kVA vs. kW: Check Both Before Connecting Outage Loads
Use kVA, kW and power factor correctly when sizing a home generator or UPS, including a worked example and motor-starting checks.
Before connecting outage loads, compare them with every limit the generator or UPS publishes. A UPS commonly has separate watt and VA limits. A residential portable generator may instead list running watts, starting watts and receptacle or circuit amperage. Passing one published limit does not guarantee that the source can run your refrigerator, furnace blower, sump pump, router or other essentials.
| Check | What it represents | Sizing rule |
|---|---|---|
| kW (kilowatts) | Real power that performs work or produces heat | Total running kW must stay below the continuous kW limit |
| kVA (kilovolt-amperes) | Apparent power: voltage × current | If specified, total running kVA must stay below the kVA limit |
| Starting demand | Brief demand as a motor starts | Must fit the model’s stated starting or surge capability |
| Runtime | Energy available over time | Calculate from Wh or kWh and load watts—not from kVA alone |
The conversion depends on power factor
Power factor (PF) is the ratio of real power to apparent power:
- kW = kVA × PF
- kVA = kW ÷ PF
- PF = kW ÷ kVA
The Department of Energy’s electrical fundamentals handbook distinguishes true power in watts, apparent power in volt-amperes and reactive power in VAR (DOE). One kVA equals 1,000 VA; one kW equals 1,000 W.
At PF 1.0, 1 kVA corresponds to 1 kW. That relationship applies to an ideal resistive load, but it is not a safe assumption for all household equipment. Motors and electronic power supplies can draw more VA than their watt consumption suggests.
| Real load | Power factor | Apparent load |
|---|---|---|
| 1.0 kW | 1.0 | 1.0 kVA |
| 1.0 kW | 0.8 | 1.25 kVA |
| 1.0 kW | 0.6 | 1.67 kVA |
Eaton gives the same watts-to-VA relationship and warns that ignoring power factor can undersize a UPS.
How to size a UPS
A UPS may have separate ratings such as 1,500 VA / 900 W. Treat them as independent ceilings:
- A 1,000 W load fails the watt limit even if it is below 1,500 VA.
- A 1,400 VA load at 0.5 PF uses only 700 W, but it approaches the VA limit.
- The connected load must fit both limits, with additional capacity for startup and operating margin.
Do not interpret 1,500 VA / 900 W as a prediction that every connected device has PF 0.6. The 0.6 ratio describes the relationship between that UPS model’s two output limits; each load has its own power factor.
For each essential load, use measured operating values when available or the equipment manufacturer’s input specifications:
- Record watts and VA if both are provided.
- If the label gives volts and amps, calculate single-phase apparent power as
VA = volts × amps. That equals watts only at PF 1.0. - If it gives watts and PF, calculate
VA = watts ÷ PF. - If PF is unavailable, check the manual or manufacturer’s data rather than assigning a convenient value. A nameplate maximum may be used as a conservative input figure.
- Add the watts for all simultaneous loads. Separately add their VA for a conservative sizing total; do not average their power factors.
Eaton recommends a UPS VA rating at least 1.2 times the supported load and additional VA capacity for equipment with high inrush, including motors (Eaton UPS guidance). The UPS manual’s load restrictions and approved applications still control; many small UPS units are not intended to start motor loads.
Worked UPS example
Suppose two measured loads will run together:
- Load A: 600 W at PF 1.0 = 600 VA
- Load B: 400 W at PF 0.8 = 500 VA
The combined requirement is 1,000 W and 1,100 VA. A 1,500 VA / 900 W UPS fails because its watt limit is too low. A hypothetical 1,200 VA / 1,200 W unit clears the bare running totals, but leaves little VA margin and may still be unsuitable if either load has substantial startup current.
Battery duration is a separate calculation. Use watts, usable battery energy and conversion losses; see amp-hours versus watt-hours.
How to size a generator marked in kVA
First look for the generator’s continuous kW rating. If only kVA appears, convert it using the power factor stated by the manufacturer for that exact output and operating mode.
For example, a 3 kVA generator rated at PF 0.8 has:
3 kVA × 0.8 = 2.4 kW
It therefore has a 3 kVA apparent-power ceiling and a 2.4 kW real-power ceiling. Do not apply 0.8 automatically to every generator. Generac uses 0.8 for the three-phase sets discussed in its mobile-generator sizing paper, illustrating why the rated PF must be tied to the particular equipment (Generac). Residential generator specifications are often presented directly as running and starting watts.
Starting power can override the running calculation
A refrigerator compressor, furnace blower, well pump or sump pump may fit after it is running but overload the source while starting. Generac defines starting wattage as the brief additional demand associated with motor-driven equipment and advises checking appliance and generator specifications rather than relying only on running watts (Generac, November 2025).
For a portable generator, add the total starting demand of the motor being started to the running demand of the other loads that will remain on. Do not also add that same motor’s running watts if its published “starting watts” figure already represents its total demand during startup. Compare the result with the generator’s stated starting capability. Starting motors one at a time can reduce coincident demand, but it cannot make an individual motor compatible with an undersized source.
For a complete running-watt and startup procedure, use the portable generator sizing guide. Also verify voltage, receptacle and circuit-current limits in the source manual; a favorable total kW figure does not override those limits.