Generator Size Calculator
Estimate minimum generator kVA from running kW or load current, power factor, starting demand, reserve, and site derating.
Calculator is for informational purposes only. Terms and Conditions
Choose the calculation setup
Use total running power or measured load current as the calculation basis.
Enter the known values
Starting demand is entered as the additional kVA above the load already included in the running total.
Solution
Preliminary nameplate capacity, load checks, warnings, and calculation steps.
Capacity checks
- Running apparent load—
- Starting-event demand—
- Estimated steady loading—
Show solution steps Review conversions, demand calculation, reserve, derating, and limitations
- Enter valid values to see the complete solution.
Source, References, and Assumptions
Calculation basis, limitations, and final verification requirements.
This simplified estimate checks apparent-power demand and the generator kW capability implied by its nameplate power factor, then applies one additional starting event, planning reserve, and a manufacturer-derived site rating factor. It does not model transient voltage or frequency dip, harmonics, load steps, fuel limits, or code-required emergency-load behavior.
- Total running input must represent loads expected to operate simultaneously.
- Starting input is one incremental kVA event above the running load already counted.
- The 20% reserve and 0.8 generator power factor are editable examples, not universal requirements.
- A 100% site rating factor assumes no manufacturer derating.
- Final selection requires applicable codes, load sequencing, manufacturer data, site conditions, and qualified engineering judgment.
Calculator guide
How the Generator Size Calculator Works
The Generator Size Calculator above estimates the minimum preliminary generator nameplate capacity in kVA from either total simultaneous running power or load current. It checks the running kW and kVA demand, adds the largest additional starting event, applies your planning reserve and site rating factor, and returns the larger of the apparent-power and engine real-power requirements.
This is more useful than sizing from watts alone because a generator must satisfy both kW and kVA limits. The result is still a screening value: final selection must be matched to an actual generator’s duty rating, fuel, site derating, transient motor-starting capability, voltage and frequency dip, load sequencing, and manufacturer data.
- Best for
- Preliminary standby, portable, and commercial generator capacity screening
- Primary result
- Required generator nameplate capacity in VA, kVA, or MVA
- Governing check
- The larger of the starting-event kVA requirement and generator kW-capability requirement
How to Size a Generator Correctly
Build the generator load from equipment that can operate at the same time, then add the starting demand that can occur on top of that running condition. Do not use the main breaker rating or the sum of every connected load unless that truly represents simultaneous operation.
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Choose running power or load current
Use From Running Power when a load schedule provides total simultaneous W, kW, or MW. Use From Load Current when voltage, phase, RMS current, and load power factor describe the same operating condition.
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Enter the simultaneous running load
Include the loads expected to operate together during the intended backup, standby, or prime-power sequence. Apply justified load shedding or diversity before entering the total.
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Enter load and generator power factor
Load power factor converts running real power into running apparent power. Generator nameplate power factor represents the candidate set’s kW-to-kVA rating ratio. Use project or manufacturer data instead of assuming 0.8 is always correct.
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Add only the extra starting kVA
If a motor already contributes 20 kVA to the running total and needs 80 kVA during starting, enter 60 kVA as the additional starting demand. Entering the full 80 kVA would double-count the motor’s running contribution.
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Apply reserve and site rating
Use a planning reserve that matches the project strategy and a site rating factor from the selected manufacturer’s altitude, ambient temperature, fuel, and duty data. The calculator’s starting values are examples, not universal requirements.
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Review both capacity checks
Confirm whether the starting-event kVA requirement or the generator kW-capability check governs. Then compare the preliminary result with real product ratings and transient performance before selecting equipment.
Generator Sizing Inputs and Outputs
The calculator supports either a running-power basis or a current-and-voltage basis. Both paths feed the same kW, kVA, starting-load, reserve, and site-derating checks.
- Input Method
- Choose running power when total simultaneous real power is known. Choose current when RMS load current, voltage, phase, and load power factor are known.
- Electrical System
- Current mode supports single-phase and balanced three-phase calculations. Balanced three-phase mode uses RMS line current and line-to-line voltage.
- Total Simultaneous Running Load
- The real power of all loads expected to operate together, not automatically the sum of every connected device or the panel rating.
