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Four outage plans, calculated
All for 24 hours with a 20% battery reserve. “Essentials” is a refrigerator, Wi-Fi router, five LED bulbs and four phone chargers. Fuel is gasoline per day at a light generator load.
| Plan | Running | Biggest surge | Energy per day | Battery | Generator | Gasoline |
|---|---|---|---|---|---|---|
| Essentials only | 255 W | 705 W | 1,930 Wh | 2,838 Wh | 1,000 W | 0.69 gal |
| Winter: plus gas furnace blower | 855 W | 2,055 W | 7,930 Wh | 11,662 Wh | 2,000 W | 2.83 gal |
| Well and sump pump home | 1,955 W | 3,955 W | 4,130 Wh | 6,074 Wh | 3,500 W | 1.47 gal |
| Summer heat: plus window AC | 855 W | 2,055 W | 6,730 Wh | 9,897 Wh | 2,000 W | 2.40 gal |
- The winter plan needs far more energy than the pumps do, because a furnace blower runs for hours while a pump runs for minutes.
- The pump plan uses little energy but needs the biggest generator, because pump start-up surges set the size.
How the plan is calculated
- Energy per day (Wh) = watts × hours per day for every ticked appliance. Energy for the outage = daily energy × outage hours ÷ 24.
- Battery (Wh) = outage energy ÷ 0.85 for inverter losses ÷ (1 − reserve).
- Inverter (W) = the larger of running load × 1.25 and the biggest surge moment ÷ the surge rating, rounded up to a standard size. The surge moment is the running load plus the extra start-up power of the biggest motor.
- Generator running watts = the larger of running load × 1.2 and the surge moment ÷ 1.25, because portable generators typically deliver about 25% more for starting than for running.
- Fuel = energy in kWh ÷ (energy per gallon × generator efficiency). A gallon holds about 35 kWh for gasoline, 40 for diesel and 26.7 for propane. Efficiency is 8%, 12% or 16% by how hard the generator works, since small loads waste a lot.
- Solar to recharge = daily energy ÷ 0.85 ÷ (peak sun hours × 0.75).
It plans for essentials, not a whole house. Real fuel use varies widely by generator and load, so check your model’s fuel chart.
Battery backup or generator?
- Battery backup is quiet, safe indoors and starts instantly, but its capacity is limited and recharging takes sun or grid power. It suits short outages and small essential loads.
- A generator keeps going as long as you have fuel and handles big loads such as pumps. It is loud, needs fuel storage, and must run outdoors.
- A battery plus solar can run a small essential load indefinitely. The “solar to recharge” result shows the panel size for that.
Using it anywhere in the world
- Fuel is shown in gallons and liters, and energy in watt-hours.
- Watts are the same on 120 V and 230 V systems, but check your appliance labels, because heaters and pumps may be rated higher on 230 V.
- Choose generator and inverter output voltage and frequency to match your appliances.
Common planning mistakes
- Sizing for running watts only. Pumps and compressors need a surge several times their running power.
- Forgetting the heating or cooling. A furnace blower or window air conditioner can use more energy than everything else combined.
- Assuming a battery stays full. Keep a reserve and plan to recharge.
- Underestimating fuel. Store enough for the longest outage you want to cover, and store it safely.
- Connecting a generator unsafely. Use a properly installed transfer switch or interlock, fitted by a licensed electrician.
Common questions
How big a battery do I need to keep a refrigerator running in an outage?
A modern refrigerator averages about 150 W while its compressor runs, around 8 hours of every 24, which is about 1.2 kWh a day. With inverter losses and a 20% reserve that takes about 1.8 kWh of battery for each day. Its start-up surge is roughly 600 W, so choose an inverter of at least 500 W.
What size generator do I need for essentials?
For a refrigerator, router, lights and phone chargers, about 1,000 W of running watts is enough. Adding a gas furnace blower raises that to about 2,000 W, and adding a well pump and a sump pump raises it to about 3,500 W, because pump start-up surges set the size.
How much fuel does a generator use per day?
For the essentials plan, the calculator estimates about 0.69 gallons of gasoline a day at light load, and about 2.83 gallons a day with a furnace blower. Real consumption varies widely by generator model and load, so use your model’s fuel chart for a better number.
Should I buy a battery backup or a generator?
For short outages and small loads, battery backup is quieter, safe indoors, and needs no fuel. For multi-day outages or big loads such as pumps and heating, a generator is usually more practical. A battery with solar panels can also run a small essential load for many days, and the calculator shows the panel size you would need.
Can I run my gas furnace in a power outage?
A gas furnace still needs electricity for its blower and ignition, typically 300–800 W with a start-up surge. A battery or generator can run it only if it is connected correctly, usually through a transfer switch or dedicated inlet installed by a licensed electrician. Never run a portable generator inside the house or garage.
Can I power my whole house in an outage?
Only with a much larger system. Central air conditioning, electric ranges and electric water heaters each use thousands of watts. Whole-house backup needs a load calculation and a transfer switch installed by a licensed electrician. Planning for essentials is far cheaper and usually enough for a few days.
Related calculators
Choose an inverter with the inverter size calculator, check battery life with the battery runtime calculator or the power station runtime calculator, and see how long panels take to recharge with the solar charge time calculator.
For fuel and runtime, see the generator runtime calculator, and to see what your appliances cost every month, the appliance energy cost calculator.
For typical watts and surge for common appliances, see the appliance wattage chart.