Quick Answer: To size a commercial backup battery, list the critical loads you want to keep running, sum their running wattage, multiply by the backup hours you need, then divide by the battery's usable depth of discharge (typically 90%). The formula is:
Required battery capacity (kWh) = (Critical load kW × Backup hours) ÷ Usable DoD
For example, a facility that needs 50 kW of critical load for 2 hours requires 100 kWh ÷ 0.90 = 111 kWh of battery . Add 10–15% margin for degradation and startup surges, and you land on a standard cabinet size.
Step 1: Define critical loads, not total load
The most common sizing mistake is multiplying the building's total contract demand by backup hours. That over-sizes the system by 3–5× and wastes money. Instead, list only the loads that must ride through an outage:
| Load category | Typical kW for a 200-person office |
|---|---|
| LED lighting (critical areas) | 8–12 |
| Servers / network / PBX | 15–25 |
| Security and access control | 2–3 |
| Refrigeration (if applicable) | 5–15 |
| Elevator (1 passenger) | 5–10 |
| Water pump / sump | 2–5 |
| Total critical load | 37–70 kW |
Non-critical loads — HVAC, kitchen equipment, EV chargers, general office outlets — should be excluded. If a load can wait 4 hours, it does not belong on the battery.
Step 2: Account for startup surges
Motors and compressors draw 3–6× their rated current at startup. A 5 kW refrigerator compressor may surge to 25 kW for 2 seconds. The PCS (power conversion system) must handle these peaks, not just the steady-state load.
Rule of thumb: size the PCS at 1.25× the critical load if the list includes motors or compressors, or 1.5× if large motors (elevators, water pumps) are present.
Example: 50 kW critical load with compressors → 50 × 1.25 = 62.5 kW PCS .
Step 3: Apply usable depth of discharge
Most LFP BESS systems are rated at 90% depth of discharge. That means a 100 kWh cabinet delivers 90 kWh of usable energy. A conservative engineer uses 85% to preserve cycle life.
| DoD assumption | 100 kWh nameplate delivers |
|---|---|
| 95% (aggressive) | 95 kWh |
| 90% (standard) | 90 kWh |
| 85% (conservative) | 85 kWh |
If your project has a 10-year warranty and you expect daily cycling, use 85–90%. For pure standby backup (occasional use), 95% is acceptable.
Step 4: The sizing formula, worked example
A manufacturing plant in Germany wants 3 hours of backup for:
- Lighting: 10 kW
- CNC machine controls: 15 kW
- Data servers: 20 kW
- Compressed air dryer: 5 kW
- Total critical load: 50 kW
Calculation:
- Energy needed: 50 kW × 3 h = 150 kWh
- Apply 90% DoD: 150 ÷ 0.90 = 167 kWh
- Add 10% degradation margin: 167 × 1.10 = 184 kWh
- PCS sizing: 50 × 1.25 = 62.5 kW → select 75 kW PCS
- Standard cabinet match: 200 kWh Outdoor Cabinet with 75 kW PCS
The nearest off-the-shelf size is 200 kWh, which provides a small buffer instead of an undersized custom build.
Step 5: What the formula does not account for
The calculation above is the starting point. Four factors need a second look:
- Round-trip efficiency. A 92% efficient system loses 8% on every charge-discharge cycle. If the battery charges from solar and discharges during an outage, multiply by 0.92.
- Cold temperature. Below 0°C, LFP capacity drops by 10–20%. If the cabinet is in an unconditioned space, upsize by 15%.
- Inverter overload rating. Check how long the PCS can run at 125% load. Some units cap at 110% for 10 minutes — too short for a compressor startup.
- Future load growth. If the facility plans to add servers or EV chargers in year 3, upsize the cabinet now. Adding a second cabinet later is more expensive than buying one larger unit upfront.
A note on single-phase vs three-phase
Most commercial backup systems are three-phase. If you are backing up a single-phase load in a small office, you can use a split-phase PCS at lower cost. Do not try to run three-phase critical machinery on a single-phase inverter — it will not start the motor.
When in doubt, ask an installer to read your facility's main breaker panel and identify which circuits are critical. A 30-minute walk-through usually resolves the sizing questions faster than an email exchange.
Ruibit supplies Dawnice outdoor cabinets and all-in-one systems from 112 kWh to 1 MWh, pre-matched with PCS sizing for common backup applications. We review the critical load list before quoting so the recommended cabinet matches the actual runtime requirement.
FAQs
Q: How long will a 100 kWh battery run a 50 kW load? 2 hours, assuming 90% usable DoD: (100 × 0.90) ÷ 50 = 1.8 hours. Add efficiency losses and it drops to about 1.6 hours.
Q: Should I size the battery for total facility load? No. Size only critical loads. Total building load includes HVAC and kitchen equipment that does not need backup and would make the system 3–5× larger than necessary.
Q: Can I extend backup time later by adding more batteries? Yes, if the system architecture supports parallel cabinets. Ask before ordering whether additional battery racks can be added to the same PCS later, and what the maximum expandable capacity is.
Q: What happens if the battery is undersized? The BMS will cut off power before the scheduled backup time elapses, potentially during a critical moment. The PCS will also alarm on low DC voltage. It is cheaper to oversize by 10% than to discover the shortfall during a real outage.
Q: Does solar charging extend backup time? Yes, if the system includes solar input. A 50 kW load backed by 20 kW of solar on a sunny day will draw only 30 kW from the battery, roughly doubling the runtime. This should be modeled separately from pure grid-outage scenarios.
