How to Determine Generator Size

The first question when buying a generator is always the same: "How many kVA should I get?" But the right answer is rarely as simple as "I'll add up my equipment ratings and pick accordingly." Concepts like single-phase/three-phase, kVA/kW, prime/standby, starting watts/running watts can turn an uninformed decision into a wasted investment.

This guide isn't a substitute for a certified electrical engineer's site survey. But at Berksan Jeneratör, drawing on hundreds of projects every year, we've compiled the key points that protect you from the most common mistakes.

Generator power ranges: which segment am I in?

Modern generators cover a wide power range. Identifying the right segment is the first step:

• 5-50 kW (roughly 6-62 kVA): the typical range for small offices, homes, villas, and small businesses.
• 50 kW-3 MW (62 kVA-3,750 kVA): the industrial generator segment. Factories, hospitals, data centers, shopping malls, and large facilities sit in this range.

If your need is close to the boundary between the two segments, moving toward the entry model of the higher segment is always the safer choice.

The most common mistake: "a small generator will do" fallacy

The most frequent mistake we see in the field is sizing the generator below the actual demand on the assumption that "it won't run continuously anyway." This approach causes two critical problems:

• The generator runs in continuous overload, its windings and bearings wear out prematurely, and its service life shortens dramatically.
• The connected equipment — especially motorized devices and sensitive electronics — suffer from low voltage and frequency fluctuations, and the generator ends up damaging the very systems it's meant to protect.

Key rule: always select a generator with capacity above the calculated demand. The general consensus is to add at least 20-25% headroom on top of the total calculated load.

Concept clarity: kVA, kW, and the power factor

The two most commonly confused units in generator selection are kVA (apparent power) and kW (active power). They measure different things, and the relationship between them is set by the power factor (cos φ) — the industry standard for which is 0.8.

• kVA: the total apparent power the generator produces
• kW: the active power that equipment actually uses to do work
• Power factor: the ratio of active to apparent power; 0.8 is the standard

The practical formulas:

kW = kVA × 0.8
kVA = kW ÷ 0.8

Example: a 100 kVA nameplate generator delivers approximately 80 kW of active power at a 0.8 power factor. If the equipment label gives a value in kW, divide by 0.8 to convert to kVA for generator selection.

Prime vs standby: which scenario uses which?

Two critical labels you'll see on a generator define which selection fits your usage scenario:

• Prime power: the power the generator can deliver continuously over the long term. Used in applications where the grid is unavailable or the generator is the primary energy source — remote construction sites, agricultural facilities, mobile applications.
• Standby power: the maximum power the generator can deliver only briefly when the grid fails. Annual operation hours are limited (typically 200-500 hours). Hospitals, data centers, offices, and stores are the typical users of this segment.

For the same generator, standby rating is roughly 10% above prime rating. A generator rated 100 kVA prime can deliver about 110 kVA standby. Reading the right label for your usage scenario is critical for the investment decision.

Single-phase or three-phase?

The answer depends on the load type:

• Single-phase generator: for small-scale loads served by a single-phase grid (homes, small offices, small shops). Generally sufficient below 10 kVA.
• Three-phase generator: mandatory for motorized equipment, large HVAC systems, manufacturing facilities, and nearly all commercial/industrial applications. Most generators above 10 kVA are three-phase.

Connecting a single-phase generator to a three-phase facility is technically not viable. This decision cannot be made without examining the facility's electrical infrastructure.

Step-by-step generator power calculation

Correct power calculation comes down to four steps:

Step 1: list the equipment to be powered

Write out every device the generator will feed. Lighting, servers, refrigerators, HVAC units, production machinery, elevators, fire panels — all of it.

Step 2: note running and starting watts for each device

From the equipment label or user manual, record two values: running watts and starting watts. Motorized devices in particular (HVAC units, refrigerators, compressors, pumps) can draw 3-7 times their normal consumption at the moment of startup.

