CONSIDERED HOME ENERGY TOOLKIT

Solar & Battery Size Finder

Not sure what size solar array or battery you actually need? Answer a few questions about your usage, when you're home, and what you're trying to get out of it — and get a sizing range to take to installers, not a single made-up number.

✓ Works from a bill, a meter reading, or a rough estimate ✓ Accounts for when you're actually home to use solar directly ✓ Separates energy sizing (kWp/kWh) from inverter power sizing (kW)
Size Finder v1.1 Added August 2026 Release notes

This is a sizing tool, not a payback calculator. It answers "what size do I need?" so you can get real quotes. Once you have those quotes, use the Home Energy Investment Calculator to model the financial payback.

1. YOUR USAGE

How much electricity do you use?

Pick whichever you actually have to hand — the more accurate the input, the more confident the recommendation.

How do I find my usage myself?

From meter readings (most reliable for a daily figure):

  1. Take a reading at the same time each morning — 7am is an easy one to remember.
  2. Do this for at least two mornings in a row and enter both above; the tool works out the daily figure for you.
  3. A single day is only a snapshot — usage swings a lot between summer and winter, so repeat it in a different season if you can, or use 12 months of bills instead for a figure that already averages this out.

From your last 12 bills: add up the kWh usage figure shown on each bill (not the £ amount) across a full year. If your bills only show £, add up the total spend instead and use the £/year option above — it'll be converted using a typical blended rate.

2. WHEN YOU'RE HOME

What's your household's daily pattern?

This is what actually determines how much battery you need — solar generated while no one's home to use it either gets stored or exported.

3. WHAT'S DRAWING POWER

What might run at the same time?

This sizes the inverter, not the battery — it's about peak power (kW), not energy over a day (kWh).

4. PLANNING AHEAD

Anything on the horizon?

Sizing for what's coming avoids paying to upgrade again in a year or two.

5. YOUR GOAL

What matters most to you?

This changes how aggressively the battery is sized.

6. YOUR ROOF (OPTIONAL)

Any constraints worth knowing about?

Skip this if you're not sure — it only narrows the solar range, it won't block a result.

Your sizing range will appear here

Start with section 1 — how much electricity you use — then work down. The recommendation updates as you go.

Confidence

Your recommended sizing

Solar array—
Home battery—
Inverter—
Typical solar generationYour household usage

Roughly why a battery helps: when the "usage" bar is taller than the "generation" bar, that gap has to come from the battery or the grid.

What this suggests

Common sizing mistakes

  • Sizing solar off a summer bill alone — UK generation is heavily skewed to summer, so a system that looks right in July can undershoot badly in winter.
  • Assuming a bigger inverter is the fix for running several big appliances at once — most UK homes only have a single-phase supply, which realistically caps out around 5kW. When demand goes above that, the extra is drawn from the grid automatically, same as it would without a battery.
  • Not accounting for a planned EV or heat pump until after the system's already installed.
  • Assuming full year-round self-sufficiency is realistic from solar and battery alone — it rarely is in the UK without heavy oversizing.
Compare real battery systemsSee named systems in this capacity range
Model the paybackOnce you have real quotes in this range
See the assumptions used
    Illustrative only, not an electrical design or financial advice. This gives a starting range for conversations with installers, not a substitute for an MCS-certified system design.
    How this tool works

    The tool resolves your annual electricity usage from whichever input you have — 12 months of bills, a single bill or spend figure, two meter readings taken 24 hours apart, or a rough estimate from household size and heating type — then adds any future EV or heat pump usage you flagged.

    Solar array size is calculated by dividing your target annual generation (a multiple of your usage that depends on your goal) by a typical UK solar yield of 900 kWh per kWp per year, adjusted for the roof orientation you selected, and capped by the roof size you selected if you gave one.

    Battery size is based on the share of your daily usage that falls outside a typical solar generation window, which depends heavily on your occupancy pattern — a household home all day self-consumes more solar directly and needs less battery than one that's out at work and home only in the evenings.

    Inverter size is calculated separately from energy sizing entirely: a fixed background load plus your two largest simultaneous appliances. The recommendation is then matched to what's actually installed in UK homes rather than a theoretical worst case — 3.6kW inverters sit under the 3.68kW (16A) threshold that can be connected without prior approval (G98), 5kW is the common step above that (needs a G99 DNO notification, typically 4–8 weeks), and true 6kW+ inverters are three-phase products that most standard UK homes can't connect at all. Where your ticked appliances add up to more than 5kW, the tool still recommends 5kW and explains that the shortfall is drawn from the grid automatically — exactly what would happen without a battery, and not something worth oversizing for.

    Every figure is shown as a range and rounded to sizes that are actually available, not a false-precision single number. A confidence label reflects how reliable your usage input is likely to be.

    Release notes and version history
    v1.1
    Realistic inverter recommendations

    The inverter recommendation previously scaled up with no ceiling, showing figures like "12kW+" for a handful of appliances ticked at once — not a realistic recommendation, since the overwhelming majority of UK homes have a single-phase supply that tops out at 3.6kW (no prior DNO approval needed, G98) or 5kW (needs a G99 notification). True 6kW+ inverters are three-phase products most homes can't connect. The tool now recommends only 3.6kW or 5kW, and where ticked appliances add up to more than that, explains that the shortfall is drawn from the grid automatically — exactly what happens without a battery, and not something worth oversizing for. Added a note that a 7kW EV charger needs its own separate DNO notification.

    v1.0
    Initial release

    A sizing tool that turns usage, occupancy pattern, appliance load and goals into a solar (kWp), battery (kWh) and inverter (kW) recommendation range, separate from the financial payback modelled by the full Investment Calculator.

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