Pure Sine Wave vs Modified Sine Wave Inverter: Which One You Need and What Size

What an inverter is

Batteries and solar panels supply direct current (DC), which always flows the same way. Wall outlets supply alternating current (AC), which changes direction many times a second. Most appliances are made for AC, so between the battery and them you need a device that converts it: the inverter. Portable power stations have one built in; if you put together your own system with separate batteries, you buy it on its own.

The three waveform types

Grid AC rises and falls smoothly, in a curve called a sine wave. Not every inverter copies it equally well. Pacific Northwest National Laboratory (PNNL), part of the U.S. Department of Energy, sorts them into three types by their output, and warns that the two that aren’t pure sine wave give lower-quality power that can potentially harm devices. Here’s what the National Renewable Energy Laboratory (NREL) says about each one in its 1998 and 2000 guides:

Mind the dates: back then, modified sine wave inverters were the most common because pure sine wave ones cost considerably more. Both types are sold today, so always check the spec sheet for which one you’re being offered. If it doesn’t say, ask before you buy.

So, pure sine or modified sine?

• Pure sine wave if you’ll plug in computers, audio equipment or any electronics you care about, or if you’re not sure what you’ll be running. It’s the only type that supplies the same power as your wall outlet.
• Modified sine wave may be enough if what you’ll connect is simple (lights, a fan) and you’ve checked it works properly on it. If an appliance doesn’t work properly on it, unplug it.
• If someone at home uses an electrically powered medical device, ask their doctor or the manufacturer what backup source they can use. That’s what Ready.gov, the U.S. government’s emergency website, recommends: have a plan for those devices before the power goes out.

Where to find the inverter’s numbers

On the label or spec sheet of any inverter or portable power station you’ll find:

• Continuous power (in W): what it supplies without stopping. According to NREL, this is the figure an inverter’s size is given by: a “1,000 W” inverter supplies 1,000 W continuously.
• Surge or peak power (in W): what it can handle for an instant. NREL explains that most inverters can handle considerably more than their continuous rating for short periods, which is useful when a motor starts.
• Input voltage (12, 24 or 48 V DC): that of the battery it connects to.
• Output voltage: it has to match the outlets in your country. Our beginner’s guide explains how to check it on your appliances’ labels.
• Waveform: pure sine wave or modified sine wave.

What size: a worked example

There are two parts to the calculation. It’s the method used, for example, by the U.S. public utility EWEB to choose a generator, and it works the same way for an inverter:

1. Continuous power = the sum of the watts of everything that will be on at the same time.
2. Surge power = the above + the start-up of the most demanding motor. A motor, like a fridge compressor, briefly draws several times its normal power when it starts.

With the appliances our power outage battery calculator loads when you open it (fridge, 4 LED bulbs, a fan, 2 chargers and the TV), everything on at once adds up to 426 W. The 200 W fridge draws about 600 W extra when it starts (three times its power, the ratio taken from EWEB’s table), so the surge is 426 + 600 = 1,026 W. You need an inverter rated for at least 426 W continuous that handles at least 1,026 W of surge. It’s worth leaving some headroom in case you plug in something else one day.

Why a large inverter usually runs on 24 or 48 V

An inverter draws as many amps from the battery as the watts it supplies divided by the battery voltage, plus a bit more for what it loses in the conversion (the calculator assumes 85% efficiency). For the 426 W in the example:

• With a 12 V battery: 426 ÷ 0.85 ÷ 12 = about 42 A.
• With a 24 V battery: 426 ÷ 0.85 ÷ 24 = about 21 A.

For an inverter supplying 1,000 W continuously, it would be about 98 A at 12 V, 49 A at 24 V and 25 A at 48 V. The more amps, the thicker the cables between battery and inverter need to be. That’s why large inverters usually run on 24 or 48 V. The inverter’s input voltage has to match your battery.

What the inverter itself uses

Converting power costs energy. According to NREL, an inverter is inefficient when supplying little power and very efficient (over 90%) when working near its maximum, and in between it varies a lot from model to model. It also uses some power when it’s on with nothing connected: NREL advises checking this no-load draw on the spec sheet and whether it has a sleep mode, which cuts it to a few watts when nothing is plugged in. Practical tip: don’t buy one much bigger than you need, and switch it off when you’re not using it.

Safety: when to call an electrician

• Plugging appliances into the outlets on the inverter or power station itself is something you can do.
• Connecting an inverter to your home’s wiring, the electrical panel or a wall outlet is a job for an electrician.
• The cables between battery and inverter carry a lot of current: they must be the gauge the manufacturer specifies and have their fuse or breaker, like the one in the photo above. If you don’t know how to choose them, have an electrician install it.
• Leave space around the inverter so air can circulate: it gets warm while working.

To find out how much battery you need besides the inverter, use our power outage battery calculator, and if you’re unsure which battery to buy, see our comparison of battery types.