Calculator
Solar Self-Consumption Calculator
Under a net metering scheme it barely matters when you use solar electricity, because every exported kilowatt-hour comes back at the same value. Under net billing, which now governs most new European connections, the exported kilowatt-hour is worth a fraction of the imported one, and self-consumption becomes the single largest lever on return.
This tool estimates the split between energy used on site and energy exported, then shows what a battery of a given size adds to it in both kilowatt-hours and money.
Estimate the split
25–35% for an empty weekday house, 50%+ with home working or a heat pump.
Enter 0 to see the no-battery case.
Used directly
1,339 kWh/yr
Generation coinciding with load
Shifted by battery
1,350 kWh/yr
Total self-consumed
2,689 kWh/yr
Exported
1,511 kWh/yr
Self-consumption ratio
64%
Share of generation used on site
Self-sufficiency
60%
Share of consumption met by solar
Annual value of generation
859
Avoided imports plus export revenue
Added by the battery
284
Per year, before battery cost
How the calculation works
Direct self-consumption is bounded by two ceilings: you cannot use more than you generate, and you cannot use more than the load actually present during daylight. Even when both are large, generation and load never align perfectly minute to minute, so the model applies a coincidence factor of 0.85 to the smaller of the two — a figure consistent with measured European household profiles.
The battery contribution is the surplus that remains after direct use, capped by the evening load and by the energy the battery can actually cycle in a year after round-trip losses. Storage cannot create demand; it can only move existing demand in time, which is why an oversized battery in a low-consumption home adds nothing.
The formula
E_day = E_consumption × daytime% E_direct = min(E_day, E_gen) × 0.85 E_surplus = E_gen − E_direct E_evening = E_consumption − E_day E_shift = min(E_surplus, E_evening, C_usable × η × 300) SCR = (E_direct + E_shift) / E_gen SSR = (E_direct + E_shift) / E_consumption Value = E_self × p_import + E_export × p_export 0.85 = coincidence factor between generation and load profiles 300 = approximate useful full cycles per year for a solar-charged home battery η = round-trip efficiency
Assumptions behind the numbers
- Annual energy balances only. A true result needs half-hourly generation and load data for a full year.
- The 0.85 coincidence factor reflects typical household profiles; a home with a large steady daytime base load will beat it, an empty weekday house will not.
- 300 useful cycles per year assumes the battery is solar-charged and rarely cycled in winter. Tariff-arbitrage charging from the grid raises this and is not modelled.
- Battery degradation, standby losses and inverter conversion losses beyond round-trip efficiency are not deducted.
- Prices are held flat. Rising import prices improve the case for self-consumption over time.
How to read the result
A self-consumption ratio of 30 to 40 percent is normal for a household without storage or deliberate load shifting. Adding a battery sized to the evening load typically takes it to 60 to 75 percent; beyond that the return per added kilowatt-hour of storage falls steeply, because the remaining surplus occurs on days when the battery is already full.
Compare the annual figure in the last row against the annualised cost of the battery. If a 5 kWh unit costs 4,000 over a ten-year warranted life, it must add more than 400 a year to break even before finance costs. Free load shifting — running the dishwasher, immersion heater or car charger at midday — often delivers a third of the same benefit for nothing. The battery sizing calculator works the capacity question from the other direction.
Worked example
A house generating 4,200 kWh and consuming 4,500 kWh, with 35 percent of load in daylight hours and no battery.
Daytime load is 4,500 × 0.35 = 1,575 kWh. Direct use is min(1,575, 4,200) × 0.85 = 1,339 kWh, a self-consumption ratio of 32 percent. Exports are 2,861 kWh. At 0.28 import and 0.07 export the generation is worth 1,339 × 0.28 + 2,861 × 0.07 = 375 + 200 = 575 a year. Adding a 5 kWh battery at 90 percent round-trip shifts a further 1,350 kWh from export to self-use, lifting the ratio to 64 percent and the annual value to about 858 — a gain of roughly 283 a year against the battery's cost.
Mistakes that skew the answer
- Confusing self-consumption ratio with self-sufficiency. The first is the share of generation used on site, the second is the share of consumption met by solar. They move in opposite directions as the array grows.
- Assuming a battery raises the ratio to nearly 100 percent. Winter generation rarely fills it and summer surplus routinely overflows it.
- Modelling with annual totals and then treating the result as precise. Half-hourly data changes the answer by several points.
- Ignoring standing charges and grid fees, which continue regardless of self-consumption in most tariffs.
- Sizing a battery to the array rather than to the evening load.
Questions readers ask
- What is a good self-consumption ratio?
- Without storage, 30 to 40 percent is typical for a household whose occupants are out during the day, and 45 to 55 percent where someone is home or a heat pump runs in daylight. With a battery sized to the evening load, 60 to 75 percent is realistic. Ratios above 80 percent generally indicate either a small array relative to consumption or a large controllable load such as electric heating.
- Does a battery always improve the economics?
- No. A battery only pays when the gap between import and export price is wide, the household has enough evening load to discharge into, and the installed cost per usable kilowatt-hour is low enough to be recovered within the warranted cycle life. Where export is compensated at or near the retail rate, storage rarely pays on energy arbitrage alone and is bought for backup instead.
- What is the cheapest way to raise self-consumption?
- Time-shifting existing loads costs nothing. Running the dishwasher, washing machine and any immersion heater during the solar window, and charging an electric car at midday rather than overnight, can add ten to twenty points to the ratio. A simple solar-diverter for hot water is the next cheapest step, typically a tenth of the cost of a battery for a meaningful share of the benefit.
- How does array size affect the ratio?
- Enlarging the array raises self-sufficiency but lowers the self-consumption ratio, because the additional generation lands in hours when the load is already covered. This is the fundamental tension in sizing: a system covering 150 percent of annual consumption may still meet only half of it hour by hour, while exporting a large volume at a low price.
- Should I model this with hourly data instead?
- If the decision involves several thousand euros of storage, yes. Annual balance models like this one are accurate enough to decide whether to investigate further, but they cannot capture seasonal mismatch or the frequency with which a battery sits full. Smart meter data at half-hourly resolution, combined with a modelled generation profile from PVGIS or PVWatts, gives a substantially more reliable answer.