Generator size in kVA = total running kW ÷ power factor1 (usually 0.8). A 10.81 kW running load, for example, needs about 13.5 kVA before the surge margin, plus 20–30% extra for future expansion. Here is the full step-by-step calculation, including starting (surge) power and power factor.
Getting the size right comes down to three numbers: the total running load in kW, the starting (surge) power2 (2–3× running for motors), and the power factor3 (0.8 for most equipment). Below we walk through each step so you can calculate your generator size accurately.
What are the key steps to calculate the total power requirement for a generator in kVA?
Follow these steps to estimate the total power your equipment and systems need.
You need the total running load, the starting power, and a power-factor adjustment. The core formula is kVA = total kW ÷ power factor.
Step-by-Step Calculation
- List all equipment and devices that the generator will power.
- Identify the running power (kW) of each item from its nameplate or manual.
- Determine the starting power for motors or compressors (air conditioners, pumps). Starting power is typically 2–3× the running power.
- Sum the running power of all equipment to get the total running load (kW).
- Add the starting power of motor-driven equipment to get the peak (surge) load.
- Adjust for power factor (PF) — usually 0.8 for mixed loads.
- Convert kW to kVA: kVA = total kW ÷ power factor.
- Add a 20–30% margin for future expansion or load spikes.
Example Calculation
- 10 computers × 0.2 kW each = 2 kW
- 2 printers × 0.5 kW each = 1 kW
- 3 air conditioners × 2.5 kW each = 7.5 kW
- 1 refrigerator × 0.3 kW = 0.3 kW
- Wi-Fi router = 0.01 kW
Total running load = 2 + 1 + 7.5 + 0.3 + 0.01 = 10.81 kW
Surge load (air conditioners): 7.5 kW × 3 = 22.5 kW
Total peak load = 10.81 kW + 22.5 kW = 33.31 kW
At a 0.8 power factor: kVA = 33.31 kW ÷ 0.8 ≈ 41.64 kVA
Add a 25% margin: 41.64 kVA × 1.25 = 52.05 kVA
In this case, a 55 kVA generator handles the running and surge loads comfortably.
How do you account for starting (surge) power and running power when sizing a generator?
Generators must be sized for both running power (steady load) and starting (surge) power, which is much higher for motor-driven equipment.
Starting (surge) power is typically 2–3× the running power for motors and compressors.
Understanding surge power
- Running power is what equipment draws during normal operation.
- Starting (surge) power is the extra current needed to start motors or compressors such as air conditioners, refrigerators and pumps.
Key considerations
- Motor-driven equipment (air conditioners, elevators) draws a surge lasting a few seconds to a minute.
- The surge is typically 2–3× the running power.
- Surge power must be factored in to avoid overload or startup failure. See also how long a diesel generator takes to start.
Example calculation for surge power
- Air conditioner running power = 2.5 kW
- Air conditioner starting power = 2.5 kW × 3 = 7.5 kW
- Total surge for 3 AC units = 7.5 kW × 3 = 22.5 kW
This surge power is added to the total running load to get the peak load, which determines the generator size.
Why surge power is critical
An undersized generator will trip or fail to start equipment, causing downtime and possible damage to sensitive devices.
What factors, such as power factor and load type, influence the kVA rating of a generator?
Power factor and load type (resistive vs. inductive) affect how efficiently a generator converts kW to kVA.
Understanding power factor and load type ensures the generator can handle both steady and fluctuating loads.
Key factors that influence kVA rating
- Power Factor (PF) — indicates how efficiently a generator’s power is used. It is typically 0.8 for office equipment; lower for inductive loads (motors), which then need a higher kVA rating for the same kW.
- Load Type (resistive vs. inductive) — resistive loads (lighting, heating) have a power factor close to 1; inductive loads (motors, air conditioners) draw more current and need more kVA.
- Load Characteristics — continuous (lighting), intermittent (printers, pumps), or variable (air conditioners).
- Generator Efficiency — a minor factor compared with the others.
How power factor impacts generator sizing
| Power Factor | kVA Required | kW Power |
|---|---|---|
| 1.0 | Lower kVA required | More efficient use of power |
| 0.8 | Higher kVA required | Standard for most equipment |
| 0.6 | Even higher kVA required | Less efficient, often for motors |
Example
At a 50 kW running load with a 0.8 power factor: 50 kW ÷ 0.8 = 62.5 kVA
At a 0.6 power factor: 50 kW ÷ 0.6 = 83.33 kVA
Why power factor matters
The lower the power factor, the more kVA you need to produce the same kW — critical for motor-heavy environments or equipment with high starting power.
Frequently Asked Questions
How many ACs can run on a 50 kVA generator?
A 50 kVA generator at a 0.8 power factor supplies about 40 kW. A typical 1.5-ton air conditioner draws roughly 1.5 kW, so it can run about 20–26 ACs — fewer if they all start at once, because of surge current.
How many ACs can run on a 7.5 kVA generator?
A 7.5 kVA generator at a 0.8 power factor supplies about 6 kW, enough for roughly 4 air conditioners (1.5-ton, about 1.5 kW each).
How do I calculate kVA?
kVA = total running kW ÷ power factor. Use 0.8 for most mixed loads, then add a 20–30% margin for expansion and load spikes.
What power factor should I use for generator sizing?
Use 0.8 for most mixed loads. Motor-heavy (inductive) loads have a lower power factor (down to about 0.6), so they need a larger kVA rating for the same kW.
Should I add a margin to the calculated kVA?
Yes — add 20–30% on top of the calculated kVA for future expansion and load spikes.
What is the difference between kW and kVA?
kW is real (usable) power and kVA is apparent power. kW = kVA × power factor, so a 50 kVA generator at 0.8 PF delivers 40 kW.
Conclusion
To size a generator, add up the running and surge load, divide by the power factor to get kVA, and add a 20–30% margin. For help choosing a unit, see our range of silent diesel generators.
