Add the appliances you want to power and find the right generator size
| Appliance | Qty | Running | Surge | Total surge |
|---|
| Generator | Running | Surge | Fit |
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Founder & CEO, Toolraxy
Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.
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Generator sizing trips up more buyers than any other spec. The nameplate says 5,000 W running, but a fridge, a well pump, and a furnace blower can each pull two to three times their rated watts for a split second when they start. Buy on running watts alone and the breaker trips every time the compressor kicks in. This calculator handles the surge side properly. It totals your continuous load, then adds the single largest startup delta to represent the realistic worst case. That combined figure, plus a safety margin you choose, becomes the recommended generator size. The amp and kVA figures matter too: they determine breaker sizes, extension cord gauges, and whether a 30 A or 50 A inlet is enough.
Tap presets from the grid to add appliances to your list.
Adjust quantities with the +/− steppers.
Edit running and starting watts if you have the appliance labels.
Set a safety margin. Twenty percent is the standard.
Choose portable or standby as your generator type.
Pick your voltage and enter a power factor if you know it.
Read the recommended size in the hero panel.
Check the size table to see which commercial generators fit your load.
The result updates as you tap.
Four calculations combine into the recommendation.
Formula: Total running watts = Σ (appliance running watts × quantity)
Formula: Peak surge watts = total running watts + largest single startup delta
Formula: Recommended running = total running watts × safety margin
Formula: Recommended surge = peak surge watts × safety margin
Formula: kVA = watts ÷ (power factor × 1000)
Formula: Amps = watts ÷ (volts × power factor)
The peak surge calculation uses the largest single startup delta rather than summing every appliance’s surge. That’s how real homes work. A fridge cycles on, then the well pump kicks in a few minutes later. The two surges rarely overlap. Modelling them separately keeps the recommendation realistic without over-sizing the generator by a factor of two or three.
Power factor matters for motor loads. Resistive devices (heaters, incandescent bulbs, kettles) run at PF 1.0. Motor loads (ACs, fridges, pumps, power tools) sit closer to 0.8. Using the correct PF keeps the kVA figure accurate and stops you buying a larger generator than you need.
Validation requires at least one appliance. The safety margin defaults to 1.20 (20%) if the field is empty. Power factor clamps between 0.5 and 1.0.
A homeowner wants emergency backup for the essentials. Here’s the list:
Refrigerator: 700 W running, 2,200 W surge, qty 1
Chest freezer: 500 W running, 1,500 W surge, qty 1
Furnace blower: 800 W running, 2,350 W surge, qty 1
Well pump 1 HP: 1,000 W running, 3,000 W surge, qty 1
Wi-Fi router: 15 W running, 15 W surge, qty 1
Modem: 10 W running, 10 W surge, qty 1
LED bulbs: 10 W running each, qty 6
TV: 100 W running, 100 W surge, qty 1
Phone chargers: 10 W running each, qty 2
Running total: 700 + 500 + 800 + 1,000 + 15 + 10 + (10×6) + 100 + (10×2) = 3,185 W
Largest single startup delta: the well pump at 3,000 W surge minus 1,000 W running = 2,000 W
Peak surge: 3,185 + 2,000 = 5,185 W
Safety margin (20%):
Recommended running = 3,185 × 1.20 = 3,822 W
Recommended surge = 5,185 × 1.20 = 6,222 W
Running kVA at PF 0.9: 3,185 ÷ 900 = 3.54 kVA
Running amps at 240V: 3,185 ÷ 240 = 13.3 A
The badge reads “Mid-Size Generator.” Looking at the size table, the 7,500 W portable (6,000 W running / 7,500 W surge) covers both the running and surge requirements with headroom. The 6,500 W unit sits just under the surge figure, so the calculator flags it as “Close” and recommends the larger option.
Match running watts to your continuous load and surge watts to your largest single startup. A home essentials setup needs a 6,500 to 7,500 W portable. Whole-home backup with AC needs a 22,000 W or larger standby. Enter your appliances in the calculator for an exact figure.
Running watts is the continuous draw while the appliance operates. Surge watts is the brief startup spike, usually two to three times higher for anything with a motor or compressor. Generators must cover both, but only the running figure counts for sustained load.
Multiply the appliance’s running watts by two to three for motor-driven loads. Refrigerators, ACs, well pumps, and power tools all surge. Resistive loads like heaters and incandescent bulbs don’t surge. The preset library includes typical surge figures.
A household fridge draws 600 to 800 W running and 1,800 to 2,200 W surge. A chest freezer is similar. A 2,000 W inverter covers a fridge on its own. Add other appliances and the requirement climbs fast.
Twenty percent above your calculated load is the industry standard. Go to 30% if you plan to add appliances later, run in hot weather, live at high altitude, or want extra headroom for larger startups. Below 10% leaves almost no margin for real-world conditions.
Portable generators are gasoline or dual-fuel units rated for intermittent use. They cost $500 to $2,000 and connect via transfer switch or cords. Standby generators are permanently installed, run on natural gas or propane, start automatically during outages, and cost $5,000 to $15,000.
Divide watts by the power factor. For resistive loads at PF 1.0, watts equal VA. For motor loads at PF 0.8, a 3,000 W load needs 3,750 VA, which is 3.75 kVA. The calculator shows the kVA figure from your PF input.
A fridge, a few LED bulbs, a Wi-Fi router, and phone chargers. Maybe a small TV. Skip anything with a heating element (kettle, space heater, hair dryer), a large motor (AC, well pump), or high-wattage tools. A 2,000 W unit is for camping, tailgating, and light emergency power.
A 1 HP well pump draws about 1,000 W running and 3,000 W surge. Larger pumps scale roughly linearly: 2 HP is 2,000 W running and 6,000 W surge. The startup spike is the reason well pumps often push the generator size up a notch or two.
A 22,000 W standby unit covers most single-family homes with central AC. Larger homes with two AC units and an electric range may need 26,000 W or more. The whole-home scenario loads a large appliance list, and the size table shows where your load lands.
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