Battery Sizing

What sizing actually means

Two numbers, decided separately.

Capacity, in kilowatt hours, is how much energy the battery holds. It sets how long the battery can carry the house.

Power, in kilowatts, is how fast it can charge or discharge. It sets what the battery can carry.

A quotation that names only capacity has answered half the question. A large battery with a modest power rating will run out of delivery before it runs out of energy — it holds plenty, but when the oven, the kettle and the immersion are on together, the grid makes up the shortfall and you pay peak rate for it.

Where the numbers come from

Not from the roof. Array size and battery size are related but they are not the same calculation, and sizing storage as a fixed proportion of generation is a shortcut that produces poor answers.

The input that matters is half-hourly consumption data from your smart meter. It shows the shape of your demand: when it peaks, how deep the evening block is, what the overnight baseline is, and how much of it falls outside generating hours. With a year of that data the sizing is arithmetic rather than judgement.

Layered on top:

• Modelled generation from the proposed array, hour by hour across the year.
• Tariff structure. A time-of-use tariff with a cheap overnight window changes the answer significantly, because the battery earns from arbitrage as well as from stored solar.
• Large loads. An EV, a heat pump, an electric shower, a hot tub.
• Export rate. The higher the export rate, the less each stored unit is worth relative to simply selling it.

The test a size has to pass

A correctly sized battery cycles. It fills, it empties, it fills again, most days of the year. Every kilowatt hour that regularly goes unused is capital earning nothing.

This is why the answer for most households is smaller than the one they arrive expecting. The evening and overnight block is what storage is for, and covering it well is usually enough. Buying enough capacity to also cover a dull week in January means buying capacity that sits idle from March to October.

Suitability and the limits of storage

Storage shifts energy by hours. It does not shift it by seasons. No domestic battery bridges the gap between UK summer and UK winter generation, and any sizing conversation that heads toward grid independence has left the realistic range.

Where a household genuinely wants resilience rather than savings, that is a different specification — backup circuits, a changeover arrangement, and a battery sized against how long an outage you want to ride out — and it should be costed as such rather than blended into the savings case.

Installation implications of the size chosen

Capacity is usually built from modules, so physical space, floor loading and wall construction scale with the answer. Clearances around the cabinet are set by the manufacturer and are not negotiable.

Two constraints deserve checking before the size is fixed. The inverter has a maximum supported battery capacity, which caps future expansion. And larger systems more often cross the threshold from a G98 notification to a G99 application requiring DNO approval before installation, which adds time to the programme.

Cost

Battery cost does not scale linearly. Installation labour, the inverter and the electrical works are largely fixed, so the second module costs less per kilowatt hour than the first. That argues for not fitting the smallest possible unit, but it does not argue for fitting the largest — the marginal kilowatt hour still has to cycle to be worth anything.

Our approach is to model your consumption against two or three capacities and show what each one saves, so the size is a decision you make on figures rather than one made for you. The usable capacity, power rating and warranty terms of whatever is proposed are stated in the written quotation.