A generator’s advertised peak rating is not a promise that it can start every motor or power every circuit together.
1. Make a deliberately limited load list
List the functions that matter during an outage: water, refrigeration, heating controls, lights and communications. Separate optional heating, cooking and other high-power loads. Record continuous power from documentation or measurements and obtain starting requirements for motors.
A staged operating plan may reduce the peak requirement. For example, postponing a discretionary appliance while the well pump starts changes the simultaneous demand. This is a planning decision, not a reason to ignore protection or equipment ratings.
2. Keep running and starting ratings separate
If a manufacturer gives total starting watts for one motor, add the other already-running loads to that total. Do not add the same motor’s running watts a second time. If the documentation instead lists “additional starting watts”, the arithmetic is different; convert it to a total before using our tool.
The calculator handles a single motor start with known total starting watts. It does not model several motors starting together, voltage dip, power factor, ambient derating or a generator’s transient response. Ask the supplier to confirm compatibility with your actual load list.
3. Choose a safe operating arrangement
Portable generators must operate outdoors, away from openings and occupied spaces, with working CO alarms in the house. A garage, shed or basement is not an acceptable operating location. Follow the manufacturer’s operating and weather-protection instructions.
For a connection to house wiring, use an arrangement specified and installed by a qualified electrician. Never feed power into a household socket. The CPSC hazard guide explains the shock and backfeeding risks, as well as safe refuelling precautions.
CPSC · Portable generator hazards ↗4. Make the outage routine usable
Keep a short procedure with the equipment: where it operates, what it supplies, who can start it and how transfer back to the normal supply works. Use the installer’s and manufacturer’s instructions rather than improvising a sequence during a storm.
Record service intervals and fuel arrangements. A successful test with a lamp does not show that the well pump will start. Have the intended load combination verified through an appropriate commissioning test and record the limitations.
Questions for the equipment supplier
| Question | Why it matters | Evidence to request |
|---|---|---|
| Can it supply the running load continuously? | Continuous and advertised peak ratings differ | Rated output under your conditions |
| Can it start this motor with other loads on? | Transient response is separate | Manufacturer compatibility confirmation |
| What happens at altitude or high temperature? | Available output may be reduced | Applicable derating data |
| How is the house isolated from the grid? | Prevents dangerous backfeeding | Electrician’s approved connection design |
| How is fuel handled and stored? | Runtime and safe operation | Operating and maintenance instructions |
Worked example
One motor starting while essentials remain on
Other running loads total 700 W. A motor uses 600 W running and has a documented total starting demand of 1,800 W.
Running combination = 700 + 600 = 1,300 W Starting combination = 700 + 1,800 = 2,500 W
These are load requirements, not a recommended generator size. Confirm duration, voltage, power factor and actual motor-starting capability with the manufacturer.
What to have ready
- Essential-load list
- Total starting requirement for each motor
- Approved electrical connection
- Safe outdoor operating location
- Service and fuel plan
Common mistakes
- Adding running watts twice to a total starting rating.
- Choosing equipment only by its peak advertising number.
- Running a portable generator in a garage.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.