Generator Size Calculator

How this calculator works

Every appliance has two numbers that matter: running watts (what it draws continuously) and starting watts (the brief surge a motor or compressor pulls in the first second or two). The calculator combines them like this:

running total = Σ (quantity × running watts)
largest surge = max (starting − running) among your appliances
peak demand   = running total + largest surge
recommended   = peak × (1 + safety margin) → rounded up to next 500 W

The default mode assumes the largest single motorstarts while everything else is already running. That's realistic: two compressors almost never start in the exact same instant, and adding every surge together would oversize the generator substantially. If you want that conservative number anyway, switch to "everything starts at once" mode and the calculator adds every surge, multiplied by quantity.

The recommendation is a starting (surge) watts rating. Generators publish both ratings — make sure the model you pick also has a running-watts rating at or above your running total, which the results state explicitly.

Worked example: storm-outage basics

Say you want to keep a refrigerator (200/1,200 W), sump pump (1,050/2,200 W), furnace blower (800/2,300 W), LED lights in five rooms (60 W each), a TV (150 W), and a Wi-Fi router (15 W) running through an outage:

• Running total: 200 + 1,050 + 800 + 300 + 150 + 15 = 2,515 W.
• Largest surge: the furnace blower needs 2,300 − 800 = 1,500 Wextra to start — more than the sump pump's 1,150 W or the fridge's 1,000 W.
• Peak demand: 2,515 + 1,500 = 4,015 W.
• Recommended size: 4,015 × 1.20 (20% margin) = 4,818 → rounded up to the next 500 W = 5,000 W starting watts, with at least 2,515 W of running capacity.

Enter those same appliances in the calculator and you'll see this exact arithmetic in the results panel.

Running vs. starting watts: getting real numbers

Motors and compressors draw 2–4× their running watts for a moment at startup, because a stopped motor is briefly close to a short circuit until it spins up. That's why a modest 1,050 W sump pump can stall a 2,000 W generator. Purely resistive loads — heaters, incandescent bulbs, toasters — have no surge at all: starting equals running.

• Check the nameplate or manual.Look for "rated watts" and "LRA" (locked-rotor amps) or a listed starting wattage. If only amps are listed, watts ≈ amps × volts (120 or 240).
• The presets here are typical planning values. A modern efficient refrigerator may run at 100 W while an older one runs at 400 W — verify yours.
• Watch for hidden loads. Furnaces have blowers, well systems have pressure switches that cycle, and refrigerators defrost on a heater cycle. Anything with a motor or compressor deserves a starting-watts entry.
• Leave starting watts blank for resistive loads— the calculator treats a blank as "no surge."

Choosing a generator type

• Portable open-frame (2,000–9,000 W): the workhorse for outage basics — fridge, sump pump, furnace blower, lights. Loud, manual start, extension cords or an inlet + interlock. Most models in the 5,000 W class cover the worked example above.
• Inverter (1,000–4,500 W typical): quieter, fuel-efficient, and clean power for electronics, at a higher price per watt. Great for partial backup and sensitive equipment; larger paired units can reach whole-circuit territory.
• Home standby (8,000–26,000 W): permanently installed, starts automatically, runs on natural gas or propane, and handles 240V loads like well pumps and central air. Requires professional installation and a transfer switch by design.

If any of your loads is flagged as 240V (well pump, central AC, electric dryer or water heater), you need a generator with a 240V outlet anda transfer setup wired for it — many smaller portables are 120V only. And regardless of type, connecting to house wiring requires a transfer switch or interlock installed by a licensed electrician; plugging a generator into a wall or dryer outlet ("backfeeding") can kill utility workers and is illegal.

Common sizing mistakes

• Sizing on running watts alone:the surge is what trips overload protection. A generator that can "run everything" may still stall the moment the well pump kicks in.
• Adding every starting surge together:the opposite error — it can double the recommendation for no real benefit, since motors don't start simultaneously. Use worst-case mode deliberately, not by default.
• Trusting generic wattage tables over nameplates: two same-size air conditioners can differ by 1,000+ starting watts depending on age and whether they have a soft-start kit.
• Forgetting electric heat: water heaters, dryers, ranges, and space heaters are enormous resistive loads (4,500 W+). Most portable-generator plans should leave them off and stagger their use.
• Sizing against the gasoline rating for a propane setup:dual-fuel units typically produce about 10% less on propane — size against the rating for the fuel you'll actually run.

Limitations

This is a load-planning estimate, not an electrical design. It doesn't model voltage drop, power factor, motor soft-starters, generator derating for altitude or temperature, or how loads split across a 240V generator's two legs — an unbalanced split can limit usable capacity before the total rating does. Preset wattages are typical planning values that vary by model and age. For a transfer switch, interlock, standby installation, or any hardwired connection, a licensed electrician sizes and wires the system to code — bring them this load list as a starting point, not a specification.