UPS Selection Calculator
Choose an uninterruptible power supply for your load: convert watts to volt-amps, select the UPS type, calculate battery capacity for the required runtime.
What the calculator computes
- Total load power in volt-amps (VA) with power factor
- Recommended UPS power with a 20–30% margin
- Nearest standard UPS size (600/800/1000/1500/2000/3000 VA)
- Required battery capacity (Ah) for the desired runtime
- Recommended UPS type: offline, line-interactive or online (double conversion)
- Runtime for the selected configuration
How to select a UPS
Step 1 — Power. Sum the power of all devices that must run from the UPS. Account for starting currents: electric motors of pumps and refrigerators consume 3–5 times the nominal at start.
Step 2 — Margin. UPS power is taken with a 20–30% margin over the nominal load: running above 80% shortens the inverter's life and the UPS shuts down at the slightest dip.
Step 3 — Type. Offline (standby) — for routers and low-power devices. Line-interactive — optimal for PCs and home appliances (protection against dips via stabilizer). Online (double conversion) — for servers, medical equipment, gas boilers with electronics: full galvanic isolation and pure sine wave.
Step 4 — Runtime. Battery capacity: C = P × t / (U × η × DoD). For example, for a 300 W load for 2 hours at 24 V, efficiency 0.9 and DoD 0.8: C = 300×2/(24×0.9×0.8) = 34.7 Ah — a pair of 17–18 Ah batteries or one 40 Ah.
Calculation formulas
Apparent power: S (VA) = P (W) / cos φ. For computers and switched-mode power supplies cos φ = 0.6–0.7; for resistive loads (heaters, lamps) cos φ = 0.95–1.0.
UPS power: S_UPS = S × 1.25 (25% margin).
Battery capacity: C = P × t / (U × η × DoD), where U is the DC bus voltage (12/24/48 V), η = 0.9, DoD = 0.8 for gel and LiFePO4, 0.5 for AGM.
Runtime inverse: t = C × U × η × DoD / P.
Standard UPS sizes table
| Size, VA | Max load, W (cos φ 0.6) | Typical applications |
|---|---|---|
| 600 VA | 300 W | Router, IP camera, mini-PC |
| 800 VA | 400 W | PC + monitor, boiler |
| 1000 VA | 500–600 W | PC + 2 monitors, boiler + pump |
| 1500 VA | 750–900 W | Gaming PC, boiler with pump |
| 2000 VA | 1000–1200 W | Server, house with pump station |
| 3000 VA | 1500–1800 W | Server rack, home workshop |
Load-specific features
- Gas boiler: only a UPS with pure sine wave (online or line-interactive with sine) — approximated sine confuses the boiler electronics. Usually 600–1000 VA for 2–6 hours is enough.
- Circulation pumps: starting current ×3–5. The UPS must withstand a peak of at least 2× the pump nominal.
- Server and NAS: online UPS with hot-swap batteries and software shutdown.
- Intercom and alarm: an offline 400–600 VA UPS is enough, runtime 4–8 hours.
How to use the calculator
- Enter the load power in watts (on the power supply / nameplate).
- Select the power factor or load type (PC, pump, boiler, heater).
- Specify the desired runtime in hours.
- Select the UPS type (line-interactive or online).
- Press «Calculate» — VA, UPS size, battery capacity and recommendations.
Common mistakes
- Buying a UPS «with a 2× margin» — needlessly expensive and does not extend runtime without external batteries.
- Ignoring cos φ: a 1000 VA UPS with cos φ 0.6 gives only 600 W — a 700 W PC power supply will overload it.
- Checking runtime by manufacturer's table without real load.
- Installing a UPS on a boiler with approximated sine — the boiler switches off with an error.
- Ignoring pump starting currents — the UPS trips into protection at every start.
UPS FAQ
What is the difference between VA and W in UPS specifications?
VA (volt-ampere) is apparent power, W is real (active) power. A 1000 VA UPS with a power factor of 0.6 has a useful 600 W. When choosing, focus on W — the actually powered load.
Which UPS is needed for a gas boiler?
Line-interactive or online with pure sine wave, 600–1000 VA, external 12 V 50–100 Ah batteries for 4–12 hours of runtime. Be sure the output waveform is Pure Sine.
How long does a UPS last during a power outage?
Typical time: an office PC at 500 W from a 1000 VA UPS — 5–10 minutes. For 2 hours of runtime for the same load you need external batteries 2×18 Ah or 1×40 Ah. The calculator will size the capacity exactly.
Can several devices be connected to one UPS?
Yes, if the total power does not exceed 70–80% of the UPS rating. Do not connect a laser printer or power tools — their starting currents exceed the rating several times.
What is an online UPS and why do servers need it?
An online UPS (double conversion) constantly powers the load from the inverter: AC → DC → AC. This gives zero transfer time, stable frequency and full isolation from the grid. For servers and medical equipment, the 4–8 ms switch of a line-interactive UPS is unacceptable.