What size inverter do I need? Your largest load plus 25%, in pure sine wave

Most households need a smaller inverter than they expect, mounted closer to the battery than they expect.

By the OFF Grid Journal staff · October 8, 2026 · 5 min read
Based on Project 22 of The Off-Grid Handbook by Garrett Dalton

Size an inverter to the largest AC load you will run, plus 25%, not to the sum of everything you own. For most emergency setups that means a pure sine wave inverter in the 300 to 600 watt class. If the load has a motor, such as a refrigerator, also check that the inverter's surge rating covers the startup draw.

That rule comes from Project 22 of The Off-Grid Handbook, which says the inverter is for "the one or two stubborn AC exceptions." Phones, lights, a radio and a router are DC natives, so the list of things that truly need 120 V AC is usually short.

How to size an inverter in three steps

1. Start from your audit. Find the wattage of each AC device you actually need during an outage, with a plug-in meter or from the nameplate. Our watt-hour audit guide walks through it.
2. Take the largest single load and add 25%. If you plan to run two AC devices at the same time, add them together first.
3. Check the surge rating. Motors pull a burst at startup. Virginia Cooperative Extension notes a motor draws about three times its running current as it starts, and the Red Cross gives the same up-to-three-times warning when sizing generators.

A quick example: a device that draws 400 watts needs at least a 500 W inverter, because 400 × 1.25 = 500. The continuous rating covers the running load, and the surge rating has to cover the startup moment of anything with a motor.

Oversizing has a cost of its own. A big inverter wants big cables, a big fuse and a battery that can feed it, so the honest audit number saves money all the way down the line.

Which devices actually need an inverter?

Most of what keeps a household running in an outage already works on low-voltage DC. The book's field note is that most "I need a big inverter" cases dissolve in the audit.

• Run on DC, no inverter needed: phones and battery banks over USB, LED lights, a 12 V radio, and the router, which the book counts among the DC natives.
• Usually need AC: devices with a standard household plug and no DC option, such as a refrigerator, some medical equipment, or a laptop charger without a 12 V version.

Every conversion from DC to AC loses some energy as heat, and an inverter that is switched on uses some power of its own. Running DC loads on DC, and switching the inverter off when nothing needs it, keeps more of the battery for the devices that have no other option.

Pure sine vs modified sine inverter

Buy pure sine wave. The book's reason is practical: electronics and anything with a motor dislike modified sine. The University of Arizona Extension makes the same point, noting that pure sine models are designed to handle electronic devices such as laptops and communication radios with minimal distortion.

What size inverter to run a refrigerator?

Measure the fridge with a plug-in meter, or read its nameplate. The running watts set the continuous rating (plus 25%), and the startup spike, which can be around three times the running draw, sets the surge rating you need.

The bigger question is energy, not watts. A modern fridge uses roughly 1,000 to 1,500 watt-hours a day, per the book, while the book's 12 V 50 Ah battery box stores about 600 usable watt-hours. An inverter can only deliver what the battery behind it holds.

So a fridge on an inverter needs a larger battery bank, often built by connecting batteries in parallel, or a power station sized for it. Ready.gov notes that a closed fridge keeps food cold for about four hours, which gives you time to start the system calmly.

Does battery voltage matter?

At 12 V, current climbs fast. The arithmetic is simple: 600 watts ÷ 12 volts = 50 amps on the DC side before any conversion losses, which is why the book specifies short, thick cable, such as 4 AWG at 600 W.

The University of Arizona Extension suggests 12 V for systems using less than about 1 kWh a day, 24 V between 1 and 4 kWh, and 48 V above that. It also notes that a 2,000-watt inverter may come in a 24 V version while a 6,000-watt one requires 48 V, so a growing AC list eventually becomes a battery voltage decision.

The Florida Solar Energy Center at the University of Central Florida shows the same effect at small scale: a 120-watt load draws 10 amps from a 12 V battery but only 5 amps from a 24 V one. Lower current means smaller and cheaper conductors, fuses and disconnects.

Inverter wiring safety

• Mount the inverter on a ventilated board within 3 to 6 feet of the battery, keeping cable runs short.
• Use cable sized per the inverter manual, short and thick.
• Put a Class T or ANL fuse sized to the inverter on the positive cable, within inches of the battery terminal.
• Torque the lugs. Loose high-current connections become heat sources.
• Load-test at half and then full rating, and feel the cables after ten minutes: warm is normal, hot is a fault.

Never wire inverter output into household wiring or an outlet. That is backfeeding, the same hazard as a generator plugged into a dryer outlet (see our generator safety guide), and the Red Cross warns it puts utility workers, neighbors and your own household at risk of electrocution. Loads plug into the inverter directly, and anything beyond plug-in use is work for a licensed electrician under the National Electrical Code.

Inverter output is real 120 V AC and can kill exactly like a wall outlet. Treat it with the same respect, and keep DC devices such as 12 V LED lighting on DC, where they need no inverter at all.