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kW vs kWh: power, energy and C-rate in plain words

kW is power, the rate at which energy flows at a moment; kWh is energy, power kept up over time: 1 kW for one hour is 1 kWh. A battery has both. Its kWh say how much it can store, its kW how fast it can charge or discharge, and the C-rate is the ratio of the two: 125 kW on 245 kWh usable is about 0.5 C, roughly a two-hour battery. Business tariffs charge for both — per kW for the peak, per kWh for the energy — so a battery is sized on both.

5 min read · Updated 2026-10-30

Power and energy

  • Energy = power × time. 100 kW for one hour is 100 kWh; 100 kW for 15 minutes is 100 × 0.25 = 25 kWh.
  • Power = energy ÷ time. 25 kWh in a quarter-hour is an average of 25 ÷ 0.25 = 100 kW.
  • That is why a 15-minute meter value can be read either way, and why reading it the wrong way makes a site four times too big or too small. See The ten data problems that size a battery for you.

On the bill

Business network tariffs charge per kW of peak and per kWh of energy, and the two can be of similar size. Illustrative, on Liander's 2026 medium-voltage tariff (Liander 2026): a month with a 400 kW peak, a 400 kW contract and 150,000 kWh drawn costs 400 × €3.57 = €1,428 for the kW max, 400 × €2.29 = €916 for the contracted kW and 150,000 × €0.0226 = €3,390 for the transport kWh.

A battery that shaves 60 kW off every month's peak changes the first two lines. It barely changes the third: the kWh are moved in time, and the battery's losses add a little.

On a battery datasheet

  • Energy: look for usable kWh, not only nominal. The sonnenPro Compact 261 has 261 kWh nominal and 245 kWh usable (sonnen datasheet).
  • Power: the maximum charge and discharge kW, usually set by the inverter. The same sonnen cabinet charges and discharges at up to 125 kW.
  • C-rate = kW ÷ kWh. On the nominal energy, 125 ÷ 261 ≈ 0.48 C; on the usable energy, 125 ÷ 245 ≈ 0.51 C. Datasheets differ in which one they use, so check before comparing.
  • Duration = kWh ÷ kW, the hours at full power: 245 ÷ 125 ≈ 1.96 hours.

Three batteries, three shapes

Which jobs need power, which need energy

  • Short, sharp peaks need power. Bäckerei Miedl in Bavaria cut its maximum grid demand by just over 70 kW with a 131 kWh battery and an 88 kW inverter, about 1.5 hours of storage (Solarserver, 17 Feb 2021).
  • Power can be the binding limit: on a dairy farm, small batteries discharging about 4 kW each could not cover a milking peak of about 25 kW (Teagasc).
  • Long peaks and connection limits need energy: the kWh above the limit, divided by the discharge efficiency. See Sizing for a connection limit.
  • Arbitrage needs energy and cycles: every kWh stored in the cheap hours is a kWh the site does not have to buy in the expensive ones (behind the meter the battery serves the site's own load; it does not sell to the grid). See Arbitrage on a dynamic contract.

The battery database lists usable kWh, power and C-rate for every product with the datasheet each value came from. The free peak-shaving calculator sizes both from a site's own load profile.

Frequently asked questions

What does 0.5 C mean?
The battery can charge or discharge at half its energy per hour, so a full charge or discharge at full power takes about 1 ÷ 0.5 = 2 hours. 1 C is about one hour, 0.25 C about four.
Which sets the power: the battery or the inverter?
The smaller of the two. SMA's Storage XL battery allows up to 1 C, but the package's 50 kW inverter limits it to about 0.25 C. SolarEdge's CSS-OD 197 comes with two 50 kW inverters, yet the cabinet's maximum C-rate caps the pair at 98.5 kW.
Why does a peak-shaving battery need so few kWh?
Because it only has to cover the energy above the target peak, and short peaks have little energy above it. A 60 kW excess that lasts 30 minutes is 60 × 0.5 = 30 kWh before losses.
Is kVA the same as kW?
Not quite. kVA is apparent power and kW real power; they are equal only at a power factor of 1. Connections are often rated in kVA or amperes, while the Dutch and German peak charges in these guides are billed in kW.

Related guides

Sources

Last checked: 2026-10-30. Every figure in this guide comes from the source linked next to it, with the retrieval date Zorowatt's research recorded (28–30 October 2026), or from arithmetic shown step by step. Product values as recorded in Zorowatt's battery database (retrieved 28 October 2026).

Stuck on something else? Ask the community.

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