- Load Current and Voltage
- In current mode, enter values from the same operating state. Use the load voltage for single-phase systems and line-to-line voltage for balanced three-phase systems.
- Load Power Factor
- The aggregate ratio of running kW to running kVA. Lower load power factor increases alternator kVA demand for the same real-power load.
- Generator Nameplate Power Factor
- The generator set’s rated kW divided by rated kVA. Use the actual candidate product rating; 0.8 is common for some industrial three-phase sets but is not universal.
- Largest Additional Starting Demand
- The extra apparent power above the running load already counted during the largest expected start or load step.
- Planning Reserve
- A user-selected allowance for future growth, uncertainty, and operating flexibility. It is not a universal code-required percentage.
- Site Rating Factor
- The usable fraction of generator nameplate output at the actual site and duty conditions. Obtain it from manufacturer data for the intended configuration.
- Required Generator Capacity
- The greater of the kVA-limited and kW-limited requirements, reported as generator nameplate apparent power.
Generator Size Formula
The calculator checks both alternator apparent-power demand and the engine real-power capability implied by the generator’s nameplate kW-to-kVA ratio.
Running apparent power from running kW
Divide total simultaneous running real power by aggregate load power factor to obtain running apparent power.
Running apparent power from current
For balanced three-phase current mode, use RMS line current and RMS line-to-line voltage. Running real power is then \(P_{\mathrm{run}}=S_{\mathrm{run}}PF_{\mathrm{load}}\).
Governing preliminary generator capacity
The first term checks running apparent load plus one additional starting event. The second checks whether the generator’s rated kW capability can carry the reserved running real-power requirement after site derating.
This is a transparent preliminary sizing model. It does not calculate transient voltage or frequency dip during a load step.
- \(S_{\mathrm{gen}}\)
- Preliminary required generator nameplate apparent power.
- \(P_{\mathrm{run}}\)
- Total simultaneous running real power in W, kW, or MW.
- \(S_{\mathrm{run}}\)
- Simultaneous running apparent power in VA, kVA, or MVA.
- \(S_{\mathrm{start,add}}\)
- Largest additional starting kVA above the running load already included.
- \(PF_{\mathrm{load}}\)
- Aggregate running-load power factor.
- \(PF_{\mathrm{gen}}\)
- Candidate generator’s nameplate kW-to-kVA ratio.
- \(r\)
- Planning reserve as a decimal; 20% corresponds to \(r=0.20\).
- \(f_{\mathrm{site}}\)
- Usable site output as a decimal; 90% available output corresponds to \(f_{\mathrm{site}}=0.90\).
Worked Example: 80 kW Load
A facility has an 80 kW simultaneous running load at 0.80 aggregate power factor. The largest expected motor start adds 50 kVA above the motor’s running contribution. Use a 20% planning reserve, 90% site rating factor, and 0.80 generator nameplate power factor.
Running apparent power
Starting-event demand
Alternator and starting kVA check
Generator kW-capability check
Result
Preliminary required generator capacity: \(200\ \mathrm{kVA}\)
The starting-event kVA requirement governs because 200 kVA is larger than the 133.3 kVA real-power requirement. The next step is to compare that value with an actual generator’s kW, kVA, site-derated rating, and allowed transient voltage and frequency dip.
Running Load vs Starting Load
Generator sizing fails most often when starting demand is ignored or counted incorrectly. Motors and compressors can require much more apparent power during acceleration than after they reach normal speed.
Running load
The electrical demand after equipment is operating normally. Build this total from loads that can run at the same time under the intended generator sequence.
Additional starting demand
The temporary kVA above the running load already counted. If a motor contributes 20 kVA while running and requires 80 kVA during start, the additional starting demand is 60 kVA.
| Load condition | Running contribution already counted | Total starting demand | Enter as additional start |
|---|---|---|---|
| Motor example | 20 kVA | 80 kVA | 60 kVA |
| Compressor example | 8 kVA | 32 kVA | 24 kVA |
Cummins’ current portable-generator sizing guidance similarly separates running demand from the largest starting requirement, while Caterpillar’s commercial sizing workflow models individual loads and load steps and can evaluate voltage and frequency dip. For overlapping starts, enter the combined additional kVA or move to manufacturer sizing software.
kW, kVA, Current, and Power Factor
Generator sets have both real-power and apparent-power limits. A generator that has enough kW can still be inadequate in kVA, particularly when load power factor is low or starting current is high.
kW is real power
kW represents the real power consumed by the load. In running-power mode, this is the primary steady load entered into the calculator.
kVA is apparent power
kVA represents the voltage-current demand on the alternator. For the running load, \(kVA=kW/PF_{\mathrm{load}}\).
