⚙️ Generator Sizing Calculator
Add the appliances you want to run on backup power, with their running watts and starting watts, to calculate the minimum generator size needed.
Running watts add up, surges do not
Every appliance on the list contributes its running load, but only one motor is assumed to be starting at any instant, so the biggest surge is added once:
total running = sum of all running watts
size needed = total running + largest starting surgeThe four defaults - fridge 150 W, sump pump 800 W, lights 300 W, furnace fan 500 W - total 1,750 W running. The sump pump's 1,300 W surge is the largest, so the minimum generator is 3,050 W. The starting column is the extra draw on top of running watts, not the total, which is why lights and other resistive loads are entered as 0.
Reading it against a generator's label
- Generators are advertised on peak watts and run on rated watts. Compare 1,750 W with the rated figure and 3,050 W with the peak one.
- Leave headroom. Running a portable flat out for days is how they die; 80% of the rated output is a sensible ceiling.
- Engines lose roughly 3% of their output per 1,000 feet of altitude, so mountain homes should size up.
Frequently asked questions
What size generator do I need to run a refrigerator?
A fridge draws about 150 W running and roughly 600 W extra for the moment the compressor starts, so it needs around 750 W to itself. A 2,000 W inverter unit covers a fridge, lights and phone charging comfortably.
What are starting watts and why are they so much higher?
An induction motor pulls two to three times its running current for a fraction of a second while it gets moving. Pumps, compressors and air conditioners all do it; heaters, kettles and LED lights do not.
How much does it cost to run a generator?
A 5,000 W petrol unit at half load burns roughly 0.75 gallons an hour. At $3.50 a gallon that is $2.63 an hour for about 2.5 kWh, near $1 a kWh - some seven times a 15 cent grid rate, which is why generators are for outages rather than everyday use.