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The lights go out during dinner, the fridge hums to a stop, and the house goes quiet while the street stays dark. An emergency generator size calculator turns that moment into a plan: list the loads to keep running, add their running watts, add the starting watts of the biggest motor, and match the total to a generator with enough running and peak watts.
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How an emergency generator size calculator works
A calculator like this asks for the running watts of each load, then adds a starting allowance for motors, and compares the totals with a generator’s running watts and peak watts. Running watts are what a load draws while it runs. Starting watts are the short spike a motor draws when it starts. A generator that covers the running total but not the starting spike stalls or shuts down when the motor kicks in, so the peak figure matters as much as the running figure.
The reference listings on this page show the shape of the answer. Each one lists a running watts figure and a peak watts figure, and the gap between them is the room for motor starts. One listing gives 2,000 running watts and 2,800 peak watts, and another gives 9,500 running watts and 12,500 peak watts on gasoline. The calculator sits between those two sets of figures: the loads decide the totals, and the totals decide the unit.
Step 1: List the loads and their running watts
Walk through the house and write down each load to keep running during an outage: the fridge, the freezer, lights, a phone charger, a fan, a sump pump, a well pump and a furnace fan. Read the running watts off each appliance label or manual, and write the figure next to the load. Add the running watts together for the total the generator must carry while everything runs at once. A plug-in watt meter between the appliance and the outlet measures real use over time, so use one when the label gives no watts figure. See the best watt meters guide for how a meter reading compares with the label.
Keep the list honest. A load that runs only now and then still counts if it runs at the same time as the rest, so add it. A load that stays off during an outage stays off the list. The running total is the floor for the generator: a unit with fewer running watts than the total cannot carry the list, no matter how high its peak watts run.
Step 2: Add starting watts for motors
Motors draw a short spike when they start, and that spike can sit far above the running watts. Fridges, freezers, pumps and fans all have motors, so mark each motor load on the list and note its starting watts from the label or manual. Add the starting watts of the biggest motor to the running total, because the other motors are already running by then and only the starting motor adds its spike. If a label gives no starting figure, the manual or the maker can give it; do not guess it.
A fridge and a freezer that start at the same moment draw two spikes at once, so plan for the worst overlap the house can produce: the biggest motor starting while everything else runs. That overlap is the number the peak watts figure has to cover.
Step 3: Match running and peak watts
Compare the two totals with the generator’s own figures. The running total must stay under the generator’s running watts, and the running total plus the biggest starting spike must stay under the generator’s peak watts. The reference listings show how wide the range runs. The Oxseryn 2800-Watts (on Amazon) unit lists 2,000 running watts and 2,800 peak watts, the PowerSmart lists 3,200 running watts and 3,800 starting watts, the Oxseryn 4400 Watts (on Amazon) unit lists 3,400 running watts and 4,400 peak watts, and the Westinghouse lists 9,500 running watts and 12,500 peak watts on gasoline, with 8,500 running watts and 11,200 peak watts on propane. See the power generator types guide for how inverter and dual fuel units differ.
| Generator | Running watts | Peak watts | Run time |
|---|---|---|---|
| Oxseryn 2800-Watts | 2,000 | 2,800 | Up to 9 hours at 25% load |
| PowerSmart 3800 Watt | 3,200 | 3,800 | Not stated |
| Oxseryn 4400 Watts | 3,400 | 4,400 | Up to 14 hours at 25% load |
| Westinghouse 12500 Watt | 9,500 | 12,500 | Up to 12 hours |
These are the makers’ stated figures. A dual fuel unit lists separate figures for each fuel, so match the fuel to use: the Westinghouse lists its gasoline figures and its propane figures side by side, and the propane figures run lower than the gasoline figures on that unit.
Step 4: Check run time, fuel and outlets
Run time sets how long the generator carries the load on one tank. The Oxseryn 2800-Watts runs for up to 9 hours at 25% load on a 1.1 gallon tank, the Oxseryn 4400 Watts runs for up to 14 hours at 25% load on a 2 gallon tank, and the Westinghouse runs for up to 12 hours . A bigger tank and a lighter load stretch the run time, and a heavier load shortens it. See how long a gas generator runs on one tank for more on tank size and load.
Match the outlets to the loads before buying. The PowerSmart panel lists a 120V 20A duplex outlet, a 120V 30A twist-lock outlet, a 12V 8A DC outlet and dual 5V 2.1A USB ports. The Westinghouse lists two 120V 20A household outlets, a 120V 30A transfer switch outlet and a 120/240V 50A RV outlet. Count the outlets the loads need, and check the manual for the fuel and oil the unit takes. A transfer switch connection to home wiring is a job for a licensed electrician.
Noise is the last figure to weigh. The Oxseryn 2800-Watts lists under 58 dBA from 23 feet away, the PowerSmart lists 59 dBA from 23 feet away, and the Oxseryn 4400 Watts lists under 72 dBA from 23 feet away. The figures come from the makers, measured at the same distance, so they compare directly. A unit that runs through the night sits close to the house, so a lower figure matters more there than on a unit that runs for an hour at midday.
Ways to keep the load small
- Run only the loads that matter during an outage, and leave the rest off until power returns.
- Stagger motor starts: start the fridge, let it settle, then start the next motor load, so the spikes never stack.
- Use the ECO mode the listing states when the load is light. The Oxseryn units and the PowerSmart list an ECO mode.
- Keep the fridge and freezer shut so they cycle less often, and move chilled items to a cooler when the outage runs long.
FAQ: power away from the grid
What size generator runs a fridge and a freezer?
Add the running watts of the fridge and the freezer from their labels, then add the starting watts of the larger motor. Match the running total to the generator’s running watts and the total with the spike to its peak watts. The PowerSmart lists 3,200 running watts and 3,800 starting watts, which shows how the two figures sit side by side on one unit.
What is the difference between running watts and peak watts?
Running watts are what the generator carries without a break. Peak watts are the short burst it can give for a motor start. The Oxseryn 4400 Watts lists 3,400 running watts and 4,400 peak watts, so the gap between the two figures is the room for starting spikes.
How long will a generator run on one tank?
It depends on the tank and the load. The Oxseryn 2800-Watts runs for up to 9 hours at 25% load on a 1.1 gallon tank, the Oxseryn 4400 Watts runs for up to 14 hours at 25% load on a 2 gallon tank, and the Westinghouse runs for up to 12 hours . A lighter load stretches the run time, and a heavier load shortens it.
Can a generator run indoors during an outage?
No. Fuel generators run outdoors only, far from doors and windows, because of carbon monoxide. The PowerSmart listing states it plainly: never use a generator indoors or in partially enclosed areas, and only use it outdoors.
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