Generator sizing starts with the loads you need during an outage. It does not start with the largest unit that fits the budget.
A qualified installer should complete the electrical and fuel calculations. Your job as the owner is to make the operating plan clear.
Decide what must keep running
Make three lists:
- Essential: equipment that must operate, such as selected heating equipment, refrigeration, a well pump, sump pump, medical equipment, or critical business systems.
- Important: loads you want when capacity allows, such as lighting, selected receptacles, internet, or part of the kitchen.
- Optional: high-demand equipment you can leave off or manage during an outage.
Do not assume “whole house” means every load can start and run at once. Ask the installer to define the operating limits.
Account for starting demand
Motors and compressors can draw more power during startup than during steady operation. Pumps, air conditioners, refrigeration, and similar loads need to be evaluated around both starting and running demand.
Load-management equipment may allow selected high-demand loads to operate without sizing the generator for every possible combination. Get a written explanation of which loads are managed and what the owner will notice in practice.
Match the electrical and fuel plan
Generator output is only one part of the system. The installer also needs to evaluate:
- existing electrical service and panel arrangement;
- transfer equipment;
- selected circuits or whole-service connection;
- gas meter, regulator, piping, propane storage, or other fuel supply;
- generator location and clearances;
- sound and exhaust considerations;
- permits and inspections; and
- manufacturer installation requirements.
A fuel system that cannot support the required input will not be rescued by a larger nameplate.
Ask for the load calculation
The proposal should identify the assumed loads and the resulting generator selection. Review it before ordering equipment.
Use these questions:
- Which loads are included?
- Which loads are excluded or managed?
- What starting demand was considered?
- What happens if two large loads call at once?
- Is future equipment included?
- What fuel assumptions were used?
- What output is available under the expected fuel and site conditions?
Keep that document with the equipment records. It will matter when loads change later.
Leave room for the real priorities
Oversizing can add equipment, fuel, installation, and maintenance cost. Undersizing can cause nuisance shutdowns or force difficult choices during an outage. The target is enough capacity for a defined plan, with reasonable margin and controls.
If a salesperson cannot show the loads behind the recommendation, the sizing is not finished.