How big should a home battery be? Avoiding the most common sizing mistake

The most common battery sizing mistake is simple:

People size the battery to match the solar system, instead of sizing it to match the household’s evening and night demand.

A home battery is mostly a timing tool. It moves solar from the midday window into the evening when you actually use it.

If you want a quick, local baseline for what solar output looks like where you live, start here: Find your postcode

We will use kW and kWh (see Glossary) and keep this practical. You will be able to read a quote and immediately see if the battery size is sensible.

Assumptions and variability

• We assume a typical UK demand shape where electricity use is higher in the morning and evening than at midday.
• We assume the primary battery value is increased self-consumption (self-use) and reduced imported kWh, not “off-grid living”.
• What varies most in real homes: occupancy, cooking pattern, and flexible loads (EV charging, immersion heater, heat pump).
• We do not assume a specific tariff or SEG export rate (see Glossary), because prices change. The structure is stable: self-used kWh usually beats exported kWh.
• For how SolarByPostcode estimates output and savings by outcode, see: Data sources and methodology

The common mistake (and why it ruins payback)

You will often hear a neat-sounding line:

“Your panels will generate X kWh a day, so you want a battery of about X kWh.”

That logic fails in three predictable ways:

1) You never have “all-day surplus”. Your home is using power while the sun is up.
2) Surplus is seasonal. Winter can shrink or remove the charging window.
3) Even if you can charge it, you might not have enough evening demand to empty it. That is wasted capacity.

The correct question is:

How much electricity do we typically use after solar drops, and how much of that do we want to cover?

That is why batteries sit in the “self-use vs export” cluster. If you have not read that guide, it is the missing value model:

• Self-consumption vs export: the UK solar trade-off that decides payback

Step 1: estimate your evening + night demand (the sizing target)

You do not need perfect half-hourly data to get a sane number. You need a realistic target window.

A useful definition for most households is:

• Evening: roughly 4pm to bedtime (cooking, lights, TV, laundry, showers)
• Night: baseline loads (fridge, standby, broadband, dehumidifier, aquarium, etc.)

If your supplier app shows usage by hour or half-hour, pick a normal weekday and approximate:

• Add up the kWh from about 16:00 to 00:00
• Add the overnight baseline from 00:00 to 07:00

That total is your “battery job”.

If your app only shows daily totals, use the timing guide first and make a rough split:

• Daytime vs evening electricity use: why self-use matters more than kWp

If you want a baseline for “how much electricity do homes actually use”, anchor here:

• How much electricity does the average UK home actually use?

Step 2: cap the battery size by your charge opportunity (surplus)

A battery must be filled before it can help. Filling depends on surplus:

• how much your solar generates during the day
• minus what your home uses during that same period

If your household is away all day, surplus is often high in spring and summer.
If you work from home or run daytime loads, surplus can be smaller, even with the same panel count.

Surplus is also site-specific. Compare a few outcodes to sanity-check the “likely surplus” story:

• https://solarbypostcode.co.uk/tr1-cornwall/
• https://solarbypostcode.co.uk/bn1-brighton-and-hove/
• https://solarbypostcode.co.uk/m1-manchester/
• https://solarbypostcode.co.uk/eh1-city-of-edinburgh/
• https://solarbypostcode.co.uk/ab10-aberdeen-city/

The practical rule: size capacity to the smaller of (evening demand, typical surplus)

Once you have the two numbers, the sizing logic becomes boring:

• If your evening + night demand is 6 kWh, there is no point buying 15 kWh “for savings”.
• If you only have 3 kWh of typical surplus to charge in winter, a 10 kWh battery will often sit underfilled in winter.

That does not mean “never buy bigger”. It means:

Bigger capacity is only useful if you can usually fill it and usually use it.

Capacity (kWh) vs power (kW): the second most common confusion

Batteries have two separate limits:

• Capacity (kWh): how much energy is stored.
• Power (kW): how fast it can deliver energy.

Capacity answers: “How long can it cover loads?”
Power answers: “Can it cover the spikes without importing?”

Short high-load spikes are where people get surprised:

• kettle
• oven / hob
• tumble dryer
• electric shower (if applicable)
• heat pump ramps
• EV charger (if you try to run it “from battery”, which is often not the plan)

If you want the inverter mental model (hybrid vs separate, why kW matters), read:

• Inverters explained (UK): the part of your solar system that decides how it behaves

Usable capacity: why “10 kWh battery” is not always 10 kWh

Quotes and marketing often show a single capacity number. In real operation, usable capacity depends on:

• reserve settings (some keep a buffer)
• how the system is configured (self-consumption vs backup)
• the battery’s allowed depth-of-discharge and protection logic

You do not need to be technical. You just need one quote question:

“What usable kWh are you modelling for normal operation, after reserve and settings?”

If the salesperson cannot answer, the model is not real.

A practical table: which battery size usually makes sense for which household pattern

This table is intentionally conservative. It is designed to prevent over-buying.

If “bill to zero” is part of your mental model, read this insight before you buy anything:

• Standing charge drag

When a bigger battery actually makes sense

“Bigger” is rational when at least two of these are true:

• you have high, predictable evening demand
• you have reliable surplus for much of the year
• you have tariffs where shifting away from peak import is valuable
• you value backup behaviour (even partial) as well as savings

But capacity stops paying when:

• you rarely fill it (winter, heavy daytime use, shading)
• you rarely empty it (modest evening demand)
• power (kW) is the real limiter, not capacity (kWh)

Backup and “power cut mode”: do not assume it is included

Some systems can provide backup power, but this is not automatic, and it is not always “whole house”.

Ask this explicitly:

• “Does this system provide backup power during a grid outage?”
• “Which circuits are backed up, and what is the max continuous kW?”

If backup is a priority, it can change battery configuration and reserve settings, which changes usable capacity for savings.

Quote questions that expose weak battery sizing

Use these questions when comparing installers:

1) What usable kWh are you modelling after reserve?
2) What continuous kW can the battery deliver (and for how long)?
3) What self-consumption increase are you assuming, and in which months?
4) How does your model behave in winter when charging opportunities shrink?
5) Are you modelling export under SEG, and at what rate? (see Glossary)
6) Do export limits apply on my network (DNO, G98/G99, see Glossary) and do they change the “surplus” picture?

If you are not already confident in comparing quotes, start with:

• How to compare solar quotes in the UK: the checklist that prevents regret

FAQs that matter for sizing

Should I size the battery to one day of solar generation?

No. You size it to the evening + night slice you want to cover, capped by typical surplus.

Is it better to buy more panels or a bigger battery?

It depends on what you are short of:
- If you have surplus but waste it on export, a battery can help.
- If you rarely have surplus, more panels (or fixing shade/orientation) can help more than storage.

These two guides cover that decision:
- Solar system sizing in the UK: choosing the right kWp without wasting money
- Oversizing your solar system in the UK: when it pays off and when it does not

Will a battery make winter “sorted”?

A battery can help on good winter days, but it cannot fix seasonality. That is the next guide:

• Winter solar generation in the UK: why batteries can’t fix seasonality

Bottom line

• Size the battery to your evening + night demand, not your panel count.
• Cap capacity by your typical surplus (especially winter).
• Treat kW and kWh as separate questions.
• Use postcode pages to sanity-check whether surplus is likely to exist where you live.

Next reads

• Winter solar generation in the UK: why batteries can’t fix seasonality
• Solar batteries in the UK: who they make sense for (and who should skip them)
• Daytime vs evening electricity use: why timing matters more than totals
• Self-consumption vs export in UK solar: how the Smart Export Guarantee changes the maths
• Solar payback periods in the UK: what actually drives the number

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