How to size a generator
Add the running watts of the loads that will operate together, identify the largest additional motor-starting demand, and include a deliberate margin without treating the result as an electrical design.
4,200 running watts
plus 1,800 startup watts
The generator-sizing calculation
Running or rated watts support loads continuously. Starting or surge watts are the additional short-duration power some motor-driven equipment needs at startup. Compare both requirements with the generator’s own continuous and surge ratings.
Planning scenarios
| Total running load | Largest added startup load | Margin | Suggested running | Suggested peak |
|---|---|---|---|---|
| 3,000 W | 1,500 W | 20% | 3,600 W | 5,400 W |
| 4,200 W | 1,800 W | 20% | 5,100 W | 7,200 W |
| 6,000 W | 3,000 W | 20% | 7,200 W | 10,800 W |
These rows demonstrate the formula only. Use equipment nameplates, manuals, or qualified measurements for actual loads.
Build the load list correctly
List only equipment you intend to operate at the same time. Record voltage, running watts, and any additional starting watts from the nameplate or manufacturer. If multiple motors may start together, a simple “largest startup load” approach may understate demand. Electronic loads, pumps, heating equipment, well systems, and central air conditioning can also have power-quality or starting characteristics that require professional review.
Watts alone do not determine whether a generator can serve a building. Voltage, phase, frequency, grounding, transfer equipment, overcurrent protection, fuel supply, placement, and installation rules also matter.
Portable generator safety
The U.S. Consumer Product Safety Commission says portable generators must be used outside only, at least 20 feet from the home, with exhaust directed away from the home and other occupied buildings. Never operate one inside a house, garage, basement, crawlspace, shed, or porch. Keep working carbon-monoxide alarms on every level and outside sleeping areas.
Never connect a portable generator to house wiring by plugging it into a wall outlet. This dangerous practice, called backfeeding, can electrocute utility workers and neighbors and bypass household protection. A building connection needs properly installed transfer equipment and qualified electrical work.
Before choosing a generator
- Prioritize loads. Separate essential equipment from optional comfort loads.
- Read every nameplate. Record running and starting requirements.
- Check simultaneous operation. Identify which motors may start together.
- Compare both generator ratings. Continuous output and surge output are not interchangeable.
- Plan installation and fuel. Use qualified professionals for building connections and permanent systems.
- Plan safe placement. Maintain required outdoor separation and working CO alarms.
Frequently asked questions
Do I add every appliance’s starting watts?
Not necessarily. A common first-pass method adds the largest additional startup load to total running watts, but simultaneous motor starts may require a more detailed assessment.
Is advertised peak wattage the same as continuous output?
No. Check both ratings. A unit may provide its peak output only briefly while its continuous rating is lower.
Can the calculator size a standby generator installation?
It can organize a preliminary load estimate. Permanent standby systems and transfer equipment require site-specific design, product selection, permits, and qualified installation.