A home battery has two ratings, and quoting one without the other is how a customer ends up disappointed. The kWh figure is how much energy the pack stores — it sets how long the system can run a load. The kW figure is how much power the system can deliver at any moment — it sets what it can run at all. A large pack behind a small inverter stores plenty of energy and still cannot start a heat pump.
Installers quote the kWh number because it is printed on the module. The power number tends to be buried further down the page, and it is the one behind most awkward post-installation calls. This guide shows where a system's real power ceiling comes from, how to check that the ratings agree, and what to put on the quote.
The two numbers answer different questions
| Rating | Unit | The question it answers | Where it is set |
|---|---|---|---|
| Energy capacity | kWh | How long can the system run this load? | The battery modules |
| Power rating | kW | What can the system run at once? | The inverter (AC side) and the battery's continuous discharge rating (DC side) |
Energy and power are not interchangeable. It is normal for a pack's nameplate energy to be several times its continuous power: a single-phase unit on the market is sold as, for example, 10 kWh of storage with a 5 kW continuous output (SunPower Reserve single-phase datasheet). The kWh tells the customer how long the lights stay on; the kW decides which appliances are allowed to start.
Where the power ceiling comes from
A system's usable power is the lowest of three ceilings, and every one of them has to clear the load before anything runs.
1. The inverter's continuous AC output
The inverter turns DC from the battery into the AC the house uses, and its continuous output rating is the first ceiling. Our hybrid inverters are single-phase 230 V units — Huawei SUN2000 and Deye SG-series — in 5, 8 and 10 kW steps, with two to three MPPT trackers and CAN/RS485 battery communication (hybrid inverters).
Read the continuous figure, not the model name. A unit sold as "10 kW" delivers its rated output under stated conditions and leans on a time-limited peak rating above them. That is why specifications are quoted with their conditions — see how to read a home battery datasheet.
2. The battery's continuous discharge rating
This is the ceiling people miss. How much current a pack can deliver at once is a property of the cells and the BMS, not of its kWh. Two packs of identical capacity can have different continuous discharge ratings, and the lower one caps the whole system.
It is not printed next to the capacity, so ask for it directly: what is the continuous discharge rating, is it per module or for the bank, and under what conditions was it measured? We publish our storage specifications with their test conditions and provide the datasheet on request.
3. The supply the system connects to
The installation has a limit too. On a single-phase 230 V supply, power is roughly voltage times current, so a 16 A circuit carries about 3.7 kW. An inverter can only deliver what the circuit feeding it — and the backup output carrying the load — can pass. Local connection rules belong in the design; the German registration path is set out in our guides to VDE-AR-N 4105 and §14a EnWG.
The system delivers the smallest of the three. If the inverter is 5 kW and the customer wants a 6 kW load, it does not matter how many modules are on the wall — the load wins.
Continuous and peak are not the same number
Two ratings sit on the same label, and customers read the bigger one.
- Continuous (rated) output is what the unit can deliver indefinitely under its stated conditions. Size loads against this.
- Peak (surge) output is a short, time-limited burst for start-up. It is measured in seconds, so it starts a motor; it does not run one.
Appliances with motors or compressors — fridges, pumps, power tools — draw a starting current several times their running current. The inverter has to supply that surge, not only the steady wattage on the appliance label. Two consequences follow: a load well inside the continuous rating can still trip on start-up, and a peak rating should never be quoted to a customer as if it were continuous.
How to check that a quote's power claim holds
| Ask for | Why it matters |
|---|---|
| Inverter continuous AC output | The first ceiling for every load |
| Inverter peak rating and its duration | Whether surge loads will start |
| Battery continuous discharge rating | The DC-side ceiling that capacity hides |
| Whether that rating is per module or per bank | A bank is not always the sum of its modules |
| The backup output rating | It can be lower than the inverter's total output |
| The load list, with start-up current for motor loads | Runtime and start-up are different checks |
Worked example (illustration, not a specification)
Take a 10.24 kWh Storage Wall module paired with a 5 kW single-phase inverter, with an assumed usable share of 80% (about 8.2 kWh).
| Load | Continuous draw | Can it run? | How long |
|---|---|---|---|
| Fridge, lights, router, TV | about 0.5 kW | Yes — well inside the 5 kW ceiling | About 16 hours on 8.2 kWh |
| Kettle, oven and hairdryer together | about 6 kW | No — above the inverter's continuous output | Not applicable |
| A single 2.5 kW load | 2.5 kW | Yes | About 3.3 hours on 8.2 kWh |
The arithmetic is honest as long as the assumption stays visible: runtime is usable energy divided by load, and the load is the part that gets underestimated. The capacity side of that calculation is covered in how to size a home battery, and what a system can back up during an outage — a different question again — is covered in what a home battery actually backs up.
What to ask before you quote
- Get all three ratings in writing: inverter continuous output, battery continuous discharge, and the backup output rating.
- Confirm the DC bus before you commit to modules — the pairing logic is in high-voltage or low-voltage.
- Check the loads against the continuous figure, not the peak, and list the start-up current for anything with a motor.
- Plan the power headroom as well as the capacity headroom if the household may grow; designing for expansion covers the module side and wall, stacked or rack the format side.
Tell us the loads and the phase
Send us the phase count at the connection point, the loads the customer wants to run — with start-up current for anything with a motor — and the inverter make and model if it is already fixed. We will confirm which configuration fits, which of the three ratings is the binding one, and which datasheet applies before you quote. If nothing is fixed yet, start from the site and the objective and we will work forward from there.
Tell us about the project, or browse the full storage range and the Storage Wall series.