When solar generation and household demand are both expressed as daily totals they can look perfectly matched, yet a home can still export most of its solar because the sun peaks at midday and people use power in the evening. This calculator overlays representative hourly curves to estimate what share of your solar you actually use on-site versus send to the grid.
How it works
Generation and load are each spread across 24 hours using a chosen shape, then compared hour by hour:
for each hour h:
self-consumed[h] = min(generation[h], load[h])
exported[h] = max(generation[h] − load[h], 0)
self-consumption % = Σ self-consumed / Σ generation × 100
annual export = Σ exported × 365Because the comparison is per hour, midday solar above the daytime load counts as export even if the daily totals are equal — which is exactly the gap a battery can close by shifting that surplus to the evening.
Why self-consumption matters more than total generation
The value of solar electricity depends on where it goes. Energy consumed on-site displaces power you would have bought from the grid at the full import price. Energy exported earns the feed-in or export tariff, which in most countries is significantly lower than the import price. The gap between those two rates is why self-consumption is the primary financial driver of a rooftop solar investment.
For example, if your import price is 28p/kWh and your export tariff is 15p/kWh, each kilowatt-hour shifted from export to self-consumption is worth an extra 13p — roughly 87% more value from the same electrons.
Typical self-consumption by household type
Self-consumption rates vary significantly based on when the home uses power:
| Household type | Typical self-consumption (no battery) |
|---|---|
| Working couple, out all day | 20–30% |
| Part-time home worker | 35–45% |
| Full-time home worker or retiree | 45–65% |
| Home with EV charged midday | 55–75% |
| Home with battery storage | 70–90% |
These figures assume a south-facing array at a moderate UK latitude. Households that can shift appliances (dishwasher, washing machine) to run during the solar peak hours see meaningful improvements without any additional hardware.
Strategies to raise self-consumption
The annual export figure the tool returns is a planning bound on what a battery could capture. But other approaches can raise self-consumption at lower cost:
- Timed appliances — running the dishwasher and washing machine between 10am and 2pm during clear days adds virtually no hardware cost.
- EV smart charging — scheduling the car to charge from midday surplus can absorb 5–15 kWh of midday generation that would otherwise be exported.
- Hot water diverter — a simple diverter diverts surplus generation into an immersion heater, effectively storing it as heat rather than exporting.
- Battery storage — most effective for households that have already exhausted the above options; sized against the annual export figure shown here.
The annual export volume in kWh is roughly the upper bound of what a battery could time-shift. Compare that energy quantity at the retail price differential against the cost and expected life of storage to assess whether a battery pencils out.