How Many Watts Do I Need?
Tell it what you are plugging in and it tells you three things: the watts you need right now, the surge you need for a split second, and how long a given generator or battery will actually last. Built for outages, food trucks, coffee carts, market stalls, RVs and jobsites.
Work out what you need
Nothing added yet.
Pick a setup above, or add your equipment one item at a time. You will get the running watts, the starting surge, and the generator or battery size that covers them.
What this assumes
- Running watts assume everything can be switched on at the same time. That is the safe way to size a generator, and it is why the number looks high.
- Starting watts are the running total plus the single largest start-up surge — not every surge added together, because two motors rarely start in the same instant.
- Energy uses each item's duty cycle: a fridge compressor runs about a third of the time, an espresso boiler idles at a fraction of its element rating between shots.
- Battery sizing allows 15% for inverter losses and keeps 15% in reserve so you do not finish the day at empty. In the cooler bands it also allows for the capacity lithium gives up, which is about 85% of rated around freezing and roughly 60% in severe cold.
- Generator sizing never plans to sit above 80% of the continuous rating, which is the same thing as adding 25% to your running total. In the warmer bands it adds roughly 2% per 10 °F, or per 5.5 °C, above 77 °F (25 °C) for the output an engine loses in hot air.
- Fuel burn is modelled from published inverter-generator run times and is accurate to roughly ±20% in the real world.
How the calculation works
Three numbers decide whether a power source will do the job, and most calculators only give you one of them.
Running watts is the total your equipment draws with everything switched on at the same time. It is the number a generator has to hold for hours without complaining. We add the full running rating of every item, because that is the safe assumption: the moment you plan around "I'd never have them all on together" is the moment someone does.
Starting watts is the brief spike as motors and compressors spin up. A fridge compressor, a well pump, an air conditioner and a blender can all pull two to four times their running rating for well under a second. The rule that matters: you add your total running watts and then one surge — the largest single start-up gap in your list. Adding every surge together is the single most common sizing error, and it routinely doubles the generator someone thinks they need.
Energy is what actually empties a battery or a fuel tank, and it is where nameplate arithmetic falls apart. Very little equipment runs flat out. A refrigerator's compressor runs about a third of an hour. A deep fryer heats hard after a basket drop, then cycles. A two-group espresso machine idles near 300 W holding its boiler at temperature and only pulls its full 2,800 W while recovering. Multiply the nameplate by the hours and you get a figure that can be three times too big — which is exactly why so many people are told they need a 5 kWh battery for a market stall that genuinely needs 1.5.
If something you run is not in the dropdown, add it yourself: the "Something else" option takes a name, the running watts from the appliance's data plate, and optionally its starting and idle watts. It then counts exactly like a catalogue item, surge rule included.
What each setup actually needs
These are computed from the same equipment data the calculator uses, at the hours shown. Load any of them into the tool with one tap on the buttons above.
| Setup | Running | Starting | Energy | Generator | Battery |
|---|---|---|---|---|---|
| Coffee cart 6 hours |
4,200 W | 7,200 W | 5.37 kWh | 7,500 W | 7,300 Wh |
| Food truck 8 hours |
9,300 W | 10,500 W | 30.8 kWh | 15,000 W | 41,600 Wh |
| Home backup 24 hours |
2,100 W | 3,700 W | 16.1 kWh | 5,000 W | 21,700 Wh |
| RV weekend 10 hours |
3,350 W | 5,450 W | 16.6 kWh | 7,500 W | 22,400 Wh |
| Market stall 5 hours |
1,350 W | 2,400 W | 3.62 kWh | 3,500 W inverter | 4,900 Wh |
| Event / DJ 5 hours |
1,450 W | 2,350 W | 5.91 kWh | 3,500 W inverter | 8,000 Wh |
| Jobsite 8 hours |
5,000 W | 8,000 W | 8.04 kWh | 7,500 W | 10,900 Wh |
| Tailgate 4 hours |
3,450 W | 3,650 W | 6.83 kWh | 5,000 W | 9,200 Wh |
Notice how far apart the running and starting columns are for the home and RV setups: that gap is a compressor, and it is the reason a generator that comfortably carries your load can still refuse to start it.
Why your answer will differ from a brochure
Manufacturer run-time claims are quoted at 25% or 50% load with a single, steady appliance. Real loads are lumpy. Four things move the answer, and the calculator lets you change all of them:
- How hard you work the equipment. A quiet Tuesday and a festival Saturday put the same espresso machine at completely different duty fractions. The "how hard it works" column changes this per item.
- Temperature. One setting, three effects. Pick the band you will be working in, in Fahrenheit or Celsius. A lithium battery delivers about 85% of its rated capacity around freezing and roughly 60% in severe cold. An engine loses about 2% of its output per 10 °F, or 5.5 °C, above 77 °F (25 °C), because hot air is thin air. And refrigeration cycles much harder on a hot day while heating does the opposite, so a fridge in a 95 °F (35 °C) truck is a different appliance from the same fridge in a cool room.
