.6 kWp on the roof, dynamic contract. With no storage, self-consumption sits at 34% and the effective electricity cost lands around €0.31 per kWh consumed. Add a correctly sized 10 kWh battery and self-consumption rises to 68%; the capacity tariff adds a second, smaller saving because evening peaks flatten; the total improvement is €940 in year one. The same home quoted a 15 kWh battery saves only €70 more — the extra €1,800 of hardware works out to a 25-year payback. Sizing, not brand, decided that outcome.

The checklist, printable

• Confirm your evening consumption from meter data before any sizing talk
• Demand the RMA process in writing: who collects, who pays shipping, how many days
• Ask for the warranty document itself, not a summary
• Verify grid-operator registration status before operation
• Compare €/kWh installed across at least three quotes

Questions readers actually ask

Is now a good time to buy, or should I wait for prices to fall? — Prices have turned upward; waiting has a cost too. Buy on the maths, not the market mood.

Is bigger safer? — No; oversizing is the most expensive mistake in our data.

Do I need three-phase? — Only for larger systems; your meter answer decides.

One level deeper

One level deeper, for readers who want it. Battery economics decompose into four independent levers: the spread between buy and sell prices (the tariff lever), the peak-shaving effect (the grid lever), the degradation cost per stored kWh (the hardware lever), and the standby losses (the honesty lever — some systems idle at 40–80 W, quietly eating €30–€60 a year). Marketing collapses all four into one payback figure. When you compare brands, demand the four levers separately; a system that wins on one and hides another is not the better buy. Our audit’s service metrics sit on top of this frame: the best economics in the class are worthless if the warranty claim takes four months.

What we would tell a friend

What we would tell a friend. Buy the audited mid-tier, size it to your evenings, put the savings in the contract not the hardware, and keep every document. The brands fight for your attention; the arithmetic works quietly in your favour if you let it.

The bottom line

The bottom line. This topic is not complicated; it is complicatedly sold. Strip the marketing, keep the arithmetic, and the right answer for your household is usually obvious within one evening of honest numbers. That is what this platform is for.