C-rate & peak power
What C-rate says about how fast you can charge and draw from a battery — and why inverters need extra headroom in the system for peak power.
What is C-rate?
C-rate (or C-value) is how fast a battery can be charged or discharged relative to its capacity. 1C means the whole capacity can be drawn in one hour. 0.5C means two hours. 2C means half an hour.
Current (A) = C-rate × capacity (Ah) Example: 100 Ah × 0.5C = 50 A continuous
Why it matters
Two 100 Ah batteries can have wildly different C-rates. A 2000 W inverter at 12 V draws around 174 A continuously at full load (including efficiency losses) — 1C on a 100 Ah battery is not enough, you need at least ~1.75C or more capacity. Use only half of it, say 1000 W, and consumption drops to about 87 A. A cheaper battery rated 0.3C only tolerates 30 A — the BMS then throttles the system and the inverter can in practice only deliver ~384 W.
- C-rate out (discharge) — how much power you can draw
- C-rate in (charge) — how fast the battery accepts energy
- A brief peak (a few seconds) usually tolerates a higher C-rate than continuous draw
Peak power vs continuous power
Appliances such as induction hobs, compressors and pumps often draw 2–6× more current at the moment they start than they do while running. That is the peak power. An inverter has to handle both the continuous power and the peak — otherwise the overload protection trips.
- Continuous: what the appliance draws in normal operation
- Peak: a short spike at start-up or under load (typically 1–10 seconds)
- Rule of thumb: choose an inverter with roughly 2× peak headroom over your most demanding appliance
How Kraftur calculates it
We add up the continuous power of your appliances and show the peak requirement based on the appliance with the highest peak. The battery and inverter recommendations take both into account.