Power factor connects them
\(PF=kW/kVA\). At 80 kW and 0.80 PF, the running apparent load is 100 kVA. At 0.90 PF, the same 80 kW load would be about 88.9 kVA.
| System | Apparent-power relationship | Voltage input |
|---|---|---|
| Single phase | \(S=VI\) | Voltage across the load |
| Balanced three phase | \(S=\sqrt{3}V_{LL}I\) | Line-to-line voltage |
Published generator ratings illustrate why nameplate power factor cannot be assumed universally. For example, Generac industrial spec sheets show three-phase ratings where kW and kVA correspond to 0.8 PF, while some single-phase ratings are listed at 1.0 PF. Use the actual candidate product’s rating.
How to Interpret the Generator Size Result
The displayed value is the minimum preliminary nameplate capacity under the entered load, reserve, power-factor, and site-factor assumptions. It is not automatically the next catalog size to purchase.
Starting kVA governs
The alternator/start-event requirement is larger than the engine kW requirement. Review motor or compressor starting data, starting method, load sequencing, and the candidate generator’s motor-starting or transient performance.
Engine kW governs
The reserved running real-power demand is larger than the kW capability implied by the entered generator nameplate power factor and site factor. A higher-kW set or different product rating is needed.
Steady utilization is low
The generator may be driven by one rare starting event rather than normal running demand. Review load sequencing and the selected manufacturer’s minimum-loading or application guidance instead of assuming low utilization is acceptable.
Units, Reserve, and Site Derating
The calculator converts supported display units internally, but it cannot tell whether a number represents the correct physical quantity. Unit prefixes, percent fields, power-factor basis, and site derating deserve a deliberate check.
kW and kVA are not interchangeable
kW is real power; kVA is apparent power. Use \(kVA=kW/PF_{\mathrm{load}}\) for the load and use the candidate generator’s actual kW/kVA ratio for \(PF_{\mathrm{gen}}\).
Percent inputs use percent values
Enter 20 for a 20% reserve and 90 for a 90% site factor. The calculator converts those values internally to 0.20 and 0.90.
Three-phase voltage is line-to-line
Balanced three-phase current mode uses \(S=\sqrt{3}V_{LL}I\). Entering line-to-neutral voltage understates the apparent load by a factor of \(\sqrt{3}\).
Site factor comes from manufacturer data
Altitude, ambient temperature, fuel, duty, cooling system, and emissions configuration can change usable output. A 100% site factor means the calculator is assuming no derating.
- \(1\ \mathrm{kW}=1000\ \mathrm{W}\) and \(1\ \mathrm{MW}=1000\ \mathrm{kW}\).
- \(1\ \mathrm{kVA}=1000\ \mathrm{VA}\) and \(1\ \mathrm{MVA}=1000\ \mathrm{kVA}\).
- \(1\ \mathrm{kA}=1000\ \mathrm{A}\).
- Reserve and site derating should not be hidden inside the running-load total; keep them visible so the sizing basis can be reviewed.
Common Generator Sizing Mistakes
Most oversized or undersized results come from the load model rather than the arithmetic. Check these issues before changing assumptions simply to obtain a preferred generator size.
Using the main breaker as generator load
A breaker rating is not proof of simultaneous demand. Build the load from equipment expected to operate together or from defensible demand measurements and load-management assumptions.
Double-counting motor start
Do not add a motor’s full starting kVA when its running kVA is already in the simultaneous total. Add only the incremental difference.
Ignoring overlapping starts
The calculator models one additional starting event. If two large loads can start together, use their combined incremental requirement or a manufacturer load-step model.
Assuming 0.8 generator PF universally
The calculator’s initial value is an editable example. Single-phase, portable, inverter, and application-specific generators can have different kW-to-kVA relationships.
Ignoring site derating
A sea-level nameplate rating may not be fully available at the actual elevation, temperature, fuel, or duty condition. Use manufacturer-supported site data.
