C-Rate

The basic maths

C-rate normalises charge and discharge speed against battery size, which lets you compare batteries of different capacities fairly.

If a battery has 10 kWh of usable capacity:

• 1C = 10 kW charge or discharge (full in 1 hour)
• 0.5C = 5 kW (full in 2 hours)
• 2C = 20 kW (full in 30 minutes)

Most residential batteries have a maximum continuous discharge rate of 0.5C–1C. A 10 kWh battery with a 5 kW inverter is running at 0.5C when the inverter is at full output.

Why it matters for home storage

The continuous power rating of a battery - what it can sustain over hours - is limited by C-rate. A 13.5 kWh Tesla Powerwall 3 has a 5 kW continuous discharge (0.37C). The SolarEdge Home Battery at 9.7 kWh runs at 5 kW continuous (0.52C).

For a household with a 3–5 kW evening load, most batteries operate well within their rated C-rate and thermal limits. Problems arise when batteries are asked to do things they weren’t sized for - like running a large ducted air conditioner (8–10 kW) entirely from a single small battery. You’ll hit either the inverter output ceiling or the battery’s BMS protection limit.

Peak vs continuous

Most manufacturers specify both a continuous and a peak C-rate. Peak rates apply for short durations (typically 10–30 seconds) and cover surge loads - motor starts, appliance inrush. A battery might have 5 kW continuous but 7 kW peak. If your home has equipment with high startup draw, check the peak spec, not just the continuous.

Heat and degradation

Higher C-rates generate more heat inside the cell. Heat accelerates electrolyte degradation and lithium plating, which is why cycling a battery at 2C constantly wears it faster than at 0.5C - even if the total energy throughput is the same. LFP (lithium iron phosphate) chemistry handles higher C-rates with less thermal penalty than NMC, one of the reasons it’s preferred in residential storage where the battery might cycle fully every day.