Generator Load & Capacity Planner
Screen generator kVA from running load, largest motor starting multiplier, power factor and reserve.
Use the Generator Size Calculator
Screen generator kVA from running load, largest motor starting multiplier, power factor and reserve.
Calculations happen locally in your browser. ToolLott does not send these inputs to a server for this tool.
Preliminary engineering calculation only. It does not replace applicable codes, manufacturer data, licensed electrical or mechanical design, protection studies, derating, fault checks, site conditions or professional verification.
How to use it
Replace the worked-example values with the actual project inputs. Review the intermediate quantities and assumptions before using the result elsewhere.
How the Generator Size Calculator works
Screen generator kVA from running load, largest motor starting multiplier, power factor and reserve. The methodology below exposes the production equation, a QA-verified worked example and the boundary between this screen and detailed design.
Calculation breakdown
ToolLott will explain the current inputs and displayed result here.
Generator screening from running kVA and motor starting multiplier
The production engine evaluates design kVA = [run kW/PF + (motor kW x start_mult/PF - motor kW/PF)] x (1+reserve). It begins by convert total running kW to apparent kVA using entered power factor., then replace the largest motor running kVA contribution with its estimated starting kVA., and finally apply the selected reserve to the estimated starting peak. This keeps the calculation auditable instead of hiding design assumptions behind a single number.
design kVA = [run kW/PF + (motor kW x start_mult/PF - motor kW/PF)] x (1+reserve)ToolLott evaluates the relationship using the entered units and full numeric precision before display rounding.
| Symbol / input | Meaning | Unit |
|---|---|---|
runningKw | Total running load (kW) | user input |
motorKw | Largest motor within load (kW) | user input |
startMultiplier | Motor starting kW multiplier | user input |
powerFactor | Assumed power factor | user input |
reserve | Additional reserve (%) | user input |
Step-by-step method
- Convert total running kW to apparent kVA using entered power factor.
- Replace the largest motor running kVA contribution with its estimated starting kVA.
- Apply the selected reserve to the estimated starting peak.
Worked example
Daniel, a project engineer, must temporarily supply a 22 kW motor plus 8 kW of smaller loads and needs to account for motor starting rather than simply adding running kW.
Example inputs
- Total running load (kW): 30
- Largest motor within load (kW): 22
- Motor starting kW multiplier: 3
- Assumed power factor: 0.8
- Additional reserve (%): 20
Calculation / processing
Running apparent load = 30 / 0.8 = 37.5 kVA.Motor running = 22/0.8 = 27.5 kVA; estimated motor start = 22 x 3 / 0.8 = 82.5 kVA; peak = 92.5 kVA.Design = 92.5 x 1.20 = 111 kVA.
Use the result as a screening or quantity-planning value and compare it with the project criteria, manufacturer data and applicable engineering requirements before making a design or procurement decision.
Assumptions
- The entered values are representative of the condition being screened and use the units shown on the page.
- The equation models only the stated relationship; omitted system effects are not silently estimated.
Limitations
- This page is not engineering certification and does not replace applicable standards, design loads, manufacturer data, site investigation or competent professional review.
- Results can change materially when real geometry, losses, transients, material properties, duty cycles, safety factors or regulatory criteria differ from the simplified inputs.
Common questions
Is this a final engineering design?
No. It is a transparent screening calculation using the entered assumptions and the stated equation.
Why does the page show assumptions and limitations?
Engineering formulas are only valid within their modelling assumptions; exposing them helps users decide what additional design checks are required.
Should I use the rounded displayed value in later design work?
Use suitable calculation precision and the governing project/standard requirements rather than relying on display rounding alone.
Methodology sources
Related ToolLott tools
A realistic way Daniel could use this tool
Daniel is a project engineer.
Daniel must temporarily supply a 22 kW motor plus 8 kW of smaller loads and needs to account for motor starting rather than simply adding running kW.
The page is prefilled around the scenario data: running kw = 30; motor kw = 22; start multiplier = 3; power factor = 0.8; reserve = 20. On calculation/processing it returns Preliminary generator rating: 111 kVA, which ties the example to a specific checkable outcome instead of an abstract promise.
The scenario resolves to Preliminary generator rating: 111 kVA. That gives Daniel the concrete quantity/text/file setting needed to account for motor starting rather than simply adding running kW, with the limits of the method still visible. The page also keeps this limitation explicit: Preliminary screening or quantity calculation only.
Fictional scenario using realistic example data. For Ready tools, the worked result is tied to the tested example shown in the tool. Replace the figures with your own inputs and independently verify important professional, financial, legal, health or safety decisions.
What this generator is for
Use it to screen generator kVA from running load, largest motor starting multiplier, power factor and reserve.
It sits within ToolLott’s Electrical collection, where you can also estimate battery banks, cable sizes, generator sizes, motor current, Ohm’s law values, solar systems, transformers and voltage drop.
Generator Load & Capacity Planner
Separate running load from a simple starting allowance, then add a margin to create a planning target.
Starting current can vary widely by motor/load type. Check the generator and load manufacturer's requirements before selection.