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.
On this page
Calculator Guide
How to Use the Generator Size Calculator
The Generator Size Calculator above estimates the minimum generator nameplate capacity in kVA for a planned load. Start with either total simultaneous running kW or measured load current and voltage, then enter aggregate load power factor, generator nameplate power factor, the largest additional starting kVA, planning reserve, and site rating factor. The result is the larger of two checks: the alternator and transient kVA needed for the running load plus one start event, and the engine kW capability implied by the generator’s kW-to-kVA rating. Use the answer as a preliminary screening value, then compare it with a real manufacturer’s continuous, standby, surge, and site-derated ratings before selecting equipment.
This method is more useful than simply adding watts because a generator must satisfy both real-power and apparent-power limits. It still cannot predict voltage dip, frequency dip, harmonics, fuel-system performance, or every possible load sequence.
Quick Answer
Convert the simultaneous running load to kVA, add only the largest extra starting kVA above the load already counted, apply the planning reserve, divide by the site rating factor, and compare that result with the generator kW-capability check. The larger value is the preliminary required generator capacity.
Equipment-selection warning
A calculated kVA value does not approve a generator, transfer switch, breaker, conductor, fuel system, exhaust system, grounding arrangement, or emergency-power installation. Final selection requires the applicable requirements, actual load sequencing, manufacturer transient data, site conditions, and qualified electrical review.
Generator Sizing Inputs and Outputs
The calculator can begin with total running power or with current and voltage. Both methods feed the same capacity checks, so the most important task is entering a realistic simultaneous load and a correctly defined starting increment.
- Input Method
- Choose From Running Power when a load schedule already provides total simultaneous W, kW, or MW. Choose From Load Current when current, voltage, phase, and load power factor are known.
- Total Simultaneous Running Load
- The real power of all loads expected to operate together. Do not enter the panel rating or the sum of every connected device unless all of them truly run at the same time.
- Current, Voltage, and Phase
- In current mode, use RMS load current and the voltage at the same operating condition. Balanced three-phase mode expects line-to-line voltage; single-phase mode uses the voltage across the load.
- Load Power Factor
- The aggregate ratio of running kW to running kVA. Use metering, a load study, or equipment data. A lower value increases alternator kVA demand for the same real-power load.
- Generator Nameplate Power Factor
- The candidate generator’s rated kW divided by rated kVA. The calculator begins with 0.8 as an editable example, not as a universal generator rating.
- Largest Additional Starting Demand
- The extra kVA above the running load already included in the total. If a motor runs at 20 kVA and starts at 80 kVA, enter 60 kVA, not 80 kVA.
- Planning Reserve
- A user-selected allowance for uncertainty, growth, and operating flexibility. The initial 20% is an example and is not a universal code or manufacturer requirement.
- Site Rating Factor
- The usable fraction of nameplate output at the actual altitude, ambient temperature, fuel, and duty conditions. Enter the manufacturer-supported percentage, such as 90 for 90% available output.
- Required Generator Capacity
- The greater of the kVA-limited and kW-limited nameplate requirements, reported in VA, kVA, or MVA. The result must still be matched to an actual product rating and transient-performance data.
- Capacity Checks
- The result card also reports running apparent load, running real power, starting-event demand, kVA-limited requirement, kW-limited requirement, and estimated steady utilization.
Generator Size Formula
The calculator performs two independent nameplate checks. One protects the apparent-power and starting-event requirement; the other checks whether the engine kW rating implied by the generator nameplate power factor can carry the running real-power load.
Running apparent power
Use this form when the running input is real power. Keep kW with kVA or W with VA so the unit scale is consistent.
Running apparent power from current
The three-phase expression assumes a balanced load and line-to-line RMS voltage. Running real power is then calculated from \(P_{\mathrm{run}}=S_{\mathrm{run}}PF_{\mathrm{load}}\).
Governing generator capacity
The first term is the alternator and transient kVA check. The second is the engine kW check expressed as the minimum generator kVA rating needed at the entered nameplate power factor.
- \(S_{\mathrm{gen}}\)
- Preliminary required generator nameplate apparent power, in VA, kVA, or MVA.
