Generator Wattage Calculator
Generators are sold by a number — watts — and bought wrong by ignoring the second number every motor hides: the starting surge. A fridge that runs at 150 W may demand 1,200 W for the seconds its compressor starts, and a generator sized to the running total alone will stall or trip the moment that compressor kicks. This calculator does the sizing the way electricians do: it totals the running watts of the appliances you enter (five common ones are prefilled with typical figures — edit any of them to your nameplates), adds the single largest surge, and recommends a generator rating with headroom to spare, stated in the same watts the machines are sold by.
The Two Numbers Every Appliance Carries
Running watts are the steady draw once an appliance is working: the figure on the nameplate, the number that decides fuel consumption and continuous load. Starting watts are the brief inrush — typically two to eight times the running figure for anything with an induction motor (compressors, pumps, fans, power tools) as the motor drags itself up to speed against a stationary load. Resistive and electronic loads (lights, heaters, TVs, microwaves) have little or no surge, which is why their two fields match in the form. Generator marketing quotes both numbers too — a "4,000 W generator" is usually 4,000 surge and about 3,200–3,500 continuous — and the sizing rule of this page compares like with like: your starting peak against its surge rating, your running total against its continuous rating, and the recommendation's 25% headroom keeps the continuous load near the 50–80% band where engines are efficient, quiet and long-lived.
Worked Example: The Storm Kit
The defaults are a sensible outage kit: fridge (150/1,200), a house's worth of LED lights (300/300), TV and router class electronics (150/150), the microwave (1,200/1,200) and a half-horsepower sump pump (800/1,800). Running total: 150 + 300 + 150 + 1,200 + 800 = 2,600 W. Surges: the fridge wants 1,050 extra watts to start; the pump wants 1,000; the largest single surge is the fridge's. Starting peak = 2,600 + 1,050 = 3,650 W. With 25% headroom: ≈4,563 W — in market language, a 4,500–5,000 W class machine (whose continuous rating, around 3,800–4,200 W, still clears the 2,600 W running load comfortably). Note what changes the answer most: deleting the microwave (a luxury load) drops the running total by 1,200 W and a whole generator class with it. Sizing is triage arithmetic — the essentials list (fridge, pump, heat's fan, lights, communications) first, the comforts second, and the calculator is where the two lists are priced against each other.
The Loads That Eat Generators
Three categories deserve their reputations. Well pumps and central air conditioners are the heavyweight motors of the house: a 1 HP well pump can surge past 4,000 W and a central AC's compressor inrush can exceed 10,000 W momentarily (soft-start kits exist precisely to tame it) — either one, added honestly to the form, can double the recommended size, and both are why whole-house standby units start around 10–14 kW rather than at portable sizes. Electric resistance heat — space heaters at 1,500 W each, electric water heaters near 4,500 W — has no surge but bottomless appetite; most portable-generator plans heat with the furnace's fan (a few hundred watts, the gas does the heating) instead of with resistance elements. Power tools on a worksite budget surge like pumps: a table saw or compressor is a starting-watts problem wearing a running-watts disguise. In every case the method is unchanged: nameplate running, honest surge, one motor starting at a time, headroom over the sum.
Fuel, Runtime and the Other Half of Sizing
Watts decide the machine; the tank decides the night. Runtime at a stated load (manufacturers quote hours at 50% load) and fuel type are the second sizing conversation: gasoline is available and storage-limited (it ages in months), propane stores forever and runs slightly derated, dual-fuel splits the difference, and diesel owns the long-runtime commercial end. A pragmatic chain of reasoning: size the watts with this page; then ask how many hours between refuellings your scenario demands (an overnight storm kit at 50% load on a mid-size tank typically runs 8–12 hours); then let the fuel answer follow from what you can store safely and legally. The exercise takes ten minutes in peacetime and repays itself the first night the street goes dark and the fridge, alone among the neighbourhood's, is still humming at the wattage this page predicted. And the safety line that belongs on every generator page: generators run outdoors only, well away from windows — carbon monoxide from a generator in a garage or near an eave has killed more people in outages than the outages themselves; transfer switches, not extension-cord improvisation, are how a house gets connected.
