UPS Battery Backup Calculator: Size and Runtime
Add up what the UPS will power, choose a UPS by its watt rating with headroom to spare, and see how much runtime a battery could give at most. Runtime from any calculator is an upper bound: the manufacturer’s runtime chart is the real answer.
The interactive calculator on this page takes a list of equipment with quantities and watts, your maximum loading, the UPS watt and VA ratings, battery energy or the battery label (volts, amp-hours, count) and the runtime you want, and shows each formula with your numbers.
Example: a small server rack
- Server 400 W + switch 150 W + firewall 30 W + NAS 40 W = 620 W. At most 80% loading, choose a UPS rated at least 775 W.
- A 1500 VA / 1000 W UPS carries it at 62% of its watt rating (power factor 0.67). Its VA figure alone would have suggested far more headroom than it has.
- With 432 Wh of battery, the runtime can be no more than 37.6 minutes at 620 W; the real figure is lower. Ten minutes needs at least 115 Wh in theory.
- An 80 W NAS and router on a 230 Wh battery could run at most 155 minutes in theory; check the UPS’s runtime chart at 80 W for the real number.
Watts are real power; VA is apparent power; a UPS datasheet gives both, and the watt rating is the limit. Loading and inverter efficiency are editable assumptions. Runtime figures here are upper bounds, never guarantees.
Minimum UPS rating by load (80% loading)
| Equipment load | Minimum watt rating | VA if power factor is 0.9 | VA if power factor is 0.6 |
|---|---|---|---|
| 100 W | 125 W | 139 VA | 209 VA |
| 250 W | 313 W | 348 VA | 521 VA |
| 500 W | 625 W | 695 VA | 1,042 VA |
| 750 W | 938 W | 1,042 VA | 1,563 VA |
| 1,000 W | 1,250 W | 1,389 VA | 2,084 VA |
| 1,500 W | 1,875 W | 2,084 VA | 3,125 VA |
| 2,000 W | 2,500 W | 2,778 VA | 4,167 VA |
| 3,000 W | 3,750 W | 4,167 VA | 6,250 VA |
Choose on the watt rating. The VA columns show why a VA figure alone can mislead: the same watts need very different VA ratings depending on the UPS’s power factor.
Runtime upper bound by battery energy and load (90% inverter efficiency)
| Battery | 100 W | 250 W | 500 W |
|---|---|---|---|
| 216 Wh (2 × 12 V 9 Ah) | 117 min | 47 min | 23 min |
| 432 Wh (2 × 12 V 18 Ah) | 233 min | 93 min | 47 min |
| 864 Wh (4 × 12 V 18 Ah) | 467 min | 187 min | 93 min |
Battery Wh × 0.9 ÷ load × 60. Real runtime is lower, and the gap grows at higher loads: see below.
Size on watts, not VA
A UPS has two ratings. VA (volt-amperes) is apparent power; watts is the real power the equipment uses. The ratio between them is the power factor. A UPS can carry no more than its watt rating, so a "1500 VA" model may only support 900 or 1000 W. APC notes that the watt-to-VA ratio is what matters most when sizing a UPS. Read the watt figure on the datasheet and compare your load with that.
Where to get the watts for each device
A power supply’s rating is a maximum, not what the device draws: a server with an 800 W supply often draws far less. Better sources, in order: a reading from a metered PDU or the UPS’s own load display; the vendor’s power calculator for that configuration (Dell EIPT, HPE Power Advisor); the datasheet’s typical and maximum consumption. For a PoE switch, include the PoE power it delivers to cameras and access points on top of its own draw. Use the CCTV planner if the load is a camera system.
Why plan for headroom
Loading a UPS close to 100% leaves nothing for growth and shortens runtime. The calculator defaults to 80% of the watt rating; change it if your policy differs. Remember that batteries lose capacity as they age, so a system that just meets its runtime target when new will fall short later.
Runtime is not proportional to load
Halve the load and the runtime more than doubles, because a lead-acid battery delivers more of its energy at low discharge rates. APC’s published figures for the Smart-UPS SMT1500RM2UC show it: 7.2 minutes at 1000 W, 25.9 minutes at 500 W, 3.6 times longer at half the load.
That is why the calculator gives only an upper bound: battery energy × inverter efficiency ÷ load. Real runtime is lower, and lower still for an older or cold battery and because the UPS shuts down before the battery is empty. For a real figure, read the manufacturer’s runtime chart for your exact model at your load.
Plan the shutdown, not just the runtime
For servers and NAS units the goal is usually a clean shutdown, not riding out a long outage. Connect the UPS’s USB or network interface to shutdown software on the servers, and set it to start shutting down while enough runtime remains for every system to finish. For longer outages, external battery packs or a generator are the answer, not a bigger UPS alone.
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Frequently asked questions
What size UPS do I need for a server?
Add the server’s actual draw to everything else on the UPS (switch, firewall, NAS), then choose a UPS whose watt rating is at least the total divided by 0.8. For 620 W of equipment that is a UPS of at least 775 W.
Is VA the same as watts?
No. Watts are the real power used; VA is apparent power. Watts = VA × power factor. Size a UPS on its watt rating, which is on the datasheet next to the VA figure.
How do I calculate UPS runtime?
Battery energy (volts × amp-hours × number of batteries) × inverter efficiency ÷ load in watts gives the most runtime possible, in hours. Real runtime is lower; the manufacturer’s runtime chart for your model gives the real figure at your load.
How long will a 1500 VA UPS last?
It depends on the model and the load. APC publishes 25.9 minutes at 500 W and 7.2 minutes at 1000 W for the Smart-UPS SMT1500RM2UC; other 1500 VA models differ.
What size UPS for a NAS and router drawing about 80 W?
Any UPS rated at least 100 W covers the load. Runtime depends on the battery: a 230 Wh battery could give at most 155 minutes in theory, and less in practice, so check the runtime chart at 80 W.
How much headroom should a UPS have?
A common planning rule is to keep the load at or below 80% of the watt rating, leaving room for growth and battery ageing. The calculator lets you change that figure.
Sources
- APC by Schneider Electric: Watts vs VA: what’s the difference anyway? (accessed 2026-10-23)
- SHI (reseller listing of APC specifications): APC Smart-UPS SMT1500RM2UC specifications (runtime 25.9 min at 500 W, 7.2 min at full load) (accessed 2026-10-23)
Related tools and guides
Updated 2026-10-22.