- \(P_{\mathrm{run}}\)
- Total simultaneous running real power, in W, kW, or MW.
- \(S_{\mathrm{run}}\)
- Simultaneous running apparent power after applying the load power factor.
- \(S_{\mathrm{start,add}}\)
- Largest additional starting kVA above the running kVA already included.
- \(PF_{\mathrm{load}}\)
- Aggregate load power factor, entered as a decimal greater than 0 and no more than 1.
- \(PF_{\mathrm{gen}}\)
- Generator nameplate kW-to-kVA ratio from the candidate manufacturer rating.
- \(r\)
- Planning reserve as a decimal; 20% is \(r=0.20\).
- \(f_{\mathrm{site}}\)
- Usable site rating as a decimal; 90% available output is \(f_{\mathrm{site}}=0.90\).
How to Size a Generator with the Calculator
Use the power method when a dependable load schedule is available. Use the current method only when voltage, current, phase, and power factor describe the same operating condition.
Choose the input method
Select running power for W, kW, or MW data. Select load current when sizing from measured or calculated amperes; then choose single-phase or balanced three-phase operation.
Enter the simultaneous running load
Include only loads expected to operate together under the intended backup or prime-power sequence. Apply any justified load shedding or diversity assumption before entering the total.
Define power factor and the start event
Enter the aggregate load power factor and the candidate generator nameplate power factor. Add only the extra kVA during the largest expected start, not the device’s full starting kVA if its running portion is already counted.
Apply reserve and site rating
Use a planning reserve supported by the project strategy and a site rating factor from manufacturer altitude, temperature, fuel, and duty data. A 100% site factor means no derating has been applied.
Review both governing checks
Do not stop at the displayed kVA. Confirm whether the alternator/transient check or engine kW check governs, inspect the starting-event demand and steady utilization, and then compare the result with a real generator’s ratings.
Generator Load Data Checklist
The formula is straightforward, but the result is only as reliable as the load schedule, starting data, and manufacturer rating information entered.
- Confirm which loads operate simultaneously during the worst intended operating state.
- Use measured demand or equipment data from the same voltage, phase, and operating condition.
- Use line-to-line voltage in balanced three-phase current mode.
- Obtain aggregate load power factor from metering, a study, or defensible equipment data.
- Convert the largest device’s starting requirement to an incremental kVA above its running contribution.
- Use the candidate generator’s actual kW, kVA, fuel, duty, altitude, and temperature ratings.
- Identify any loads that can start together or that must be sequenced by controls or a transfer scheme.
Worked Example: Generator Size from Running kW
A facility has an 80 kW simultaneous running load at 0.80 aggregate power factor. Its largest expected motor start adds 50 kVA above the motor’s running contribution. Use 20% planning reserve and a 90% site rating factor.
Running apparent power
Starting-event demand
Alternator and transient kVA check
Engine kW check
Select the larger requirement
Result
Preliminary required generator capacity: 200 kVA
The alternator and transient kVA check governs. The next step is to find a generator whose manufacturer-rated kVA, kW, site-derated output, and allowed voltage and frequency dip all satisfy the project.
Verification check
A 200 kVA generator at 0.80 nameplate power factor corresponds to 160 kW at nameplate. At a 90% site factor, that becomes 144 kW and 180 kVA available. The reserved running real-power requirement is only \(80\times1.20=96\,\mathrm{kW}\), while the reserved starting-event requirement is \(150\times1.20=180\,\mathrm{kVA}\). The kVA side therefore lands exactly at the 200 kVA nameplate requirement.
Using these same calculator inputs produces 200 kVA, identifies the alternator and transient kVA rating as governing, and reports 100 kVA running apparent load, 150 kVA starting-event demand, and approximately 50% estimated steady utilization.
How to Interpret the Generator Size Result
The displayed value is a minimum preliminary nameplate capacity under the entered assumptions. It is not automatically the next product size to buy, because actual generators have separate ratings for kW, kVA, duty, fuel, temperature, altitude, and transient response.
