Generator Power Calculator: Running and Starting Watts

How to use this generator power calculator

1. List the equipment that must run together. Enter quantity and electrical input watts per unit.
2. Enter total starting watts from equipment documentation or measured data. This is the complete starting value, not the extra watts above running power. For a load with no starting increase, enter the same value twice.
3. Select whether one unit, one entire row, or all units start together. Enter a planning margin and calculate.
4. Compare alternatives, such as separating motor starts or removing a nonessential load. Give the inventory to the generator supplier for final sizing.

The initial loads are invented examples for checking the arithmetic. They are not appliance ratings or product recommendations. All listed loads are assumed to run together after starting.

Running watts and starting watts: the formulas

For quantity q, running watts P and total starting watts S per unit:

Running load = Σ(q × P). The extra starting demand per unit is S − P.

• One unit at a time: starting estimate = running load + largest (S − P).
• One load group at a time: starting estimate = running load + largest [q × (S − P)]. All units in one row start together while the other rows are running.
• All units together: starting estimate = Σ(q × S). This assumes the entered starting peaks overlap.

Each margin-adjusted result equals its unadjusted result × (1 + margin ÷ 100). The margin is a user assumption, not a universal generator sizing rule. Watts ÷ 1,000 gives kW.

Worked generator load examples

These examples use hypothetical electrical input values and no margin unless stated.

What size generator should I buy?

Use this result as a watt-based load inventory. It does not select a generator model. A manufacturer’s sizing process also needs apparent power and starting kVA, starting method, allowable voltage and frequency dip, load steps, phase balance, duty rating and site conditions. A generator with a matching advertised watt rating may still fail to start a motor.

Motor shaft horsepower is not electrical input watts. Do not substitute HP × 746 for electrical demand without accounting for efficiency. Running power factor cannot reliably convert a transient motor starting-watt estimate into starting kVA.

Generator load inventory questions

Should I add all starting watts?

Only if the relevant starts overlap. For controlled sequential starts, add the largest applicable starting increment to the running total. The one-unit option assumes every other unit can already be running.

What if I only know motor starting amps?

Do not enter amperes in a watts field. Obtain the manufacturer’s starting data and system voltage/phase information for a generator sizing review. This tool deliberately does not assume an inrush power factor.

Can I estimate fuel consumption here?

No. Fuel consumption depends on the generator model and load-dependent fuel curve. Electrical load alone does not determine liters or gallons per hour.

Does a 20% margin guarantee reliable starting?

No. It is an editable arithmetic reserve. Confirm transient performance and the applicable continuous, prime or standby rating with the supplier.

Unit reference: the U.S. EIA explanation of watts, kilowatts and kilowatt-hours distinguishes instantaneous power from energy accumulated over time. The load inventories and worked scenarios above are this calculator’s own arithmetic examples.