Usable battery capacity

Usable battery capacity is the energy the control system allows to move between its upper and lower state-of-charge limits. It is the relevant starting point for sizing, but it is not always the energy that reaches household loads after conversion losses.

The word “capacity” needs a boundary and a condition.

Nominal and usable capacity

Nominal or gross capacity describes the stored-energy rating before the product’s operating reserve is applied. Usable capacity is the portion made available in normal operation.

If a battery has a nominal capacity C and permits a depth of discharge DoD, an initial estimate is:

usable DC energy = C × DoD

This is only an estimate where the manufacturer already quotes a usable figure, applies additional dynamic limits or defines capacity at a particular temperature and discharge rate.

Do not apply the depth-of-discharge percentage twice. Some data sheets quote gross capacity plus DoD; others quote usable capacity directly.

Operating reserve

A lower state-of-charge limit protects the cells and may retain energy for backup. The owner or installer can sometimes raise that reserve.

For a battery with published usable energy U and an owner reserve R, the planned energy above reserve is approximately:

planned energy = U × (1 - R)

Controller definitions vary. A displayed zero per cent may be the bottom of the allowed operating window rather than a physically empty cell. An emergency reserve can also sit outside the ordinary user range.

Record whether a percentage is measured against nominal, usable or displayed capacity.

DC capacity and delivered AC energy

Battery data sheets may rate stored energy on the DC side. Household loads use AC after the inverter. Energy is lost in:

cell resistance
battery electronics
DC cabling
inverter conversion
pumps, fans or heaters where fitted
standby consumption

Delivered AC energy can therefore be lower than usable DC capacity. The difference depends on power level, temperature and equipment state, so a single generic deduction should not be applied to every product.

Ask whether a quoted system capacity is measured at the cells, battery terminals or AC output. Compare products on the same boundary.

Power changes accessible energy

Capacity is measured in kWh; power is measured in kW. A battery can have enough energy for a load in theory but still be unable to supply its instantaneous power.

High discharge power can increase losses and may cause the BMS to reach voltage or temperature limits sooner. Low loads can also be inefficient because inverter standby use becomes a larger share of the output.

Sizing should check both:

energy needed across the intended period
continuous and surge power of simultaneous loads

Temperature limits

Battery capacity and charge acceptance change with temperature. Lithium batteries commonly restrict charging when cells are cold, while discharge may remain available over a wider range.

The product’s minimum and maximum temperatures refer to cells or ambient conditions as stated in its documentation. An outside enclosure can experience different cell temperature from the air, particularly in sun or after sustained operation.

Where the system includes a heater, account for its energy use and control behaviour. Do not assume an IP-rated battery can deliver its published capacity at every outdoor temperature.

Ageing

Available capacity normally declines with calendar time and cycling. A warranty may promise a minimum retained capacity under specified use, but it is not a forecast that every battery follows exactly.

Actual ageing depends on:

time at high or low state of charge
cell temperature
charge and discharge rate
energy throughput
number and depth of cycles
control and firmware limits

A ten-year financial model should not use first-day usable capacity for every year. Apply a documented degradation assumption and test a worse case.

Modular systems

Adding modules can increase energy, power or both, depending on the architecture. Modules connected in parallel generally add capacity and current capability. High-voltage stacks connect modules in series to reach the required voltage and often require a prescribed module count.

Check:

permitted minimum and maximum modules
whether mixed generations or ages are approved
matching and balancing procedure
inverter power limits
communications topology
usable capacity per approved configuration

Adding capacity behind an unchanged inverter may extend duration without increasing household output power.

Backup sizing

For backup, the useful question is how long the battery can support maintained loads above its protected reserve.

An approximate duration is:

hours = delivered energy available ÷ average maintained load

Real loads vary. Refrigerators, pumps and motors cycle and can have starting surges. Solar may recharge the battery during an outage only if the installed islanding arrangement supports it and can restart from the expected state.

Test a critical-load schedule rather than dividing the battery rating by a whole-home annual average.

Reading the portal

State of charge is an estimate produced by the BMS. It is not a direct fuel gauge. Calibration, balancing, temperature and firmware can change its behaviour.

To assess delivered capacity:

Define the start and stop state of charge.
Use a consistent AC or DC meter boundary.
Record charge and discharge energy separately.
Exclude unrelated solar, grid or load flows.
Note temperature, power and reserve settings.

One short discharge is not a conclusive health test. Use the manufacturer’s diagnostic process if capacity appears to have changed materially.

Capacity boundary and handover records

The design should state:

nominal and usable capacity
measurement boundary
normal and backup reserve
permitted depth of discharge
inverter charge and discharge power
temperature restrictions
expected ageing assumption
modular expansion limit
warranty retention and throughput conditions

This prevents a gross cell rating from being presented as energy guaranteed at the socket.