Stopping at the steady kVA result
A generator may satisfy steady kW and kVA but still have unacceptable voltage or frequency dip during a motor start or abrupt load step.
Calculator Limits and Manufacturer Sizing
This is a design-sensitive preliminary estimator. It is intentionally transparent, but it cannot reproduce the transient electrical and engine behavior of a specific generator set.
One governing start event
The model adds one largest incremental starting demand to the simultaneous running load. It does not independently sequence multiple overlapping starts.
Aggregate running power factor
The running system is represented by one aggregate power factor. Nonlinear loads, harmonics, UPS systems, rectifiers, and VFDs can require more detailed alternator and waveform analysis.
Scalar site derating
The calculator applies one site rating factor. It does not derive altitude, ambient-temperature, fuel, cooling, emissions, or duty derates automatically.
No transient voltage/frequency model
The calculation does not predict voltage dip, frequency dip, recovery time, excitation response, engine acceleration, or load-step stability.
No installation design
The result does not size transfer switches, breakers, conductors, grounding, fuel storage or piping, ventilation, exhaust, sound attenuation, or installation clearances.
No universal duty rating
Standby, prime, continuous, and other manufacturer ratings are application-specific. Match the intended operating duty to the selected product’s published rating.
Sources and Calculation Basis
The article and calculator were checked against current generator-manufacturer sizing guidance and published generator rating examples.
- Caterpillar — Commercial Generator Sizing Calculators — supports load-by-load and load-step sizing, voltage- and frequency-dip review, spare capacity, and manufacturer product matching.
- Cummins — What Size Portable Generator Do You Need? — supports identifying simultaneous loads, separating running and starting power, and selecting a generator whose running and surge ratings exceed the applicable load requirements.
- Cummins — Generator Sets and Power Suite — identifies Power Suite as a generator-set sizing and specification tool for engineering applications.
The 80 kW worked example was independently checked by both the governing formula and the site-derated kW/kVA capacity of the resulting 200 kVA generator. The arithmetic reproduces the live calculator result exactly to the displayed precision.
Generator Size Calculator FAQ
These questions address the issues that most often change a generator-sizing result or its practical interpretation.
What size generator do I need for an 80 kW load?
There is no reliable answer from 80 kW alone. You also need load power factor, generator nameplate power factor, starting demand, site derating, and any planning reserve. In the worked example on this page, 80 kW at 0.80 load PF with 50 kVA additional starting demand, 20% reserve, and a 90% site factor produces a 200 kVA preliminary requirement.
Should I add every appliance or motor starting wattage?
Only if those starts can overlap. This calculator assumes one largest incremental starting event above the running load already counted. If multiple starts can occur together, use their combined additional demand or model the actual load sequence with manufacturer sizing software.
Why does the calculator size in kVA instead of only kW?
The generator engine must supply real power in kW, while the alternator and load current are constrained by apparent power in kVA. A generator can pass the kW check but fail the kVA or motor-starting requirement, so both ratings matter.
Is a 0.8 generator power factor always correct?
No. A 0.8 kW-to-kVA ratio is common for some industrial three-phase ratings, but it is not universal. Use the actual candidate generator’s published kW and kVA ratings, especially for single-phase, portable, inverter, or application-specific units.
Can I use the main breaker size as the generator load?
Usually not. Breaker ampere rating is a protection and equipment value, not proof of simultaneous electrical demand. Build the generator load from actual connected loads, measured demand, equipment data, and the intended load-management sequence.
Does the calculator include altitude and temperature derating?
Only through the site rating factor you enter. The calculator does not calculate the factor from altitude or temperature automatically. Obtain usable output or derating data from the selected generator manufacturer for the actual site, fuel, duty, and configuration.
How much extra generator capacity should I add?
There is no universal reserve percentage. The calculator lets you enter a planning reserve for growth and uncertainty, but the appropriate allowance depends on load certainty, future expansion, operating strategy, redundancy, and the selected generator manufacturer’s application guidance.
Why can a manufacturer recommend a larger generator than this calculator?
Manufacturer tools can model specific alternators, engines, excitation systems, motor-starting methods, voltage and frequency dip, nonlinear loads, load-step sequencing, and recovery limits. Those transient constraints can require a larger set even when the simplified steady kW and kVA checks pass.