Battery power rating

Charge, discharge and surge power, and how to match them to grid-connected and backup loads.

Battery power is the rate at which a storage system can charge or discharge, measured in kW. Energy capacity is the amount it can store, measured in kWh.

A large energy capacity does not guarantee that the system can run a large load. A high power rating does not say how long it can sustain that load. Both are needed for a useful design.

Charge and discharge ratings can differ

Datasheets may state separate maximum charge and discharge powers. They may also distinguish:

continuous power
short-duration overload or surge power
battery-side DC power
inverter AC input or output power
grid-connected power
emergency or island-mode output

These are not interchangeable. A peak figure without its permitted duration and recovery condition is not a usable design value. A DC figure before inverter losses is not the same as AC power available to household loads.

The lowest limit wins

The working system is constrained by the lowest applicable rating among:

cells and battery modules
BMS
internal fuses, contactors and busbars
DC cables and isolators
inverter or power-conversion equipment
AC circuit and phase arrangement
firmware and temperature limits
grid-connection and export settings

Adding battery modules may increase energy and sometimes battery-side power. It does not automatically raise AC output if the inverter, cabling or approved configuration remains the limiting part.

C-rate

C-rate relates power to battery energy capacity. In simplified terms:

C-rate = power in kW ÷ energy capacity in kWh

A 10kWh battery discharging at 5kW is operating at 0.5C. This is useful for comparing the relative duty on cells, but it does not replace the product ratings. Gross or usable capacity, temperature and manufacturer limits can change the basis of the calculation.

Do not assume a “typical” domestic C-rate or increase inverter settings from a calculation. The approved battery-inverter combination must enforce the manufacturer’s charge and discharge limits.

Match normal household demand

For grid-connected self-consumption, power determines how much household demand the battery can offset at once. If the home load exceeds battery output, the grid supplies the balance. That is normal and does not mean the battery is undersized if the larger loads are brief.

Use measured load data where possible. Identify the demand that genuinely coincides during the intended discharge window rather than adding every appliance nameplate as though all run at once.

Charging power also matters. A battery intended to fill during a short low-price period or capture a brief solar surplus needs enough charge power, after conversion and site limits, to accept the required energy in that window.

Backup power is a separate rating

During a power cut, the grid cannot make up a shortfall. The backup output must carry the total connected load, including start-up and transient demand, while maintaining voltage and frequency.

Check:

continuous island-mode output
overload magnitude and allowed duration
motor-starting evidence for pumps and compressors
single-phase or three-phase behaviour
whether loads are spread across phases that remain supplied
operation at low state of charge and temperature
protection with the inverter’s limited fault current

A grid-connected inverter rating does not prove the same EPS output. Whole-home backup also needs load control so a cooker, EV charger, immersion heater or heat pump cannot overload the island accidentally.

Shared hybrid-inverter capacity

In a DC-coupled system, PV and battery may share a hybrid inverter. The inverter can have different limits for PV input, battery charge, battery discharge and total AC output. Strong solar generation may use part of an AC output limit that marketing material appears to allocate to the battery.

In an AC-coupled system, the battery has its own inverter, but the property connection, export limit and backup architecture can still constrain combined operation with PV.

Ask for operating examples showing simultaneous PV generation, battery charging or discharging and household load. A list of separate maxima may describe conditions that cannot occur together.

Temperature, state of charge and age

Battery power is often derated near high or low state of charge and outside the preferred temperature range. Cell imbalance or a BMS alarm can reduce it further. The headline rating is therefore not available under every condition.

Internal resistance generally rises as cells age, which can reduce power capability as well as usable capacity. Warranty tests may focus on remaining energy and not promise the original peak output. Check how the manufacturer defines power performance and what evidence is available in logs.

Grid connection and export

An export-capable battery inverter is generation equipment for DNO purposes. G98 or G99 applies according to the equipment, registered capacity, type-test evidence and the rest of the generation at the premises. A G100 scheme may cap net export below installed inverter capacity.

An export limit is not automatically a limit on supplying the home’s own loads, but the product and approved control arrangement decide how much can be delivered behind the meter. Grid charging is an import-load question and must fit within the site’s supply capacity.

What to put in the design record

gross and usable battery energy
maximum and continuous charge power
maximum and continuous discharge power
surge level, duration and test condition
DC, AC, grid-connected and island-mode definitions
temperature and state-of-charge derating
simultaneous PV and battery limits
module-expansion effect on power and inverter limits
DNO registered capacity and export limit
load schedule for normal and backup operation