Whether it's a construction site, an event, agriculture, or emergency power for a building: a power generator is only as well-sized as the loads it is intended to power. "2,000 watts" on the type plate is often not enough information – because consumers draw very different currents depending on their design, have starting peaks, may require clean sine wave voltage, or should not be operated unilaterally on three-phase current.
In this article, we show the most important types of consumers – and what they mean for the selection of your generator.
1) The most important terms: Watt, kVA and why they are not the same
-
W (Watt) / kW = Active power: This is the power that "really works" (heat, light, movement).
-
VA / kVA = Apparent power: This is the power that the generator must provide electrically – including reactive components (phase shift) and sometimes distorted current draw.
-
Power factor (cos φ or PF): Ratio of kW to kVA. The smaller the factor, the more kVA the generator must supply, even though "less kW" reaches the device.
Rule of thumb: Generators are often rated by kVA, consumers often by W/kW. Depending on the type of consumer, this can vary greatly.
2) Types of consumers – and what they "demand" from the generator
A) Ohmic (resistive) consumers: "simple" – but often powerful
Typical: Heaters, kettles, toasters, hot plates, incandescent lamps
Characteristic: Relatively constant power consumption, hardly any starting peaks.
Meaning for generator selection:
-
These devices are predictable: Continuous power ≈ required generator power.
-
Caution: Heating loads are often high – operating multiple heaters quickly leads to large kW/kVA requirements.
Practical tip: Ohmic loads are easy to handle – but they consume a lot of power. It's better to plan with realistic simultaneity (don't assume "everything at once" if it's not necessary).
B) Inductive loads (motors): The classic cause of starting problems
Typical: Pumps, compressors, high-pressure cleaners, concrete mixers, saws, fans, refrigerator/freezer
Characteristic: During startup, motors require briefly several times their normal operating power (starting current / starting kVA).
Meaning for generator selection:
-
Not just the running power, but the starting power is crucial.
-
If the generator is too small, what often happens is:
-
Motor doesn't start, just "hums"
-
Generator voltage briefly drops (lights flicker, electronics reset)
-
Circuit breakers trip
-
Practical tip (very important):
Always plan motors with a starting reserve. Many guides use starting factors/coefficients here (e.g., compressors and refrigeration units significantly higher than pure heating loads).
C) Capacitive & electronic consumers: few watts, but "demanding"
This is surprising for many: modern electronics often draw current not sinusoidally (switched-mode power supplies, chargers, LED drivers). This leads to:
-
sometimes unfavorable power factors
-
harmonics
-
sensitivity to voltage dips or frequency deviations
Typical:
-
Computers, servers, network/routers
-
Chargers, workshop chargers
-
LED lighting (depending on driver)
-
Controls, sensitive measurement/control technology
Meaning for generator selection:
-
For sensitive electronics, voltage quality is often more important than "even more watts".
-
Inverter generators usually deliver very stable voltage/frequency (constant 230 V/50 Hz, clean sine wave) and are therefore often the first choice for sensitive consumers.
Practical tip: If you want to supply electronics, controls or IT: prioritize clean output voltage (inverter/high-quality regulation) rather than dimensioning "as cheaply as possible".
D) Mixed loads: The normal case – and the real challenge
In practice, almost always mixed loads are connected to the generator:
-
Light + refrigerator + heating + chargers
-
Construction site: Saw + angle grinder + compressor + construction floodlight
-
Emergency power: Heating pump + cooling + communication + possibly gate drive
Meaning for generator selection:
-
You need to accurately combine the simultaneous running power plus the largest starting peak.
-
Also important: Which consumers start simultaneously? (e.g. compressor + high-pressure cleaner is critical)
3) Correctly considering starting power: Starting coefficient & starting peaks
A proven approach is to work with a starting coefficient (multiple of the nominal power at start-up). Example: A device may draw 700 W during operation, but briefly needs 2,000 W at startup.
Here's how to proceed in practice:
-
Collect all loads that should run simultaneously.
-
Per consumer:
-
Running power (W)
-
Starting factor/starting power (if motor/compressor)
-
-
Calculate the sum of running powers
-
Additionally, plan for the most critical starting peak (or several, if realistically simultaneous)
Rule of thumb: It's better to plan for some reserve – a generator that constantly runs at its limit is louder, less efficient, and reacts worse to load changes.
4) Single-phase vs. three-phase (230 V vs. 400 V): Avoiding unbalanced loads
Anyone with a 400 V three-phase generator often connects many 230 V consumers "somehow". This is precisely where unbalanced loads threaten: an uneven load on the phases. In extreme cases, this can lead to voltage shifts and endanger both the generator and the consumers.
Practical tip:
-
If you only want to operate 230 V consumers, a single-phase generator is often the better, more robust choice.
-
If you need three-phase current: distribute loads cleanly phase by phase (and observe the rules/manufacturer's instructions).
5) Selection Checklist: How to find the right generator size
Step 1: Create a consumer list
-
Which devices should run?
-
Which of them run simultaneously?
-
Are there motors/compressors?
Step 2: Classify consumer type
-
Ohmic (heating/light)
-
Inductive (motor)
-
Electronic/nonlinear (switched-mode power supplies/LED/IT)
Step 3: Calculate power accurately
-
Sum up running power
-
Add starting peaks (realistic simultaneity!)
-
Plan for reserve
Step 4: Decide on voltage quality
-
Sensitive electronics/controls → rather inverter or high-quality regulation
-
Robust tools/heating loads → often conventional, correctly sized is sufficient
Step 5: Clarify connection & phases
-
230 V or 400 V?
-
Consider unbalanced load issue
-
Suitable sockets, distributors, protection concept (professionally installed!)
6) Common mistakes (and how to avoid them)
-
Only add watts and ignore starting peaks
-
Buying a three-phase generator, although only 230 V loads are planned
-
Operating electronics on generators with poor voltage quality
-
Not planning for reserve: Generator constantly runs at its limit
-
Neglecting load distribution at 400 V (unbalanced load)
Conclusion: The "right" power generator depends on your consumers – not vice versa
If you know the type of consumer, starting behavior, and voltage requirements, generator selection becomes much easier (and safer). Particularly important are:
-
Motors/compressors due to starting peaks
-
Electronics/IT due to voltage quality
-
Three-phase current/unbalanced load due to load distribution
If you wish, you can derive a reliable dimensioning from your consumer list in a few steps – or seek advice so that the generator and consumers work reliably together in the long term.