Altitude Steals Watts Too
One derating catches mountain buyers by surprise: a naturally aspirated generator engine breathes thinner air as altitude rises and loses roughly 3% of its power per 1,000 feet (manufacturers publish their own curves; the figure is the working rule). The worked example's recommendation, 4,563 W at sea level, therefore becomes a shopping figure of about 4,563 / 0.85 ≈ 5,400 W of sea-level rating at 5,000 feet — a full class larger, for the identical fridge and pump. Heat nibbles similarly (output and cooling both suffer on the hottest, stilllest outage days, exactly when the machine is wanted), and propane typically derates a dual-fuel unit by roughly a tenth against its gasoline figure. None of this changes the method: compute the load, then divide by the conditions, then buy the class that survives both.
Related Tools
More from the Everyday Life Calculators hub: the live Battery Charge Time Calculator for the stored-energy side of outage planning, the Data Usage Calculator for the other household utility measured in a monthly budget, and the Leap Year Calculator for calendar maintenance while the power is on.
Frequently Asked Questions
What size generator do I need for a house?
It depends entirely on the list, which is the calculator's whole thesis: an essentials-only portable setup (fridge, lights, electronics, a pump) lands near the worked example's 4,500–5,000 W class; adding well pump or furnace-heavy loads pushes toward 7,500–10,000 W; true whole-house standby with central air starts around 14 kW and climbs. Enter your actual appliances — the answer is the list, priced.
Why not just add up all the starting watts?
Because motors do not start in the same second. Summing every surge assumes the fridge, pump and furnace fan all kick within the same second — a coincidence rare enough that sizing excludes it (and if it happens, the generator's brief overload tolerance and the motors' own stagger cover most cases). The standard method — running total plus the largest single surge — is the industry compromise between stall-outs and buying a power plant.
Can I run a generator at 100% of its rating?
Briefly, at its surge rating, by design; continuously, no — the continuous (rated) watts are the ceiling for steady running, and sustained operation near even that ceiling shortens engine life and sours fuel economy. The 50–80% load band in the recommendation is where noise, consumption and longevity all behave; the 25% headroom in this calculator is chosen to land typical use in that band.
What does an inverter generator change?
The quality of the power and the manners of the machine, not the sizing arithmetic: inverter models produce cleaner sine-wave output (kinder to electronics), throttle down at light load (quieter, thriftier), and cost more per watt. The watts in this calculator are the watts either way — an inverter unit of the recommended class simply delivers them more politely.
My appliance lists amps, not watts. How do I convert?
Watts = volts × amps for the simple resistive case: a "12 A" appliance on a 120 V circuit is about 1,440 W. For motor loads the nameplate running amps × voltage gives a fair running-watts estimate, and the surge can be estimated at 2–3× running for typical household motors when no figure is published — enter the estimate in the starting field and let the headroom forgive the approximation.
How long will a tank of fuel last?
Manufacturers quote runtime at 50% load for a reason — it is the sizing band this page recommends — and mid-size portables typically deliver on the order of 8–12 hours on a tank of roughly 4 gallons in that band, burning around half a gallon an hour. Treat the quote for your model as the authority and this calculator's load estimate as the input: a unit loafing at 30% load sips well below its 50% figure, and one pinned at 90% drinks above it, which is one more dividend of the headroom rule.
How do I handle 240 V appliances?
A portable generator's 240 V outlet serves them (well pumps, some water heaters and dryers), but the wattage bookkeeping is the same — watts are watts at either voltage — with one caveat: on many portables the 240 V load draws from both 120 V legs, so a monster 240 V appliance can unbalance the machine. Keep such loads few and honest in the list, and match the generator's 240 V continuous rating against them specifically.