Generator Watts Calculator

Use-case preset:

Start from the closest inventory, then edit the rows below.

Load inventory:

One row per load: name,count,running watts,starting watts,priority,voltage.

Priorities: critical, essential, comfort, optional, or shed.

Startup overlap:

Match the start sequence you can actually operate.

Continuous load target:

Reserve operating headroom for uncertain and future load.

Startup reserve:

Use more reserve when motor ratings or extension-cord voltage drop are uncertain.

Site derating:

Manual allowance; do not guess manufacturer derating rules.

Current estimate voltage:

Match the circuit voltage for the ancillary current estimate.

Current power factor:

Ancillary conversion only; it does not alter the watt rating recommendation.

Display precision:

Visible decimal places only; rating-class rounding remains unchanged.

Metric	Value	Context	Copy
{{ row.metric }}	{{ row.value }}	{{ row.note }}

Load	Count	Running	Starting	Extra surge	Priority	Voltage	Copy
{{ row.name }}	{{ row.count }}	{{ row.running }}	{{ row.starting }}	{{ row.extra }}	{{ row.priority }}	{{ row.voltage }}

Scenario	Watts	Action	Copy
{{ row.scenario }}	{{ row.watts }}	{{ row.action }}

A generator must carry the loads that run together and survive the brief surge when a motor, compressor, pump, or tool starts. Those are separate requirements. A unit can have enough continuous watts for steady operation yet stall, trip, or drop voltage when the largest starting load turns on.

Good sizing begins with an inventory from equipment nameplates or current manufacturer data. Running watts describe steady demand. Starting watts describe the momentary demand for one item, including its running portion. The difference between the two is the extra startup surge added to the simultaneous running total.

Generator sizing quantities and decisions
Quantity	Question it answers	Common mistake
Running watts	Can the generator carry the steady simultaneous load?	Leaving no headroom for uncertainty or future loads.
Starting watts	Can it support the chosen startup overlap?	Adding a full starting rating on top of running load instead of only the extra surge.
Derating	How much rated output remains at the site?	Guessing a reduction instead of using manufacturer guidance for altitude, temperature, or fuel.
Voltage and connection	Can the generator supply the required circuits and equipment?	Assuming a watt total proves outlet, phase, transfer, conductor, grounding, or bonding compatibility.

A watt estimate does not design an electrical installation. It does not size conductors, breakers, inlets, transfer equipment, fuel supply, grounding, bonding, or neutral arrangements, and it cannot predict exact inrush or voltage drop. Portable generators also introduce carbon-monoxide, fire, shock, and backfeed hazards. Manufacturer instructions, applicable electrical rules, and qualified installation advice remain essential.

How to Use This Tool:

Replace every preset value with ratings for the exact equipment that may run.

Choose a Use-case preset, then edit the Load inventory. Each row needs name, count, running watts per item, starting watts per item, priority, and voltage.
Select Startup overlap to match the operating plan. Choose one largest item at a time, a complete row starting together, or all listed surges overlapping.
Set the Continuous load target and Startup reserve. Enter Site derating only from generator manufacturer guidance for the actual altitude, temperature, and fuel.
Compare Recommended running watts with a candidate generator's continuous rating and Recommended starting watts with its surge rating. Then verify voltage, receptacles, transfer equipment, grounding, fuel, ventilation, and installation requirements separately.

Interpreting Results:

Recommended running watts is the upward-rounded class needed to keep the simultaneous load at or below the selected continuous target after derating. Recommended starting watts is the greater of that running class and the reserve-adjusted startup requirement.

Positive running and starting reserve show capacity above the modeled demand, not a safety certification. Verify that the candidate generator publishes both required ratings under the expected conditions. If the estimate depends on staggering starts, the operator must follow that sequence; otherwise use a more conservative overlap mode.

Technical Details:

The model sums every selected running load, then adds only the startup increment required by the chosen overlap rule. Continuous headroom, startup reserve, site derating, and rating-class rounding are applied afterward.

Formula Core:

The primary sizing equations use watts throughout.

\begin{matrix} R & = & \sum_{j} n_{j} r_{j} \\ x_{j} & = & n_{j} (s_{j} - r_{j}) \\ S & = & R + X \\ W_{run} & = & \frac{R}{l (1 - d)} \\ W_{start} & = & \frac{S (1 + m)}{1 - d} \end{matrix}

n_j is item count, r_j and s_j are per-item running and starting watts, R is total running demand, X is selected extra startup surge, S is momentary startup demand, l is the continuous target as a decimal, m is startup reserve, and d is site derating.

Voltage and power factor affect only the ancillary apparent-power and current estimates.

\begin{matrix} kVA & = & \frac{R}{1000 PF} \\ I & = & \frac{R}{V PF} \end{matrix}

Rule Core:

Generator startup overlap rules
Startup overlap	Extra surge `X`	Use when
One largest item	Largest per-item value of starting watts minus running watts	Only one individual motor or appliance starts at a time.
Largest complete row	Largest count × per-item startup increment	Every item represented by one row may start together.
All listed surges	Sum of every row's startup increment	Startup overlap cannot be controlled or is intentionally modeled conservatively.

Each row's starting watts must be at least its running watts.
Raw running and starting requirements below 3,000 W round upward to the next 100 W. Values from 3,000 W upward round to the next 500 W.
Recommended starting watts can never be lower than recommended running watts.
Continuous target accepts 50% to 100%, startup reserve 0% to 50%, and site derating 0% to 40%.

For the home-essentials example, six loads total 4,000 running watts. The refrigerator contributes the largest individual extra surge at 1,500 W, giving 5,500 W startup demand. An 80% continuous target and 10% startup reserve produce a 5,000 W running recommendation and a 6,500 W starting recommendation after class rounding.

Safety and Accuracy Notes:

Never operate a portable generator indoors, in a garage, or near doors, windows, or vents where exhaust can enter occupied space.
Never connect a generator directly to a building electrical system unless a qualified electrician has installed appropriate transfer equipment.
Keep the generator dry, follow manufacturer grounding and bonding instructions, and use intact cords and outlets rated for the load and environment.
Replace every preset rating with current equipment data. Confirm exact inrush, voltage, phase, waveform sensitivity, fuel, altitude, and temperature requirements with the manufacturers.

Worked Examples:

Derated pump and lighting load

Two 100 W lights and one pump rated at 500 running watts and 1,500 starting watts total 700 W running and 1,700 W at startup. With all surges included, a 70% continuous target, 20% startup reserve, and 10% site derating, the raw needs are about 1,111 W and 2,267 W. Rating-class rounding produces recommendations of 1,200 W running and 2,300 W starting.

References:

Using Portable Generators Safely, Occupational Safety and Health Administration.
Generators and Engine-Driven Tools, U.S. Consumer Product Safety Commission.
What Wattage Generator Do I Need?, Honda Power Equipment.