- Soft starters. Fitting one to an air conditioner cuts the start-up surge by roughly half without changing running watts — often the difference between needing a 3,500 W generator and a 2,200 W one.
Start with your situation
Generator size for a house
Work out the running watts, the starting surge and the generator size for your home's essential circuits.
Food truck and coffee cart
Fryers, griddles, espresso machines and refrigeration, sized properly with idle draw and duty cycles included.
RV and camping
Roof air, converter, water pump and microwave, sized against 30 amp service and the surge that actually stops you.
Battery runtime
Enter your battery size and your equipment to see real runtime, after inverter losses and appliance cycling.
Guides
Starting Watts vs Running Watts
Starting watts vs running watts, settled: add every running watt, then add one surge, the largest single start-up gap. Never add all the surges up.
How Many Watts Does a Refrigerator Use?
How many watts does a refrigerator use? About 150 running watts, a 750 W start-up surge and an average near 55 W, which is 1.31 kWh a day.
How Long Will a Generator Run on a Tank?
How long will a generator run on a tank of gas? It depends on load, not tank size. See the fuel burn per hour and run time for every size.
What Size Generator for RV AC?
What size generator to run RV AC? A 13,500 BTU roof unit needs 1,400 running watts but surges to 3,500 W. A soft start kit changes the answer.
What an Espresso Machine Really Draws
How many watts an espresso machine uses in real service, what size generator a coffee cart needs, and the battery math for a six hour market shift.
Generator or Battery Power Station?
Generator vs power station: compare cost per watt, cost per stored kWh, refuel time and where you are allowed to run each one.
Appliance Wattage Chart
An appliance wattage chart for home, food truck, RV and jobsite gear, with running watts, starting surge, average draw, watt-hours and amps.
Frequently asked questions
How many watts do I need to run my house?
Less than most people expect. A refrigerator, a chest freezer, a gas furnace blower, a sump pump, lights, internet and a TV come to about 2,100 running watts, with a starting spike near 3,700 W when the sump pump kicks in. That is a 5,000 W generator. Add a well pump and you are at 5,000 running watts; add central air conditioning and you are past 9,500. The tool works it out from your actual list, because the single biggest motor sets the starting figure.
Do I add up the starting watts of everything?
No, and this is the mistake that sells people generators twice the size they need. Every motor surges for a fraction of a second at start-up, but two motors almost never start in the same instant. The accepted method is to add all the running watts together, then add only the single largest start-up surge on top. This calculator does exactly that and tells you which appliance set the peak.
Why is the energy figure lower than watts times hours?
Because almost nothing runs flat out. A refrigerator's compressor is on roughly a third of the time, a fryer's element cycles once the oil is hot, and an espresso boiler idles at about a tenth of its rated draw between shots. Multiplying nameplate watts by hours gives a number that can be three times too high, which is how people end up buying far more battery than they need.
Generator or battery power station — which should I buy?
Generators are cheaper per watt and refuel in seconds, so they win for cooking equipment, long outages and anything drawing kilowatts. Batteries win where noise, fumes or indoor use rule an engine out: indoor events, permitted markets, hospital-adjacent work, overnight medical equipment and RV parks with quiet hours. Above roughly 3 kWh of stored energy a battery costs several times a generator of the same output.
How long will my generator run on a tank?
Switch to the "How long will it run?" tab and pick your generator and load. As a rule, a portable generator burns about 0.13 gallons of gasoline per kilowatt-hour it delivers, plus a small idle burn. A 2,200 W inverter unit carrying 550 W runs about eight hours on its single gallon; a 7,500 W unit at half load empties a 6.6-gallon tank in roughly nine.
Does my coffee machine really need 1,800 watts all day?
No. A commercial espresso machine pulls its full element load only while the boiler recovers after a shot or a steam cycle, and idles near 180 W the rest of the time. Over a six-hour market shift that is the difference between 10.8 kWh on paper and roughly 3 kWh in reality. The calculator models idle draw, active draw and how hard you work it separately.
My appliance is not in the list. Can I enter my own watts?
Yes. Choose "Something else (enter my own watts)" at the top of the equipment dropdown and give it a name and its running watts. Starting watts and idle watts are optional, and so is how much of the time it actually works, which is what separates a steady 2,000 W load from one that cycles. Read the figures off the data plate on the appliance. Your item is then treated exactly like a catalogue entry, and it travels with the shareable link.
Is this calculator free, and does it store my equipment list?
It is free and runs entirely in your browser — nothing you enter is sent to a server or stored anywhere. The "Copy a link to this setup" button encodes your list in the web address itself, so you can bookmark it or send it to a colleague and they will see the same result.