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)
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):
- Take a reading at the same time each morning — 7am is an easy one to remember.
- Do this for at least two mornings in a row and enter both above; the tool works out the daily figure for you.
- 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—
Solar inverter guidance—
Battery / home output—
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.
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.
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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.
Solar inverter sizing and battery/home output are treated as two different questions. Solar inverter guidance is based on the suggested panel capacity and roof orientation, using an indicative DC:AC ratio range; the final design must also respect the chosen inverter's MPPT/input limits, acceptable clipping and the site's DNO connection/export approval. Battery/home output guidance instead uses a fixed background load plus your two largest simultaneous selected appliances. The familiar 3.68kW-per-phase G98 threshold is shown as a connection-planning point, not as a claim that larger single-phase inverters do not exist.
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.