How to size battery storage for a commercial building
In short
Size storage from the load you want to keep running and for how long, not from your monthly electricity bill. The sequence is: list the circuits that must stay live and their kW, choose a backup duration, convert that to usable energy, then divide by the usable state-of-charge window to get nameplate capacity. The converter is sized separately, from peak and starting current.
The method, in four steps
- 01
List the critical circuits
Not the whole building. Walk the distribution board and mark each circuit as critical, deferrable or abandonable during an outage. Record kW for each. This list is the foundation of every number that follows, and it is the step most often skipped.
- 02
Choose a backup duration
This is a commercial decision, not an engineering one. Two hours covers the gap between grid failure and generator start; six hours covers an overnight outage. Ask the client what failure they are insuring against, then write the number down — because it is the largest single driver of cost.
- 03
Convert to usable energy
Usable energy = critical load (kW) × duration (h) ÷ converter efficiency. Divide the result by the usable state-of-charge window to get nameplate capacity. The window is typically 80–90% for a new LFP system, and narrower if the design reserves capacity for backup.
- 04
Size the converter separately
The converter is sized on peak load and starting current, not on energy. A system can have enough kWh and still trip, because the inverter cannot supply the starting current of a compressor. These are two independent checks and both must pass.
The formula, written out.
Usable energy (kWh) = critical load (kW) × duration (h) ÷ inverter efficiency
Nameplate capacity (kWh) = usable energy ÷ usable state-of-charge window
Every term in that formula is project-specific. That is why a runtime figure quoted without the assumptions behind it cannot be checked — and why we do not quote one.
Worked example
A commercial building with 60 kW of critical load, a two-hour requirement, 92% inverter efficiency and an 85% usable window:
| Step | Calculation | Result |
|---|---|---|
| Energy delivered to load | 60 kW × 2 h | 120 kWh |
| Account for conversion loss | 120 ÷ 0.92 | 130.4 kWh drawn from the battery |
| Account for usable window | 130.4 ÷ 0.85 | 153.4 kWh nameplate |
| Converter check | Peak load, not average | Converter ≥ peak, plus starting margin |
This is an illustration of the method, not a recommendation for your site. The two percentages above are assumptions; your design may use different ones, and the datasheet governs.
The assumptions that change the answer
| Assumption | Effect if you take it too optimistically |
|---|---|
| Usable state-of-charge window | System runs out earlier than the design intends |
| Inverter efficiency at partial load | Efficiency is lower at light load; a nominal figure overstates output |
| Ambient temperature | High cell temperature reduces available capacity and accelerates ageing |
| Battery age at end of service | Capacity fades; a system sized to new-cell capacity under-delivers in year five |
| Load growth | The critical load list is a snapshot; expansion erodes the margin |
| Starting current | Motor starts can trip a converter that is correctly sized on energy |
What sizing cannot fix
- A site with no space for the enclosure or no cable route from the electrical room.
- A load whose starting current exceeds the converter's overload window — that needs soft starters or a different topology.
- An outage longer than storage can economically cover — that is a generator or dual-source problem.
- A tariff where storage cannot capture value, for example a flat rate with no demand charge and no export limitation.
What we need to do this for you
Send four things: the load schedule with critical circuits marked, the backup duration you are designing to, the site country and grid status, and the highest ambient temperature the equipment room reaches. With those, the calculation above can be run against your project instead of an example.
Send your load schedule