Step 3: convert amps to watts (if needed)

If the equipment label gives only amps (A), use these formulas to convert to watts:

• Resistive loads (heaters, bulbs, irons): Watts = Amps × Volts
• Reactive loads (motorized devices): Watts = Amps × Volts × Power Factor

Single-phase grid voltage in Türkiye is 220 V, three-phase grid voltage is 380 V.

Step 4: total the load and add headroom

Sum the running watts of all devices. Then add the difference between starting and running watts of the device with the highest startup demand (worst-case scenario). Finally, add 20-25% headroom to that result and divide by 0.8 to convert to kVA.

Power consumption reference table

If you can't find your equipment's manual, the table below offers approximate starting and running watts for common devices. For exact calculations, always refer to the device label or consult a professional electrician.

Device	Starting Watts	Running Watts
Circular saw	2,400	1,200
Drill	1,800	720
Electric saw	2,400	1,200
Electric lawn mower	4,320	1,440
Electric pressure washer	3,600	1,200
Paint sprayer	1,080	360
Water pump	3,000	1,000
Wet/dry vacuum	2,500	888
Hoist	5,400	1,800
Air compressor (avg.)	4,000	2,000
Furnace fan (1/4 hp)	1,000	600
Furnace fan (3/5 hp)	2,350	875
Central A/C (10,000 BTU)	2,200	1,500
Central A/C (20,000 BTU)	3,300	2,500
Central A/C (24,000 BTU)	4,950	3,800
Central A/C (32,000 BTU)	6,500	5,000
Central A/C (40,000 BTU)	6,700	6,000
Disc grinder	4,000	2,000
12" concrete cutter	3,600	1,800
Reciprocating saw	3,100	1,560

The striking takeaway from the table is this: the starting power of motorized devices can be 2-3 times their running power. When sizing a generator, not just the running power but the highest startup peak must be factored in.

Calculation scenarios: motor load conditions

Depending on equipment type and the number of motors running simultaneously, there are three different calculation scenarios:

• When a single motor runs: total running watts + the starting–running watts gap of the largest motor.
• When multiple motors run simultaneously: account for the starting watts of each motor separately. This is the most critical scenario and is frequently neglected.
• When there are no motorized loads: the sum of total running watts is sufficient; no starting multiplier is applied.

In multi-motor applications like manufacturing facilities, hotel kitchens, or cold storage, the calculation must be verified with an expert — otherwise two HVAC units engaging at the same time can trip the generator and shut it down.

The benefits of right-sizing your generator

Following this guide and selecting a properly sized generator delivers tangible benefits:

• Prevention of unexpected system failures
• Elimination of overload-driven shutdowns
• Extended generator service life
• Manufacturer performance warranty stays valid
• Predictable maintenance costs
• Protection of the equipment it powers
• Operator and user safety

Why the right supplier matters as much as the right generator

A generator is an investment that has to engage at the critical moment. That's why your choice of supplier matters as much as the brand of the generator itself. When evaluating a supplier, ask:

• How many years of industry experience do they have?
• Do they provide service and maintenance after installation? What cities does their service network cover?
• How quickly are spare parts supplied?
• Do they offer site analysis and project-specific engineering support?
• What do their reference projects and customer satisfaction look like?

A generator's service life is 15-20 years. Selecting a supplier that won't disappear during that period matters far more than the lowest initial price.

Conclusion: the right size means the right investment

Calculating generator power isn't just arithmetic — it's an engineering decision that combines equipment knowledge, conceptual clarity, and field experience. Decisions made without understanding the kVA-kW distinction, without accounting for starting watts, and without separating prime from standby end up with either an undersized system or an unnecessarily oversized investment.

At Berksan Jeneratör, we evaluate the equipment list, starting watts, and usage scenario for every client through site analysis. The right calculation means a generator that doesn't fall short at the critical moment and runs economically for years.