What Size Generator Do I Need?

The formula, in three steps

Generator sizing has no hidden variables. Three steps, and you can check any claim on any brochure in under a minute:

• 1. Total the running watts. Running (continuous) watts are what each appliance draws while it is actually working — the figure on its data plate. Add everything you want on generator power, multiplying by quantity where you own more than one. That sum is your continuous load: the number the machine has to hold for hours at a stretch.
• 2. Add one starting surge. Motors — fridge compressors, sump and well pumps, AC units, saws — pull two to three times their running watts for the first second. They do not all start at once, so you do not stack them. Add only the largest starting figure in your list:

  total surge watts = total running watts + largest starting surge

• 3. Multiply by 1.20. The 20% buffer is the recommended minimum generator rating. It keeps the machine off 100% continuous load and pays for voltage sag under heat, altitude, dirty filters and the next appliance you switch on.

Worked example: sizing for a storm outage

Five appliances, the list most people actually need when the power goes out:

Now the arithmetic:

• Running total: 700 + 1,050 + 50 + 15 + 1,000 = 2,815W
• Plus the biggest single start (the fridge at 2,200W): 2,815 + 2,200 = 5,015W surge demand
• Times the buffer: 5,015 × 1.20 = 6,018W

Portable generators are sold in round sizes, so round up: a 6,500W machine, or 7,500W if you want the sump pump cycling without watching the meter. Note what the arithmetic did not do — it never added the sump pump's 2,150W or the microwave's 1,500W start. Those spikes happen at different moments, and the generator meets each one on top of whatever is already running.

Why only one starting surge is counted

Start-up current is real but brief: a compressor pulling 2,200W draws it for one to three seconds, then falls back to its 700W running figure. Because the appliances in a house start at scattered moments, the grid-sized worst case — every motor locked rotor at once — is not the case a portable generator has to survive.

The difference is not academic. Stacking all three motor starts in the storm list above (fridge +1,500W, sump +1,100W, microwave +500W) gives 5,915W of "surge" instead of 5,015W, and after the buffer 7,098W instead of 6,018W — roughly 1,100W of extra generator you would buy, carry and refuel for a scenario that happens once a decade. Sizing on running watts alone has the opposite failure: the first time the fridge compressor kicks, the breaker trips and the food starts warming.

Typical wattages for common appliances

Industry-average figures — the exact numbers the calculator pre-fills. "×" is the starting multiplier, which is why motor loads dominate the answer:

Resistive loads — lights, heaters, routers, coffee makers — have no starting surge, so their running and starting figures are identical. If your list is nothing but those, your running total already is the surge total. The same running watts also decide what those appliances cost on mains power; the appliance bill calculator turns them into a monthly figure at your own rate.

Why the 20% buffer is not optional

A generator held at 100% of its rating runs hot, drops voltage every time a motor starts, and delivers the kind of soft power that shortens the life of whatever is plugged in. The nameplate itself is optimistic: output is measured at sea level on a cool day, and roughly 3% of power is lost per 1,000 feet of altitude, with more lost in high heat or with a dirty air filter. Then there is the label problem — a machine sold as "5,000 watts" is usually 5,000 peak and about 4,000 running, which is why the sizing formula works in peak watts and the meter in the calculator compares against your generator's peak rating.

Twenty percent lands the machine at about 75–80% load: voltage stays steady, fuel use per kilowatt-hour is at its best, and the next motor to start finds headroom instead of a tripped breaker.

Sizing by scenario

Three lists, calculated with the same formula — each one is a single click inside the tool:

The jobsite row explains why contractors rent 12,500W machines for three tools: the 1 HP air compressor alone starts at 4,500W, and with both saws running the continuous load is already 5,000W. The RV row explains the opposite trap — the 13,500 BTU roof unit starts at 2,800W, so a 3,500W inverter only covers it if the fridge and every other load are switched off at that instant (2,800W start + about 250W of lights, chargers and TV = 3,050W).

How to work it out with the calculator

The Portable Generator Wattage Sizing Calculator runs the three steps live, in your browser — nothing you type is sent anywhere:

1. Start from a loadout or tick your own. Storm Outage Essentials, RV Camping and Construction Jobsite fill the list in one click; otherwise tick any of the 17 appliances across four groups, or search them by name.
2. Set quantities. Two refrigerators is 2 × 700W running — the stepper on each row multiplies both the running figure and the starting one.
3. Read the three tiles. Total running watts, peak surge watts (running + largest start) and the recommended minimum, already rounded up to the nearest 10W.
4. Add what is missing. Custom entries take a name and a running figure; starting watts are optional and only needed for anything with a motor.
5. Test a machine you already own. Enter its peak rating and the strain meter compares surge demand against it: green to 75%, yellow to 95%, red beyond — with an explicit shortfall in watts if you are undersized.
6. Copy and share. The summary button produces plain text for an email or quote, and the URL keeps your preset, so ?preset=storm reopens the exact same list for someone else.

Five sizing mistakes

• Stacking every starting surge. Over-sizes the machine by a thousand watts or more and still does not protect against anything real.
• Forgetting the invisible loads. A well pump, furnace blower or sump pump runs unattended in a basement — if it is on the same circuit, it belongs in the list.
• Buying on running watts alone. The first motor start then trips the breaker, usually at the worst moment of the outage.
• Trusting the headline number. "5,000W" on the box is peak; continuous is typically about 80% of it. Sizing math is done in peak watts, then checked against continuous.
• Ignoring derating. Altitude, heat and a clogged filter all cost output — the reason the buffer exists is that real-world ratings are always a little lower than the sticker.