Free tool
How many kVA do I need in a generator?
Short answer
Add the running power, add a 25 % margin, check the peak when the largest motor starts (about 3 times its power) and divide by the power factor (0.8). Example: 2 × halogen floodlight (500 W), 1 × drill / rotary hammer (800 W), 1 × 2 hp air compressor (1500 W) → ≈ 7.9 kVA; from the catalog the GPS9700A fits. Reference estimate: your equipment's nameplate rules.
Grupo ConstruMarket · How many kVA do I need?
You need (reference estimate)
Generator of at least
7.9 kVA
set by the largest motor's start
Running
3.3 kW
with margin: 4.13 kW
Starting peak
6.3 kW
largest motor 1.5 kW × 3
Catalog generators
Continuous 3 kW · max 3.3 kW · 230 V (50 Hz) / 120 V (60 Hz)
Doesn't cover running + margin
Continuous 5.5 kW · max 6 kW · 120 V · 60 Hz
Doesn't cover the start
Continuous 6 kW · max 6.5 kW · 230 V / 400 V · 50 Hz
Doesn't cover the start
Spec: 50 Hz. The grid in El Salvador, Guatemala, Nicaragua and Panama is 60 Hz: confirm your equipment accepts it.
Three-phase: for three-phase motors.
Continuous 6.8 kW · max 7 kW · 120 / 240 V
Doesn't cover the start
- Smallest that fits
Continuous 9 kW · max 9.7 kW · 120 / 240 V
Fits · running load ≈ 46 %
Quote GPS9700A Continuous 9 kW · max 10 kW · 230 V / 400 V · 50 Hz
Fits · running load ≈ 46 %
Spec: 50 Hz. The grid in El Salvador, Guatemala, Nicaragua and Panama is 60 Hz: confirm your equipment accepts it.
Three-phase: for three-phase motors.
Quote GS10003G
Reference estimate. List powers are typical: use your equipment's nameplate. Start motors one at a time, the largest last. For fixed installations or panels, an electrician makes the connection.
How it's calculated
- Running power = Σ power × quantity of each load.
- Starting peak = running + (starting factor − 1) × the largest motor (motors start one at a time).
- Recommended kVA = the larger of running × (1 + margin) and the peak, divided by the power factor; rounded up to 0.1 kVA.
- A generator fits if its published continuous power covers running + margin and its published max power (or continuous, if no max is published) covers the peak.
- The smallest that fits is recommended: an oversized unit costs more and runs lightly loaded.
Assumptions and sources
- List powers (floodlight, grinder, compressor…).Source: typical values from general-use generator sizing tables; your equipment's nameplate rules and you can edit them.
- Starting factor 3 (options 1, 2, 3 and 5).Source: general practice: motors start at 2 to 3 times their running power; some more.
- Power factor 0.8 and 25 % margin.Source: general sizing practice (usual margin 20–25 %).
- Generator output.Source: catalog spec sheets (published continuous and max power). A product without published continuous power is not used.
- Generator rating taken as kVA at PF 1.Source: this tool's conservative assumption: the spec sheet doesn't publish the rated power factor.
FAQ
How do I calculate how many kVA I need in a generator?
Add the running power of everything you'll plug in, add a margin (25 % by default), compute the peak when the largest motor starts (its power times the starting factor) and keep the larger of the two. Divide by the power factor (0.8 by default) to go from kW to kVA.
What is a motor's starting factor?
When starting, a motor draws several times its running power for a few seconds; general practice takes 2 to 3 times and some equipment needs more. That's why a compressor or a pump sets the generator size even if they use little while running. Start them one at a time, the largest last.
What's the difference between kW and kVA?
kW is the power that does work; kVA is the apparent power the generator must deliver. kVA = kW ÷ power factor. With motor loads the power factor is around 0.8, so 8 kW need about 10 kVA.
How much margin should I leave?
As general practice, 20 to 25 % over the running load: the generator runs cooler, handles loads added later and doesn't fall short with altitude or heat. The calculator lets you pick 0 to 30 %.
Does a 50 Hz generator work in Central America?
The grid in El Salvador, Guatemala, Nicaragua and Panama is 60 Hz. A 50 Hz generator only suits equipment that accepts it (check the nameplate). The calculator shows the frequency and voltage each spec sheet publishes.
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