About Watts I Need

What Watts I Need does

There are two questions behind every generator or battery purchase, and they need different arithmetic. The first is "how big does it have to be", which is a question about watts: the steady load your equipment presents, and the momentary spike when a motor starts. The second is "how long will it last", which is a question about energy: watt-hours drawn out of a tank or a battery over a shift, a night or an outage.

Most calculators answer the first question badly and the second one worse. They add every appliance's nameplate rating together, add every start-up surge on top of that, and then multiply the total by the number of hours. All three steps are wrong in the same direction, and the result is a recommendation one or two sizes too large. This site was built because that arithmetic sells people equipment they do not need and, occasionally, tells a vendor a battery will get them through a market day when it will not.

How the math works

Every answer comes from three figures, computed in src/assets/js/power.js and shared between the page you see and the tables printed in the guides.

• Running watts. The sum of every item's running rating, assuming they can all be on at once. That is deliberately conservative: it is the load a generator must hold indefinitely.
• Starting watts. The running total plus the single largest start-up gap in your list, not the sum of all of them. Two motors starting in the same instant is bad luck, not something to design around. The tool names the appliance that set your peak.
• Energy. Each item's average draw multiplied by its hours, where average draw is idle + duty x (running - idle). This is the part other calculators skip, and it is usually the difference between a battery you can carry and one you cannot.

On top of that: a generator is never sized to sit above 80 percent of its continuous rating, which is the same thing as adding 25 percent to your running total. Battery capacity allows 15 percent for inverter conversion losses and keeps a further 15 percent in reserve so you do not plan to finish at zero. Ambient temperature is applied three ways: usable battery capacity follows a curve from 100 percent in mild conditions down to about 85 percent around freezing and 60 percent in severe cold, engines lose about 2 percent of their output per 10 °F, or 5.5 °C, above 77 °F (25 °C), and refrigeration and heating duty cycles scale by 1.5 percent per Fahrenheit degree away from 70 °F in opposite directions. The tool asks for a temperature band rather than an exact figure, because nobody knows the precise temperature they will be working in, and computes each band at its midpoint. Both Fahrenheit and Celsius are shown on every band. Altitude also derates an engine, by roughly 3.5 percent per 1,000 feet above 3,000 feet, but the tool does not ask for it: apply that one yourself if you are at elevation. Fuel burn is modelled as a small idle consumption that scales with engine size plus roughly 0.13 gallons per kilowatt-hour delivered, fitted to published inverter generator run-time figures.

Numbers are rounded only when they are displayed, never during the calculation. Watts round to whole numbers below 10, to the nearest 5 below 100, to the nearest 10 below 1,000 and to the nearest 50 above that. Energy switches from watt-hours to kilowatt-hours at 1,000. Durations show as minutes below an hour, hours and minutes below two days, then days. You should never see a raw decimal on this site; if you do, it is a bug worth reporting.

Where the data comes from

• Household appliance wattages. U.S. Department of Energy guidance on estimating appliance energy use, and FTC EnergyGuide typical values. Reviewed September 2026, refreshed annually.
• Commercial food-service equipment. Manufacturer nameplate ratings for typical single-phase 120 V and 240 V units. Reviewed September 2026.
• Portable power station specifications. Manufacturer-published capacity, continuous output and surge figures for well-known models, used to define size classes. Verified September 2026. Model revisions change these, so check the maker's own page before buying.
• Generator fuel consumption. Fitted to Honda's published EU2200i run times, which give roughly 0.12 gallons per hour at 25 percent load and 0.28 at the rated 1,800 W. Conventional fixed-speed engines are modelled about 15 percent thirstier, which matches published run times for 7 to 9 kW units within about 15 percent.
• Fuel energy content. U.S. Energy Information Administration figures: gasoline about 114,000 BTU per gallon, propane about 91,500, diesel about 138,000. This is why propane buys fewer hours per gallon.
• Duty cycles. Rules of thumb for how much of a running hour each item spends at full draw. These are the least precise inputs on the site and they are labelled as estimates wherever they materially change an answer.

Editorial standards

• Every number is sourced or computed from the same data files the tool uses, so the guides and the calculator cannot disagree. A data correction updates every page at once.
• We say when something is typical rather than exact, and we tell you how to get the exact figure for your own case, which is usually the data plate on the back of the appliance.
• We do not recommend products and there are no affiliate links on this site. Naming a model as an example of a size class is not an endorsement.
• Advertising is separate from content. Ads are served by Google and clearly labeled; advertisers have no input into what we publish and no page is written to suit them.
• Corrections are welcome and dated. Each page shows when it was last updated.

What the tool cannot do

It is a sizing aid, not an electrical design. It will not tell you how to connect a generator to your house: that is a transfer switch or interlock, it is an electrician's job, and back-feeding a generator through an outlet can kill a lineworker. It does not model three-phase supplies, motor power factor, harmonic distortion or voltage drop over long cable runs. It does not know your local code, your permit conditions or your venue's decibel limit. It assumes a healthy battery at its rated capacity, and batteries lose capacity as they age.

It also cannot know how hard you personally work your equipment. The duty-cycle defaults describe a normal service. A festival Saturday is not a normal service, which is what the "how hard it works" control is for. When the answer is close to a size boundary, buy the larger one.

Who runs it

Watts I Need is built and maintained by an independent software engineer as a small, focused utility. It is free to use, runs entirely in your browser, stores nothing you type, and is supported by unobtrusive display advertising. Reach us at hello@wattsineed.com or via the contact page.