Summary

Battery storage sizing gets treated as a simple "bigger is better" upsell far too often, and that's poor advice for the customer's wallet and payback period. The right battery size depends on three things: how much electricity the household actually uses in the hours when solar isn't generating (typically evening and overnight), how much surplus solar is genuinely available to charge the battery on an average day, and whether the customer's goal is solar self-consumption, tariff arbitrage (charging on a cheap overnight rate to use during expensive peak hours), or backup power during an outage — because each goal points to a different sizing answer.

Oversizing a battery for a household with modest solar generation and average consumption means paying for capacity that rarely gets used, extending the payback period well beyond the battery's warranty life. Undersizing means the battery is empty by mid-evening every day, delivering only a fraction of its potential saving. Getting the sizing calculation right — grounded in the household's actual daily consumption and solar generation profile, not a generic "most popular size" recommendation — is the difference between a system that pays for itself and one that doesn't.

This calculator sets out the sizing method step by step, with two fully worked examples: one for a solar-PV household sizing for self-consumption, and one for a household without solar sizing purely for tariff arbitrage and backup. It also covers the regulatory thresholds (DNO notification, fire safety location rules) that interact with sizing decisions.

Key Facts

Quick Reference Table — Typical Battery Sizing by Household Profile

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Household Profile Daily Consumption Solar PV Sizing Driver Typical Battery Size
Small flat/2-bed, no solar 4-6 kWh None Tariff arbitrage only 3.6-5 kWh
3-bed semi, no solar 8-10 kWh None Tariff arbitrage / partial backup 5-8 kWh
3-bed semi, modest solar 8-10 kWh 3-4 kWp Solar self-consumption 6.5-10 kWh
4-bed detached, solar 10-14 kWh 4-6 kWp Solar self-consumption + some arbitrage 10-13.5 kWh
All-electric home, EV, solar 15-20+ kWh 5-8 kWp Solar + EV load shifting 13.5 kWh+ or modular stack
Household wanting outage backup priority Varies Varies Backup duration over pure economics Sized to critical loads × desired hours, not average consumption

Detailed Guidance

The sizing method, step by step

BATTERY SIZING METHOD
======================
1. Establish daily household electricity consumption (kWh/day)
   → from bills (annual kWh ÷ 365) or a smart meter app average

2. Split that consumption into "daylight/solar hours" vs
   "evening/overnight hours" — this is the gap a battery fills

3. If solar PV is fitted: estimate daily generation
   (kWp × specific yield ÷ 365 — see [how many solar panels do i need calculator](/knowledge/solar-pv/how-many-solar-panels-do-i-need-calculator))

4. Estimate direct self-consumption during daylight (typically
   30-40% of generation for an occupied home) — the remainder
   is exported surplus, which is what the battery can capture

5. Battery target = evening/overnight consumption the household
   wants to cover from stored energy (solar surplus or cheap-rate
   grid charge), NOT the household's total daily consumption

6. Divide the target by round-trip efficiency (0.90-0.95) to
   get the raw storage capacity required

7. Round UP to the nearest standard product size — never
   specify a non-standard capacity; manufacturers sell fixed
   sizes or modular units that stack in fixed increments

8. Sanity-check against winter solar availability (if solar-
   driven) and against PAS 63100:2024 location constraints
   (if the property has limited non-habitable space for the unit)

Worked example 1 — solar self-consumption sizing

Household: 3-bed semi-detached, occupied during the day (mixed working-from-home pattern), annual consumption 3,650 kWh (10 kWh/day average). Solar PV: 4kWp system, specific yield 850 kWh/kWp/year (a reasonable mid-range assumption for a well-orientated UK roof — see how many solar panels do i need calculator for yield ranges).

Step 1 — Annual and daily solar generation: 4 kWp × 850 kWh/kWp = 3,400 kWh/year → 3,400 ÷ 365 = 9.3 kWh/day average generation

(Note: this is a full-year average. Actual daily generation ranges from under 2kWh/day in December to over 20kWh/day on a clear June day — size for the average, but tell the customer the battery will rarely be full in winter and may spill exported solar even with a battery in summer.)

Step 2 — Direct self-consumption without a battery: Assume 35% of solar generation is used directly as it's produced (daytime appliances, working from home): 9.3 kWh/day × 0.35 = 3.3 kWh/day direct self-use Remaining surplus available to charge a battery = 9.3 − 3.3 = 6.0 kWh/day average surplus

Step 3 — Evening/overnight consumption to cover: Total daily consumption (10 kWh) minus direct daytime self-use (3.3 kWh) = 6.7 kWh/day is the household's evening/overnight load that a battery could, in principle, cover from stored solar.

Step 4 — Compare surplus available to load to be covered: Average surplus available (6.0 kWh/day) is close to, but slightly below, the evening/overnight load (6.7 kWh/day). This tells us the battery should be sized around the available surplus, not the full evening load — there usually isn't more solar surplus to store than what's actually generated.

