Batteries
Battery usable capacity
- where
- usable capacity (kWh)
- nominal battery capacity (kWh)
- allowed depth of discharge (%)
- energy delivered per full cycle after losses (kWh)
- round-trip efficiency (%)
Explain this equation
Works out how much of a battery's headline capacity you can actually use, and how much comes back out after charging and discharging losses.
Batteries are not normally run completely flat, so only the depth of discharge DoD of the nominal capacity is usable. Every kWh put in loses a little in the inverter and the cells on the way in and out; the round-trip efficiency RTE is the fraction that comes back.
Two percentage reductions applied one after the other.
Sense check: a 10 kWh battery at 90% DoD gives 9 kWh usable, and with 90% round-trip efficiency about 8.1 kWh delivered per cycle.
Watch out: some datasheets already quote usable capacity. Do not apply the DoD twice.
How much of a battery's nameplate capacity is really available to the home on each cycle.
Enter your values to see the result.
Good to know
Many datasheets quote both nominal and usable capacity; if usable is already given, enter it with DoD = 100%. Round-trip efficiency is AC energy out divided by AC energy in; MGD 003 only credits batteries of at least 80%. Applying the full round-trip figure to the usable capacity is a cautious way to estimate the energy the home actually gets back.
Common mistake
Quoting the nominal capacity to a customer as the energy they can use each evening.
Limits
New-battery figures at about 25 °C. Capacity falls with age and cold, and the inverter's standby use is not included. Installation should follow the IET Code of Practice for Electrical Energy Storage Systems and MCS requirements.
Sources
- OpenSolar: How to use the MCS calculator (MGD 003 battery conditions: at least 80% round-trip efficiency at 25 °C; self-consumption capped at 95%)
- MCS MGD 003 Issue 2.0 (April 2022): Determining the electrical self-consumption of domestic solar PV, with and without battery storage
- IET Code of Practice for Electrical Energy Storage Systems, 2nd edition
- Module manufacturer datasheet: STC values (Voc, Vmp, Isc, Imp, Pmax), temperature coefficients of Voc, Pmax and Isc, NOCT/NMOT and dimensions
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.