Load ledger
One row per device. Copy whatever the rating panel actually gives you. If it lists volts and amps, use those. If it lists VA or watts, use those. If it lists both watts and VA, enter both and the true power factor is read back to you.
| # | Device | Qty | Load type | Nameplate reads | Volts | Amps | VA | Watts | PF | Load % | Surge × | On UPS | Remove |
|---|
Load % lets you enter a diversity factor. Nameplate current is a worst case, and most gear draws well under it in service. Leave it at 100 for a nameplate sizing, or dial it down if you have measured the actual draw with a clamp meter.
Measured load log
Nameplate is a worst case. A log is the truth. Bring in a CSV of watts over time from a clamp meter, a smart plug, or WATTMETER, and set the ledger from what the load actually does instead of what the label threatens.
Applying scales the Load percent column on every included row so the ledger total lands on that number. It is a proportional split, so it assumes the log covers all of the rows you have switched on. Size on the 95th percentile, not the mean: the mean will leave you short during the exact busy minute the power goes out.
Ledger totals
Sizing assumptions
A UPS carries two ratings and you have to satisfy both. The VA rating is the current limit of the output stage. The watt rating is the thermal limit of the inverter. A 1500 VA / 900 W unit will trip on watts long before it runs out of VA if your load is power factor corrected. The gauge below shows which of the two runs out first.
Candidates
Put the units you are actually considering side by side. Runtime is scaled from whatever single point the vendor publishes, so copy that point off the datasheet. The chosen row is the one drawn on the gauge below.
| On gauge | Unit | VA | Watts | Its PF | Runtime at (W) | for (min) | VA used | W used | Runtime here | Verdict | Remove |
|---|
The binding constraint
Notices
Cell and chemistry
Battery bank from a target runtime
Aging factor 1.25 is the usual end of life allowance (IEEE 485 practice): size the bank so it still makes runtime at 80 percent of its original capacity. Temperature factor above 1.00 buys capacity back in a cold room.
Discharge curve
Runtime against load for the bank sized above. The knee is the point of the whole exercise. Halving the load buys you far more than double the runtime.
Runtime of a UPS you are looking at
Vendors publish one or two runtime points. Enter one and this scales it to your load.
Feeding the UPS
A UPS is a continuous load, so the branch circuit is sized at 125 percent of the draw and the draw includes the charger pulling the battery back up right after an outage. Sizing the circuit to the steady state alone is how you find out what your breaker sounds like at 2 in the morning.
What the current actually looks like
Non linear loads do not draw a sine wave. They draw a spike near the crest, and those spikes land in the neutral instead of cancelling there. Both matter to what you buy and what you pull.
Spec sheet
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One line diagram
Panel to breaker to receptacle to UPS to load, with the battery on the DC side. This is the page you hand to whoever is pulling the wire. It prints with the spec sheet.