Step 5 — Apply round-trip efficiency: To deliver 6.0 kWh of usable evening energy, allowing for a 90% round-trip efficiency: 6.0 ÷ 0.90 = 6.7 kWh raw storage required

Step 6 — Round to standard product size, with headroom: 6.7 kWh sits between common 6.5kWh and 10kWh product sizes. Given the seasonal variability (summer surplus will be considerably higher than the 6.0 kWh average), recommending a 10 kWh battery gives useful headroom to capture more of the summer surplus, while the household will simply not fill it in winter — which is normal and expected.

Conclusion: A 10kWh residential battery is the sensible recommendation for this household, balancing average-day sizing against seasonal surplus capture, and matching a common standard product size.

Worked example 2 — tariff arbitrage sizing (no solar)

Household: 2-bed terrace, no solar PV, wants to move to a time-of-use tariff (e.g. a plan with a cheap overnight rate and standard/peak daytime rate) and use battery storage to shift consumption to the cheap overnight window. Annual consumption 2,900 kWh (7.9 kWh/day average).

Step 1 — Identify the load to shift: The household doesn't need to cover 100% of daily use — only the portion they want charged at the cheap overnight rate and discharged during peak-rate hours. Assume they want to cover the evening peak period (roughly 4pm-9pm) plus early morning, estimated at 45% of daily consumption: 7.9 kWh/day × 0.45 = 3.6 kWh/day target load to shift

Step 2 — Apply round-trip efficiency: 3.6 ÷ 0.90 = 4.0 kWh raw storage required

Step 3 — Round to standard product size: A 5kWh battery is the natural fit — slightly above the calculated requirement, giving some margin for occasional higher-consumption evenings and battery degradation over the warranty period.

Conclusion: For a no-solar, tariff-arbitrage-only household, a much smaller battery (5kWh) than the solar self-consumption example is entirely appropriate — sizing to the actual shiftable load, not to total daily consumption, avoids a costly oversized recommendation.

Sizing for backup power priority

Some customers want the battery primarily as an outage backup, not for economic optimisation. This changes the sizing question entirely: instead of "how much surplus is available," the question becomes "how many hours of which specific circuits does the customer want to keep running during a power cut?"

Common sizing mistakes

  1. Sizing to total daily consumption instead of the shiftable/coverable portion. This consistently oversizes the recommendation and inflates the quote unnecessarily.
  2. Ignoring round-trip efficiency. A 10kWh battery does not deliver 10kWh of usable output — expect roughly 9-9.5kWh delivered from a well-specified modern LFP system.
  3. Using an annual average without flagging seasonal variability to the customer. A battery that looks perfectly sized on paper may sit half-empty for four winter months — set this expectation up front.
  4. Not checking PAS 63100:2024 location constraints before recommending a larger unit. A bigger battery has a bigger footprint and heavier weight; confirm a compliant location (garage, utility room, external enclosure — never a loft, bedroom, or unprotected escape route) exists before finalising the spec — see battery storage.
  5. Conflating solar self-consumption sizing with backup sizing. These are different calculations with different answers; be clear with the customer about which goal is driving the recommendation.

Frequently Asked Questions

Should I always recommend the biggest battery the customer can afford?

No. Oversizing beyond what the household's actual daily surplus or shiftable load can fill delivers no additional saving — the extra capacity simply never gets used. Size to the calculated requirement, round up to the nearest standard product for reasonable headroom, and let the customer add a second modular unit later if their circumstances change (EV purchase, extension, home office).

Does a bigger solar PV system mean I need a bigger battery?

Not proportionally. A bigger PV array increases summer surplus more than it changes the winter picture, and UK solar generation is heavily skewed to April-September. Size the battery on the household's evening/overnight consumption and the realistic average daily surplus, not on the PV system's peak capacity (kWp) alone.

How do I account for an EV charger when sizing a battery?

Treat EV charging as a separate, usually much larger, load that's typically better served by a smart/scheduled charger on a cheap overnight tariff than by battery storage — moving 30-40kWh into an EV overnight is far beyond what's economically sensible to size a home battery around. Battery storage and EV smart charging are complementary systems with different sizing logic; don't try to size one battery to cover both daily household shifting and EV charging unless the customer specifically wants a much larger modular system and understands the cost implications.

What if the customer's consumption data is unreliable or estimated?

Use at least 12 months of actual billing data or smart meter half-hourly data where available, rather than a single estimated annual figure — consumption varies significantly by season and household occupancy pattern. If only an estimated annual figure is available, apply a seasonal weighting (roughly 60% of annual consumption falls in the October-March half of the year for a typical UK home with gas heating) rather than a flat daily average, particularly for solar sizing.

Regulations & Standards