kW vs kWh vs kVA vs Ah: the four units that size a battery

Published on October 21, 2026

Most sizing errors in storage procurement are not deliberate obfuscation by the supplier — they are two sides talking in different units. "This battery is 5 kilowatts." "Our cells are 100 amp-hours." "The inverter is 50 kilovolt-amperes." Each sentence can point at something completely different. Get kW, kWh, kVA and Ah straight, and you can read any quote sheet — and spot a dressed-up proposal on your own.

1. What each unit actually answers

• kW (kilowatt) — power: how much work per second. This is how inverter and PCS ratings are stated and how a battery rack's maximum discharge power is specified. It answers: can this system actually start that air conditioner?
• kWh (kilowatt-hour) — energy: how long that work can be sustained. A 10kW inverter with a 20kWh battery is a 10kW tap feeding a 20kWh tank. Neither number is meaningful on its own.
• kVA (kilovolt-ampere) — apparent power: the physical size of the machine. kVA is always greater than or equal to kW, and the ratio is the power factor. A nameplate of 50kVA at 0.9PF means 45kW continuous.
• Ah (ampere-hour) — charge: how much electricity the cells hold. Ah on its own is meaningless — you need the nominal voltage: Wh = Ah × V.

2. The four mistakes that size a battery wrong

Mistake 1: treating kW as capacity

Power is work per second; capacity is total work. kW has no time unit, kWh is energy. When a supplier says "this is a 5 kilowatt battery", the first question is: do you mean 5kW of output power, or 5kWh of capacity? Those two can differ several times over in price, and every downstream comparison is built on whichever answer you accept.

Mistake 2: reading Ah without the voltage

For 100Ah cells: a 12V pack is 1200Wh (1.2kWh), a 48V pack is 4800Wh (4.8kWh), a 51.2V pack is 5120Wh (5.12kWh). The same "100Ah LFP battery" can differ by more than 4x in energy, purely because of the voltage platform. This is exactly why "same Ah" never means "same battery".

Mistake 3: sizing kWh but not kW

Enough energy, not enough power, is the classic backup failure. A 10kWh battery behind a 3kW inverter can theoretically run over three hours, but it will not start a 5kW pump. Backup systems are short on power, not energy — the inrush of a motor load runs 3 to 7 times its steady-state current. The full selection flow is in our UPS versus storage for backup power guide.

The opposite mistake is just as expensive: 100kWh behind a 125kW inverter has energy to spare but demands a 1.25C discharge rate, which noticeably accelerates cycle-life degradation on many cell types. See discharge rate and cycle life.

Mistake 4: ignoring the kVA / kW gap

UPS and inverters are rated in kVA, and motor loads (pumps, fans, compressors) carry extra reactive demand on top. A quote that states only kW and omits the power factor tends to fall short the day real inductive loads are energised. Require kVA, PF, and whether a figure is continuous or peak in the same line of the quotation.

3. One table to fix the conversions

1 kW	1 kilowatt of power = 1000 J/s; run it for one hour and you have consumed 1 kWh
1 kWh	1 kilowatt-hour = 1000 W × 1 h = 3.6 MJ — an amount of energy, not a rate
1 kVA	1 kVA of apparent power ÷ 0.9 power factor ≈ 0.9kW of usable real power
1 Ah	1 ampere-hour of charge at 51.2V nominal ≈ 51.2Wh; at 12V it is only 12Wh
1C	1C empties the rated capacity in one hour; 0.5C takes two hours, 1C takes one

4. Three worked examples

Case 1: 30kW theatre load, 2 hours of backup

Required energy = 30kW × 2h ÷ 0.95 system efficiency ÷ 0.8 depth of discharge ≈ 79kWh. In practice, plan 80 to 100kWh with margin, and confirm the inverter delivers at least 30kW continuous and around 45kW peak to absorb motor starting current.

Case 2: 10kWp solar with a 15kWh battery

A 10kWp array yields roughly 1050kWh a year in most of Europe. Only the portion not consumed directly ever reaches the battery, so the usable charge window is narrower than the array size suggests. The common pairing is a 5kW hybrid inverter with 15kWh of storage: solar feeds the house and trickles into the battery by day, the battery covers the evening peak for two to three hours. Note that kWp is the peak output of the modules and belongs in a completely different table from the battery's kWh.

Case 3: what a 5kWh battery can actually carry

First, how long: 5kWh × 0.9 usable ≈ 4.5kWh. Second, how hard: that is set by the inverter rating, not the battery. A 5kW inverter with 5kWh gives roughly one hour at full power, which is exactly 1C. A 2.5kW inverter stretches it to about 1.8 hours at 0.5C, which is far kinder to cycle life. Settings for hybrid inverters are covered in the hybrid inverter selection guide.

5. Six questions that align every unit

• ① Is the power figure kW or kVA? Is it continuous or a 5-second peak? Ask for both.
• ② Is the capacity nameplate kWh or usable kWh? A 100kWh system at 80% DoD, 95% inverter efficiency and 25°C delivers about 72kWh, not 100kWh.
• ③ What is the battery voltage platform and amp-hour rating? Amp-hours only compare on a matched voltage platform.
• ④ What is the permitted charge and discharge C-rate? This decides whether the battery can be paired with an inverter that large (rate and cycle life).
• ⑤ What is the capacity derating at ambient temperature? LFP capacity drops in cold conditions, and nameplate figures are usually measured at 25°C.
• ⑥ What are the inverter efficiency, battery charge efficiency and cut-off voltage? These three numbers decide how long the system actually runs, and they are often missing from a quotation.

6. Four traps to avoid

• Trap 1: "both quote 15kWh, so take the cheaper one." Nameplate capacity can differ by more than 20% in real usable terms once DoD, efficiency and temperature derating are applied. Compare usable kWh, and usable kWh in year five.
• Trap 2: "same Ah, same battery." Ignoring the voltage platform is the most common pricing mistake in the industry.
• Trap 3: "the inverter is 10kW and the battery is 10kWh, that matches." That ignores three deratings: 0.8 DoD, 0.95 efficiency and temperature derating. Realistic usable energy is about 7.6kWh.
• Trap 4: "lower kW is always cheaper." In backup applications, insufficient kW makes the scheme non-compliant. In peak-valley arbitrage the opposite applies: too little kW directly caps your daily revenue. The arithmetic is set out in our C&I storage payback guide.

7. Bottom line

Units are the most basic and most frequently skipped step in a storage purchase. Send the six questions above to every supplier and you will eliminate most of the unsound proposals before the price is even discussed. If you want a first-pass capacity number for your own load, the free backup capacity calculator will give you a figure you can check against the supplier's parameters. Whether the final power and capacity pairing also holds up for grid operation is covered in our grid-forming storage guide and home battery buying guide. Our hybrid inverter and home battery range lists kW, kWh and the permitted discharge rate in the spec sheet, so you can check our numbers against any quote.

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