A generator load list records what you need to power, each item’s running demand, its starting requirements, and which loads may operate together. Build it around the equipment you intend to back up—not the sum of your circuit-breaker ratings. The useful result is a set of operating and starting scenarios for preliminary generator sizing, rather than one total that assumes every appliance behaves the same way. Cummins identifies both load characteristics and connection sequence as important sizing inputs.
Build an Inventory of the Loads You Intend to Back Up
List each appliance or equipment assembly once. Include supporting equipment needed to deliver the service: gas heating, for example, may still require electrical power for its blower, controls, and pumps.
Use the following fields as a working load-list template:
| Field | What to record |
|---|---|
| Equipment and quantity | Description, model, number of units, and circuit or location |
| Electrical supply | Rated voltage, frequency, and single-phase or three-phase supply |
| Running input | Electrical input watts or kW; also record amperes, kVA, and power factor when available |
| Starting requirement | Manufacturer-specified starting current, starting kVA, or starting watts, with the source and definition |
| Starting method | Across-the-line, soft starter, VFD, or another documented method |
| Operating behavior | Continuous operation, automatic cycling, intermittent use, and relevant operating modes |
| Priority | Must-run, deferrable, or excluded from backup |
| Control arrangement | Manual scheduling, automatic start delay, interlock, or load-shedding control |
| Evidence | Nameplate photo, manual reference, manufacturer data, or measurement conditions |
Manufacturer documentation should take precedence over generic appliance wattage charts. Keep missing values marked unknown rather than entering zero. Generac’s home-sizing guidance likewise recommends identifying essential equipment and checking nameplates or manufacturer specifications for running and starting requirements.
Use Electrical Input Data, Not Breaker Size
A breaker rating is not a measurement of equipment demand. Likewise, a motor’s horsepower rating describes mechanical output, not its complete electrical input requirement.
Keep these distinctions visible in the list:
- Watts and kW describe real electrical power.
- kVA describes apparent power; it is not interchangeable with kW unless the applicable power factor is known.
- kWh describes energy used over time, not instantaneous load.
- Power factor and motor efficiency describe different relationships; neither should be substituted for the other.
Cummins defines power factor as the ratio of kW to kVA and treats motor characteristics separately when assessing generator loading. Do not assign one assumed power factor to every item simply to complete the spreadsheet.
Avoid double-counting a packaged system and its components. For example, determine whether an HVAC unit’s documented input already includes its fans before adding those fans as separate loads.
Record Motor Starting Requirements Separately
Pumps, compressors, refrigeration equipment, and other motor-driven loads can impose a starting demand substantially different from their running demand. Record the starting method and the available manufacturer data rather than applying a universal multiplier.
A soft starter or VFD changes the starting behavior, but its effect depends on the equipment, settings, and driven load. Reduced starting current alone does not establish successful acceleration or generator compatibility. Cummins discusses reduced-voltage starting, available accelerating torque, and nonlinear drive loads as separate application considerations.
Evaluate Starting Events, Not a Sum of Every Surge
Do not add every motor’s starting requirement together unless those motors can actually start together. Conversely, do not assume that only the largest motor matters in every operating condition.
Check the events your system can produce:
- Initial pickup after transfer to generator power.
- A cycling motor restarting while other loads remain connected.
- Multiple automatic loads restarting together after an outage.
- A scheduled load step after earlier loads have stabilized.
Starting the largest motor first can reduce the initial load step in some arrangements. It does not eliminate the need to check that motor restarting later against the existing running load. Cummins explicitly distinguishes initial sequencing from the later restart of cycling motors.
Assign Priorities and Make Them Enforceable
For optional backup loads, use three practical tiers:
| Tier | Meaning | Planning treatment |
|---|---|---|
| Must-run | Loads you intend to keep available during the outage | Include them in the relevant operating and restart scenarios |
| Deferrable | Useful loads that can wait or operate on a schedule | Document when they may run and what must be off first |
| Shed or excluded | Loads disconnected first, or not supplied during backup operation | Identify how they will be disconnected or prevented from operating |
Priority depends on the site. A well pump may be essential where it supplies the only water source; cooling may become a higher priority during extreme heat.
A priority label does not control equipment. An automatic transfer switch does not, by its name alone, establish that individual loads will be shed. Verify the actual switch, controller, compatible load-management equipment, and configured behavior. Generac’s described load-management arrangement uses separately installed equipment to interrupt lower-priority loads.
These planning tiers do not replace legally required emergency-system classifications or permitted load-shedding arrangements. For required systems, the designer must establish the applicable local requirements and equipment obligations.
Build Realistic Operating Scenarios
Create separate scenarios where operating conditions differ materially—for example, winter heating, summer cooling, overnight essentials, and temporary operation of a high-demand appliance.
For each scenario:
- Identify the equipment that may run simultaneously.
- Sum its running electrical input.
- Identify the starting events possible against that running background.
- Record any scheduling or automatic controls needed to keep the scenario valid.
Do not reduce the running total merely because an appliance cycles. Duty cycle can help estimate energy use, but a cycling appliance can still draw its full operating input while on. Several thermostatic loads may also call for power together after an interruption; Cummins notes that cyclic loads can materially affect generator sizing.
Example: Avoid Counting a Motor Twice
Assume a hypothetical operating scenario with these electrical inputs:
| Load | Running input |
|---|---|
| Lighting and electronics | 300 W |
| Refrigerator | 200 W |
| Pump | 1,000 W |
| Total running load | 1,500 W |
Assume the pump’s documented total starting input is 3,000 W—not an additional 3,000 W—and the refrigerator is already running without a simultaneous restart.
For this event:
[
P_{\text{event}}=P_{\text{running total}}-P_{\text{pump running}}+P_{\text{pump starting}}
]
Here, each (P) is electrical input power in watts:
[
P_{\text{event}}=1{,}500-1{,}000+3{,}000=3{,}500\text{ W}
]
The pump contributes an additional 2,000 W above its running input. Adding the full 3,000 W to the running total would count its 1,000 W running contribution twice.
These invented values demonstrate bookkeeping only. A 3.5 kW event estimate does not prove that a particular generator can start the pump: starting kVA, acceleration time, voltage dip, frequency response, and the behavior of connected equipment still require assessment. Cummins identifies these transient characteristics as important to generator selection.
Use the Completed List for Preliminary Sizing
The completed load list should preserve three distinct outputs:
- The running demand for each operating scenario.
- The starting events and load steps that require evaluation.
- The controls and assumptions that make those scenarios achievable.
When comparing a candidate generator, check its specified voltage, phase, frequency, applicable duty rating, and available output for the chosen fuel and site conditions. Also check per-leg or per-phase loading rather than relying only on total kW. Generator ratings, load imbalance, altitude, temperature, and transient performance can all affect the result.
Give the load list and supporting equipment data to the installer or generator supplier for model-specific assessment. A load inventory supports preliminary planning; it is not a substitute for a required dwelling service-load calculation, installation design, or local compliance review. Revisit it when equipment, seasonal priorities, or load-management settings change.