120V vs 240V: what's the difference, and what needs 240V?

Most of what you plug in runs on 120 V. A few of the most important things in an outage, like the well pump, run on 240 V. A generator without a 240 V outlet can't run them at all, however many watts it makes.

How a US home gets 120 V and 240 V

Power comes into a US home on three wires: two "hot" legs and a neutral. This setup is called split-phase.

• One hot leg to neutral = 120 V. Lights, outlets, fridges and TVs use this.
• Hot leg to hot leg = 240 V. Big loads span both legs.

Why bother with 240 V? It moves the same power with half the amps. A 4,500 W water heater draws 18.75 A at 240 V. At 120 V it would need 37.5 A and much thicker wire. Watts, volts and amps explains the math.

What needs 240 V?

Common 240 V loads in a US home:

• Well pumps. Most submersible pumps are 240 V. In Franklin Electric's standard 4-inch line, only the 1/2 HP motor comes in a 115 V version.
• Electric water heaters. Usually 240 V, with a 3,000 to 4,500 W element.
• Central air conditioners and heat pumps. The outdoor unit is 240 V.
• Many mini splits. Larger ones are 240 V. Some 9,000 to 12,000 BTU models run on 115 V, so check yours.
• Electric dryers, ranges and EV chargers.

Things that are usually 120 V: fridges, freezers, sump pumps, gas furnaces and their fans, most window ACs (very large ones can be 240 V), microwaves, TVs and lights.

How to tell if something is 120 V or 240 V

Look at the breaker. This is the quickest check.

• Single breaker (one handle): 120 V.
• Double-pole breaker (two handles tied together, taking two slots): 240 V. It connects to both hot legs, and both trip together.

You can also read the appliance's nameplate. It will say 115 V or 120 V, or 208/230 V or 240 V. For a well pump, check the control box or pressure switch label. Don't open covers on live equipment. If in doubt, ask an electrician.

What a generator needs for 240 V loads

Watts alone aren't enough. To run a 240 V load, the generator must put out 240 V, which means:

1. A 240 V outlet. On portable generators, that's usually the 4-prong twist-lock L14-30 (30 A, up to 7,200 W) or the 14-50 (50 A, up to 12,000 W). Both carry two hots, a neutral and a ground, so they give 120 V and 240 V. See generator plug types.
2. A transfer switch or interlock, installed by a licensed electrician. A well pump or water heater is hard-wired, so there's nothing to plug in. The switch connects the generator to those circuits safely. See interlock vs transfer switch.
3. Enough watts for the start-up. A 240 V motor still has a surge. See running vs starting watts.

Many small inverter generators are 120 V only. Their outlets are 5-20 household outlets and sometimes a TT-30 RV outlet, and both are 120 V. Some can be paired with a second unit using a parallel kit that has a 240 V outlet. Westinghouse, for example, says the parallel kit for its iGen5000DFc "includes a 240V outlet."

Never backfeed

Never plug a generator into a dryer outlet or wall outlet with a "double male" cord to reach your 240 V circuits. It can send power back down the utility line and electrocute a line worker. It can also start a fire. Use a transfer switch or interlock, every time.

Which guide next?

• Generator for a well pump
• Generator for an electric water heater
• Will a generator run central air?
• Will a generator run a mini split?

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.