What the result means
The recommended kVA is the larger of the apparent-power/start-event requirement and the engine real-power requirement after reserve and site derating. A candidate set must pass both, not merely match one number.
What changes it most
When starting kVA governs, each additional 10 kVA of running or incremental starting demand adds \(10(1+r)/f_{\mathrm{site}}\) to the nameplate requirement. At 20% reserve and a 90% site factor, that is about 13.3 kVA.
Fast sanity check
The result should never be below the site- and reserve-adjusted starting event. Also verify that generator rated kW, calculated as kVA multiplied by nameplate power factor, exceeds the adjusted running kW requirement.
Power-factor sensitivity
In running-power mode, apparent load varies inversely with load power factor. Reducing power factor from 0.80 to 0.72 increases running kVA by about 11.1% before any other changes.
Site-factor sensitivity
Capacity also varies inversely with site factor. Changing from 100% available output to 90% increases the required nameplate rating by about 11.1%.
Reserve sensitivity
Reserve is linear through the \((1+r)\) multiplier. Increasing reserve from 20% to 30% raises the same base requirement by about 8.3%, because \(1.30/1.20=1.0833\).
Suspicious result patterns
A result below the unadjusted starting-event kVA, a dramatic jump caused by a unit change, or very low estimated loading should trigger an input and manufacturer-data review.
What to do next
Round only after reviewing actual manufacturer ratings. Compare the calculated requirement with the candidate set’s continuous or standby kW and kVA ratings, transient voltage and frequency dip, fuel rating, site derating, load-step capability, and intended duty.
Preliminary Calculation vs. Manufacturer Generator Sizing
The calculator is intentionally transparent and useful for screening, but final generator selection may require a manufacturer model that represents the actual engine, alternator, excitation system, controls, and load steps.
Use this calculator for
Early load screening, comparing reserve and site-factor assumptions, checking whether kW or kVA governs, reviewing one dominant starting event, and identifying inconsistent unit or power-factor inputs.
Use manufacturer sizing software for
Multiple load steps, exact motor-starting methods, voltage and frequency dip limits, nonlinear loads, UPS and VFD behavior, harmonic effects, engine recovery, alternator selection, paralleling, and critical applications.
Generator Sizing Units and Conversion Traps
The calculator converts supported units internally, but the selected unit must match the source data. The most consequential errors are mixing kW with W, kVA with VA, percent with decimal notation, or line-to-line with line-to-neutral voltage.
kW and kVA are not interchangeable
kW is real power and kVA is apparent power. For a load, \(kVA=kW/PF_{\mathrm{load}}\). For a generator nameplate, rated \(kW=kVA\times PF_{\mathrm{gen}}\).
Starting input is incremental kVA
Enter the extra kVA during starting, not the full starting kVA, when the device’s running contribution is already part of the simultaneous load total.
Percent fields use percent values
Enter 20 for a 20% reserve and 90 for a 90% site rating factor. The calculator converts these values to 0.20 and 0.90 internally.
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 apparent power by a factor of \(\sqrt{3}\).
Common Generator Sizing Mistakes
Most large sizing errors come from the load model, not the arithmetic. Double-counted starting demand and unjustified default assumptions can move the result far more than rounding.
Do
- Build the running total from loads expected to operate simultaneously.
- Use locked-rotor, inrush, or manufacturer starting data for the dominant start event.
- Check candidate generator kW, kVA, site rating, fuel, and transient performance separately.
- Model deliberate load sequencing or load shedding when the controls will enforce it.
Don’t
- Use the main breaker or panel rating as though it were measured simultaneous load.
- Add a motor’s full starting kVA on top of a total that already includes its running kVA.
- Assume every generator has a 0.8 nameplate power factor or a 100% site rating.
- Assume every motor starts alone when two or more starts can overlap.
Troubleshooting Unexpected Generator Size Results
Use the governing-check label and capacity checks to find the source of an unexpected answer before changing assumptions merely to obtain a preferred generator size.
Result is much larger than expected
Check whether W was entered as kW, kVA as MVA, or total starting kVA as the additional starting increment. Also confirm that the site factor is a supported manufacturer value rather than an overly severe guess.
