Self-consumption and payback
Self-consumption and export estimate
- where
- generation used on site (kWh/yr)
- annual generation (kWh/yr)
- self-consumption fraction (%)
- generation exported (kWh/yr)
- share of demand met by solar (%)
- annual electricity demand (kWh/yr)
Explain this equation
Splits annual solar generation into what the home uses directly and what is exported, and shows how much of the home's demand solar covers.
The self-consumption fraction f is the share of generation used in the home at the time it is produced, taken from the MCS self-consumption tables for the occupancy and demand. Applying it to the generation gives the energy used on site; the rest is exported. Dividing the on-site use by the home's annual demand gives the share of demand met.
Straight percentage splits of the generation figure.
Sense check: 3,500 kWh at 40% is 1,400 kWh used and 2,100 kWh exported, covering about 47% of a 3,000 kWh demand.
Watch out: a battery, hot water diverter or daytime occupancy raises f considerably. Use the table for the actual arrangement.
Splits annual PV generation into what the home uses directly and what is exported, from a self-consumption percentage.
Enter your values to see the result.
Good to know
Take the percentage from the MCS MGD 003 look-up tables, which depend on annual demand, generation, occupancy archetype (home all day, half day or out all day) and any battery. MGD 003 caps self-consumption at 95%, and only counts a battery that meets its efficiency and charge-rate conditions (such as at least 80% round-trip efficiency).
Common mistake
Assuming all generation is used in the home; without a battery a typical household uses well under half of a 4 kWp system's output.
Limits
Annual totals only. The fraction is an estimate from typical profiles, not a measured figure. Diverters, EVs and heat pumps change it.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.