Sizing battery storage: kW, kWh and C-rate
The right storage size does not come from gut feeling. Power, capacity, charging windows and operating strategy must match the load profile.
kW: how strongly the storage can intervene
The power of a storage system is stated in kilowatts (kW). It describes how much power the storage can deliver or absorb at a given moment. For peak shaving, the kW power is especially important, because the battery must discharge exactly when the site load exceeds a limit.
kWh: how long the storage can work
Capacity is stated in kilowatt-hours (kWh). It describes how much energy can be stored. A storage system with high power but too little capacity can only absorb a peak briefly. A storage system with high capacity but too little power cannot reduce the peak strongly enough.
C-rate: ratio of power to capacity
The C-rate describes how fast a storage system is charged or discharged relative to its capacity. It matters for service life, heat generation, cost and operating strategy. Not every application needs the same C-rate.
Sizing is an optimisation problem
Sized too small, the storage misses its effect. Sized too large, it ties up capital unnecessarily. WERAX therefore looks not for the maximum but for the economically sensible storage size.
Which factors come into play
- Peak height and peak duration
- Intervals between peaks
- PV generation and charging windows
- Grid-connection capacity and grid limit
- Tariff model and demand charge
- Operating strategy, degradation and warranty terms
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