Runtime calculators start with Wh—you often need the reverse. Define how long critical load must survive and what watts it draws, then back into the battery capacity your UPS or DC plant must store.
Benefits
- Inverts the runtime formula: required Wh ≈ load (W) × target backup time (h), before efficiency margin.
- Supports sizing new UPS purchases when the datasheet lists VA but not enough internal Wh for your shutdown window.
- Frames parallel battery or tray upgrades when existing packs fall short of load × minutes.
How it works
- Set the backup interval you need—graceful shutdown minutes or full ride-through until generator transfer.
- Measure or sum steady load watts on the protected bus (not UPS VA rating).
- Multiply W × h for raw Wh, then divide by 0.8–0.9 (or OEM factor) to account for inverter loss and cutoff headroom.
FAQ
How do I estimate UPS battery capacity from load?
Multiply load watts by required hours of backup. Example: 200 W for 2 h needs 400 Wh minimum—often 450–500 Wh installed after efficiency derating. Add margin if temperature or aged cells reduce usable capacity.
Can I use this to pick a replacement battery cartridge?
Yes. Compare your calculated Wh (with derate) to OEM cartridge Wh ratings. Match voltage and form factor; never install higher voltage than the UPS BMS expects without engineering review.
What if load varies during an outage?
Size for the highest sustained watts during the outage window, or model phases separately (e.g., servers at idle shutdown vs. full draw). Variable load is why field tests still matter after paper sizing.
Technical specifications
- Sizing: required Wh ≈ load (W) × backup time (h) ÷ efficiency factor.
- Efficiency factor: typically 0.8–0.9 unless OEM runtime chart applies.
- Validation: required Wh ≤ usable Wh of installed pack at end-of-life derating.
- Companion: use the UPS Runtime calculator forward-check after selecting a Wh target.
From SLA minutes to watt-hours
A 30-minute graceful-shutdown SLA at 350 W implies 175 Wh before derating—roughly 195–220 Wh nameplate depending on inverter efficiency. Facilities tickets should cite both W and minutes so procurement does not oversize VA while undersizing Wh.
End-of-life derating
Lead-acid and lithium packs lose usable Wh with age and cycle count. Size new installs at 80–85% of calculated fresh capacity if the asset must meet the same SLA in year five, or plan cartridge replacement intervals explicitly.