Choosing an inverter
From the little inverter you plug into a cigarette lighter to the big unit that powers a house at 120/240 V: here are the five main kinds, what each one can run, and how to find the size you need. Voltages here are North American (120 V, 120/240 V): elsewhere it's 230 V, and you need an inverter made for your country.
Tool: What size do you need?From simplest to most complete
Each step does everything the one before does, and more. Pick the first one that does everything you need.
1. Small modified sine wave inverter
- Runs
- Lights, phone and laptop chargers, a small TV.
- Voltage and size
- 12 V, about 150 to 1,000 W. Often plugged into a cigarette lighter (150 W at most) or wired straight to the battery.
- The upside
- Cheap and simple. Fine in a pinch.
- Watch out
- Its “stair-step” current makes motors, fridges, fans and microwaves run hot or hum. Some devices don't work at all: CPAP, tool chargers, dimmers, clocks.
In orange, its stair-step current. Dotted, the smooth curve your devices expect: the whole gap turns into heat and hum. 2. Pure sine wave inverter
- Runs
- Anything that plugs into a household outlet, within its watts: fridge, pump, CPAP, tools, microwave.
- Voltage and size
- 12, 24 or 48 V, about 300 to 3,000 W.
- The upside
- Current as clean as the grid's. It's the minimum for a real system.
- Watch out
- It doesn't charge the batteries: you need a separate charger for the generator.
A smooth curve, the same as in your household outlets. 3. Inverter-charger
- Runs
- A system with a generator or shore power: plug in the generator and it powers your outlets and charges the batteries at the same time.
- Voltage and size
- 12, 24 or 48 V, about 1,000 to 5,000 W.
- The upside
- One unit, and switching from one source to the other happens on its own (transfer relay).
- Watch out
- Check that it accepts your generator: some drop out on a contractor generator with less stable power.
4. All-in-one: inverter-charger + MPPT controller
- Runs
- A cabin or a house: panels, batteries, generator and outlets all come into the same box.
- Voltage and size
- Mostly 24 or 48 V, about 3,000 to 6,000 W, at 120 V.
- The upside
- Less wiring and a single setup. Often cheaper than three separate units.
- Watch out
- It often draws several tens of watts at idle, even at night when nothing is running. If one part fails, everything stops. Check the maximum panel voltage it accepts.
5. All-in-one 120/240 V with neutral
- Runs
- What needs 240 V: well pump, dryer, stove, big tools, or a whole house panel.
- Voltage and size
- 48 V, 6,000 to 12,000 W and more. Either a 120/240 V “split-phase” unit, or two units built to be paired for 240 V.
- The upside
- You power a real house panel, with both legs and its neutral.
- Watch out
- Two ordinary 120 V inverters don't make 240 V: you need models designed for it. The neutral and grounding must be done to code: that's an electrician's job.
What size do you need?
Check only the devices that can run at the same time. Motors (fridge, pump, tools) need a big kick of current to start: the inverter has to handle it.
840 W
continuous
2,040 W
surge at start-up
An inverter of at least 1,500 W continuous, pure sine wave.
A 12 V battery works.
At full load, it draws about 139 A from your batteries at 12 V.
Your devices have motors or sensitive electronics: get a pure sine wave.
Watts are ballpark figures: your device's nameplate has the real ones. We add a 25% margin, and assume only one motor starts at a time.
Words to know
- Continuous watts
- What the inverter delivers for hours. It's the real number to compare.
- Surge watts
- What it handles for a few seconds to start a motor. Often printed bigger on the box.
- Pure or modified sine
- Pure: current like the grid's. Modified: stair-step current, cheaper, that many devices don't like.
- Idle draw
- What the inverter burns just being on. Over a night, a few tens of watts can drain a good part of a small battery.
- Transfer relay
- In an inverter-charger: when the generator starts, it switches from battery to generator on its own.
- Split-phase 120/240 V
- Two 120 V “legs” and a neutral, like in a house panel. Between the two legs, you get 240 V.
Classic traps
- Trusting the big number on the box: a cheap “3,000 W” is often 3,000 W surge and 1,500 W continuous.
- Wiring a big 12 V inverter with wires that are too thin: 3,000 W at 12 V is close to 280 amps. You need very thick cables and a fuse near the battery.
- Leaving a big inverter on all night for nothing: its idle draw drains the batteries.
- Making 240 V with two 120 V inverters that aren't built to be paired: they aren't synchronized, it doesn't work.
- Connecting the inverter to a house panel without an electrician: neutral, grounding and isolation from the grid must be done to code.
To calculate
Next step
Choosing a generator
Quiet, contractor or propane, and a tool for the size and the gas it will burn per day and per month.