What Size Generator Do I Need?

A 5,000-watt portable generator comfortably runs the essentials most households reach for during an outage: a refrigerator (700W), a sump pump (800W), a gas furnace blower (800W), LED lighting, a Wi-Fi router, phone chargers, and a laptop — roughly 2,600 running watts, with room to spare for a microwave or coffee maker in rotation. What it will not run: central air conditioning, electric heat, or an electric range. Use the calculator above with your exact list to be sure — the 5,000–5,500W class is the sweet spot for essentials-only backup.

Running watts (also called rated watts) is the continuous power an appliance needs to keep operating. Starting watts (surge watts) is the brief extra burst — two to three times higher — that electric motors need for the first few seconds to get spinning. A refrigerator runs on 700 watts but needs about 2,200 watts to start its compressor. Generators are rated for both, and undersizing the surge side is the most common mistake: the generator may run your fridge fine until the compressor kicks on, then trip its breaker. Our calculator adds only the single largest surge to your running total, because in practice motors rarely all start in the same second.

Usually not. A typical 3-ton central AC needs about 3,500 running watts but up to 9,000 starting watts — that surge alone exceeds what most portable generators can deliver, and it would consume nearly the entire capacity of a 9,500W unit. If central AC during an outage is a must, you are in standby-generator territory: a permanently installed unit sized by a licensed electrician, often 14–22kW. For portable backup, plan around a window unit (about 1,200 running watts) for one cool room instead.

Inverter generators produce cleaner, more stable power (low total harmonic distortion), run quieter, and sip fuel at light loads because the engine throttles down. They are the right pick for RVs, camping, tailgating, and anything with sensitive electronics — and for anything under about 2,500 watts, inverter is the default choice. Conventional portable generators cost less per watt and dominate the 5,000W-plus classes, where clean power matters less for motors and appliances. If your list is all motors and lights, conventional is fine; if laptops, RV electronics, or quiet campgrounds are in the picture, go inverter.

Together they need about 1,500 running watts (800 + 700) and roughly 3,000 peak watts once you add the refrigerator's 1,500-watt starting surge. With the standard 25% headroom, that lands at 3,875 watts — round up to a 4,000W unit, which in practice means the 5,000–5,500W portable class, since generators are sold in standard sizes. Add a few hundred watts for lights and phone chargers and you are still comfortably inside that class.

A truly whole-house setup — central AC, electric water heater, electric range, dryer — typically needs 14,000 to 22,000+ watts, which means a permanently installed standby generator, not a portable. Most portable-generator owners aim smaller: the 7,500–9,500W class covers "most of the house minus central AC and electric heat" — well pump, fridge, freezer, furnace blower, lights, and a few kitchen appliances in rotation. Be honest about what "whole house" means to you, run the calculator with your real list, and if the answer exceeds 9,500 watts, call an electrician about standby.

Mildly oversizing (the 25% headroom in our math) is smart. Grossly oversizing wastes money twice: you pay more upfront, and generators run most efficiently at 50–80% load — a 12,000W unit loafing along at 2,000 watts burns more fuel per useful watt and can suffer from carbon buildup over time. Bigger units are also louder, heavier, and thirstier. Size for your actual list plus headroom, not for the biggest unit you can afford.

A 13,500-BTU RV air conditioner — the most common size — needs about 1,500 running watts and a 4,500-watt starting surge. Add a battery converter (500W) and basics and you are looking at roughly 6,500 watts with headroom, which points at the 7,500–9,500W portable class. Many RVers instead pair a 3,500–4,500W inverter generator with a soft-start kit on the AC, which tames the starting surge enough to run one AC unit on a smaller, quieter generator. For RV use, inverter models are worth the premium: cleaner power for your electronics and far less noise at the campground.

If you want to power hardwired circuits (furnace, well pump, wall outlets) rather than running extension cords to individual appliances, yes — and it must be installed by a licensed electrician. A transfer switch (manual or automatic) safely isolates your house from the grid so your generator cannot backfeed power lines, which can kill utility workers. Never plug a generator into a wall outlet ("backfeeding") — it is illegal in most places and genuinely dangerous. For a simple essentials kit (fridge, phones, lights via extension cords), no transfer switch is needed.

It depends on size and load. A small 2,500W inverter sips about 0.2–0.3 gallons per hour at moderate load (a gallon of gas ≈ 3–5 hours). A 5,500W portable at half load burns roughly 0.5–0.7 gallons per hour — figure 8–12 hours on a typical 6–8 gallon tank. A 9,500W unit working hard can drink over a gallon an hour. This is another reason not to oversize: fuel consumption tracks the generator's size more than your actual load. For long outages, plan fuel storage (stabilized gas, rotated yearly) or consider a dual-fuel (gasoline/propane) model — propane stores indefinitely.