Generator Size Calculator

What Size Generator Do I Need?

Buying a generator that is too small means it stalls the moment your refrigerator compressor kicks on — or worse, it runs overloaded until it dies mid-outage. Buying one far too large means you paid for capacity you will never use and burn extra fuel every hour it runs. The right size starts from your actual loads.

This generator size calculator takes the guesswork out: check off the appliances you want to power, and it totals their running watts (what they draw continuously) and starting watts (the brief surge when motors start), then recommends a generator size with proper headroom — plus an estimate of how long a tank of fuel will last.

Running Watts vs. Starting Watts

Every appliance has two wattage numbers, and confusing them is the most common sizing mistake.

• Running watts (rated watts) are what the appliance draws while operating normally. A refrigerator might run at 700 W.
• Starting watts (surge watts) are the short spike — usually a few seconds — needed to start an electric motor. That same refrigerator can surge to 2,200 W when the compressor kicks in.

Resistive loads like space heaters, coffee makers, and incandescent lights have no surge: starting watts equal running watts. Anything with a motor or compressor — refrigerators, freezers, well pumps, sump pumps, air conditioners, furnace blowers — surges to 2–3× its running watts. Your generator must cover the highest combined demand your home will ever present at one instant, not just the average.

How to Use the Calculator

Step 1 — Start from a scenario. Tap “Outage essentials”, “Work from home”, “Whole-house basics”, or “Jobsite” to pre-select a typical load list — or start from scratch.

Step 2 — Fine-tune the checklist. Check everything you want running during an outage, setting quantities where needed (two refrigerators, four phone chargers). Typical running and starting watts are pre-filled from manufacturer data.

Step 3 — Add custom appliances. Got something unusual — a medical device, a 3D printer, a pool pump? Add it with its nameplate running and starting watts (check the label or manual).

Step 4 — Enter fuel details (optional). Add your tank size in gallons to get a runtime estimate at your total load.

Step 5 — Read your results. You will see running watts needed, starting watts needed, the recommended generator size, estimated runtime, and a table showing which common generator classes cover your load. Everything updates live as you change selections.

How the Calculation Works

Total running watts is simply the sum of every selected appliance’s running watts, multiplied by quantity.

Total starting watts equals total running watts plus the single largest startup surge (starting − running) among your appliances. This reflects reality: you will not start every motor in the same second, but you must survive the worst single surge on top of everything already running.

Recommended size takes the larger of those two totals, adds a 25% safety headroom, and rounds up to the nearest 500 W — matching how generators are actually sold (3,500 W, 5,500 W, 7,500 W, and so on). The headroom accounts for nameplate optimism, voltage sag under load, and the fact that you will inevitably plug in one more thing.

Runtime estimate assumes gasoline at roughly 0.2 gallons per hour per 1,000 W of load (propane runs about 10% less efficiently). Real consumption varies with the generator’s efficiency curve, altitude, and temperature — treat it as a planning figure, not a promise.

Worked Example

A family wants to keep the essentials running during a winter outage: refrigerator (700/2,200 W), gas furnace blower (800/2,350 W), sump pump (800/2,300 W), LED lights (100/100 W), Wi-Fi router and laptop (150/150 W), and phone chargers (60/60 W).

• Running watts: 700 + 800 + 800 + 100 + 150 + 60 = 2,610 W
• Largest single surge: furnace blower, 2,350 − 800 = 1,550 W
• Starting watts: 2,610 + 1,550 = 4,160 W
• Recommended: 4,160 × 1.25 = 5,200 → rounded up to 5,500 W

A 5,500 W portable generator covers this household with headroom. With a 4-gallon tank at 2,610 W load, estimated runtime is about 7 hours 40 minutes per tank — so they should keep roughly 12–13 gallons on hand for a 24-hour outage (stored safely, with stabilizer).

Generator Sizing Tips

• Stagger motor starts. If you are near your generator’s limit, unplug the fridge while the well pump runs, then swap. You do not need to cover every surge simultaneously.
• Do not forget the “invisible” loads. Furnace blowers, garage door openers, and septic pumps are the appliances people overlook — and they all have motors.
• Size up, not down. An overloaded generator’s voltage sags, which is what actually kills refrigerator compressors and electronics. A generator running at 50–70% load is in its happy place.
• Inverter vs. conventional. For phones, laptops, and modern electronics, an inverter generator delivers cleaner power (low total harmonic distortion). The watt math is the same either way.
• Mind the fuel math. A 7,500 W generator at half load burns roughly 0.75 gallons per hour — about 18 gallons a day. Fuel logistics, not wattage, is what ends most outage plans.
• Plan the connection legally. Backfeeding through a dryer outlet (“suicide cord”) is illegal and can kill utility workers. Use a transfer switch or interlock kit installed by a licensed electrician.

Fuel Runtime: What Affects It

The calculator’s runtime is a straight-line estimate, but real-world burn rates bend with conditions. Generators are least efficient at very light loads and most efficient around 50–75% load. Cold weather thickens oil and increases warm-up consumption; altitude above 3,000 feet derates both power output and efficiency (roughly 3.5% per 1,000 feet). Ethanol-blended gasoline goes stale in 3–6 months — rotate stored fuel or use stabilizer, and never refuel a hot generator.

Disclaimer

Wattage figures are typical values compiled from manufacturer data and may differ from your specific appliances — always check nameplates and manuals. This calculator provides planning estimates only, not engineering or electrical advice. Generator installation, transfer switches, and interlock kits must be installed by a licensed electrician in compliance with local codes. Carbon monoxide from generators kills hundreds of Americans every year: never operate a generator indoors, in a garage, or near open windows, and install working CO detectors in your home.