What is the C-rate and how does it affect sizing?

The C-rate is the speed of charging or discharging relative to the capacity. 0.5C means that the storage system can be fully charged or discharged in two hours — for example, a system with 1 MWh capacity at 500 kW of power. 1C means one hour, 0.25C four hours. The C-rate determines the ratio of power (MW) to capacity (MWh), and these two numbers together describe the storage system: neither is enough on its own to evaluate an offer. For intraday arbitrage and peak shaving in an industrial setting, 0.5C is typically sufficient, because in the HUPX price pattern the expensive and the cheap hours come in blocks of several hours — there is no point in emptying the storage in one hour. A higher C-rate requires a larger converter and stronger cooling, which makes it more expensive, and it also degrades the cells faster. Conversely: with a C-rate that is too low, the consumption peak cannot be shaved, however large the capacity. The right ratio should be derived from the site’s 15-minute consumption profile, not from a rule of thumb.