Result is suspiciously low
Verify that the starting increment is not incorrectly left at zero, all simultaneous loads are included, load power factor is not assumed too high, and site derating has not been omitted.
Engine kW rating governs
The running real-power requirement is larger than the kW capability implied by the entered generator power factor. Review a higher-kW model or a product with a different kW-to-kVA rating.
Starting event dominates
Verify locked-rotor or inrush data, starting method, voltage, and whether the load can be sequenced. A manufacturer transient model is the appropriate next step for a large motor, compressor, transformer, UPS, or abrupt block load.
Low steady-utilization warning
The calculator flags low estimated loading as a review point. Check whether the generator is oversized because of one rare start event, and compare the load profile with the specific manufacturer’s minimum-loading guidance.
Another sizing tool gives a different answer
Compare whether the other tool uses total starting watts, incremental starting kVA, one or several simultaneous starts, a different power factor, an additive derating rule, or a detailed voltage-dip model.
Assumptions, Sources, and Limitations
This is a Tier 3 design-sensitive estimate. It provides a transparent first-pass kW and kVA screen, but it cannot establish code compliance or guarantee that a selected generator will accept every load step.
One governing start event
The model adds one largest incremental starting demand to the simultaneous running load. It is not valid when several high-inrush loads can start together unless their combined increment is entered.
Aggregate steady-state power factor
The running load is represented by one aggregate power factor. Nonlinear loads, harmonics, UPS systems, rectifiers, VFDs, and rapidly changing load behavior may require a more detailed model.
Scalar reserve and site factor
Reserve and site derating are applied as simple multipliers. Actual altitude, temperature, fuel, emissions configuration, duty, and cooling limits must come from the selected manufacturer.
What the calculator cannot verify
The result does not calculate voltage dip, frequency dip, recovery time, harmonic heating, short-circuit capability, protection coordination, transfer-switch rating, conductor ampacity, grounding, fuel supply, ventilation, noise, emissions, or installation clearances.
Technical basis and final-design review
Caterpillar’s commercial generator sizing guidance distinguishes simple estimates from detailed sizing that evaluates load steps and voltage and frequency dip, and it identifies running demand, starting demand, site conditions, and future expansion as relevant inputs. Cummins portable-generator guidance explains the practical running-load plus largest-starting-load workflow. Generac’s mobile generator FAQ illustrates the distinction between kW, kVA, and power factor for its mobile equipment. Use the actual selected product data rather than treating any example rating as universal.
Generator Size Calculator FAQ
These answers address the most common interpretation questions that are not resolved by a single kW or kVA result.
What size generator do I need for an 80 kW load?
There is no single answer from 80 kW alone. At minimum, you also need load power factor, generator nameplate power factor, the largest additional starting kVA, planning reserve, and site derating. In the worked example above, those assumptions produce a 200 kVA preliminary requirement.
Should I add every appliance or motor starting wattage?
Not unless those starts can occur together. This calculator assumes one largest incremental start above the running load already counted. When multiple starts overlap, enter their combined additional kVA or use a manufacturer load-step model.
Is a 0.8 generator power factor always correct?
No. A 0.8 kW-to-kVA ratio is common for some three-phase generator ratings, but it is not universal. Use the actual candidate generator’s nameplate or datasheet kW and kVA ratings, especially for single-phase, portable, inverter, or application-specific equipment.
Why does the calculator report kVA instead of only kW?
The engine must supply real power in kW, while the alternator and load current are constrained by apparent power in kVA. A generator can be adequate on one rating and inadequate on the other, so the calculator checks both.
Can I use the main breaker size as the generator load?
Usually not. A breaker rating is a protection and equipment value, not proof of simultaneous demand. Build the generator load from actual connected loads, demand measurements, nameplate data, and the intended load-management sequence.
Does the calculated result include altitude and temperature derating?
Only through the site rating factor you enter. The calculator does not derive that factor from altitude or temperature. Obtain the usable output percentage from the selected manufacturer for the actual fuel, duty, ambient temperature, elevation, and configuration.