Inverters convert direct current (DC) to alternating current (AC).  Years ago, such technology was expensive.  Now you can get a 400 Watt inverter for under $40.  Generally, there are two types on the market.  Those that produce a Pure Sine Wave (PSW), and those that produce a Modified Sine Wave (MSW).  The MSW type inverters are far less expensive, and work with most equipment.  Despite what some think, MSW will work with most items.

Inductive loads don't work as well, or not at all, on MSW.  These include transformers, motors, variable speed devices, microwave ovens, etc.

Things that do work rather well on MSW in my experience are computers, lights, TVs, most electronics, etc.

A microwave oven will work on MSW, but it takes longer, and may have a slightly different sound.  I had a conversion van that I installed a microwave oven and a MSW inverter.  5 years later, I traded it in, but kept the inverter.  The microwave still worked perfectly after all that time of running on MSW exclusively.  Yes, it did take a little longer, but it did work, and the microwave suffered no damage.

At work we run a lot of telemetry, radio equipment, medical equipment, and computers, all from MSW inverters.  No noise, and no noticeable difference in operation.


There's $100K worth of sensitive equipment in the base of this research chair, all running on MSW power.  Any RF noise would trash our ECG and Cranial Doppler signals.

There's no guarantee that a MSW inverter will run all your equipment without issues.  But my point is don't assume it won't run your equipment.  I've been using MSW inverters since the mid 90s.  Very few items I've run across that showed any noticeable difference in performance.


A Very Popular Vector Inverter (MSW)

I bought this at Target for $29.95 on sale.

Basic front panel.  Two outlets, on/off switch, green power light, red overload light.

Fan, fuses, and power connections in the back.  This came with a cigarette lighter adapter, and clamps to attach directly to the battery.  A word of caution here...  400 Watts is 33.3 Amps, far more than a cigarette lighter outlet can handle.  Many are fused at 10-15 Amps.  So if you connect to such an outlet, you will be limited to 120-180 Watts, before the fuse blows.  Make sure you also use the appropriate sized wire to handle this current.

Net folklore says you can't run a computer from a MSW inverter.  Rubbish!  The switching power supply used in computers is not very sensitive to power fluctuations.  I've used my personal laptop with a MSW inverter for the past 3 years.  Even longer with the computers at work.

Here's how a MSW emulates a sine wave.  Some of the newer, non sine wave inverters, actually have many more steps.  But this is the most common waveform you'll see with an inexpensive inverter.

This is the actual waveform from this inverter.  This is a fantastic little inverter.  I plan on installing this in my SUV, giving me AC power anywhere I go.


My First Inverter, A 1500 Watt Model (MSW)

This one has seen close to 100K miles in my previous conversion van.  Paid about $300 back in 1995, which was a bargain at the time.

As a test of its ability, I connected it to the battery in my SUV.

I operated the garage door opener with it.  There was no noticeable difference in performance than when the opener is connected to the grid.

It pulled about 40 Amps DC.  About 480 Watts.  I attempted to use my 400 Watt inverter as well, but it instantly went into overload.  The 1500 Watt inverter easily took the load.

The opener motor is a 1/2 HP model.  The door, though balanced with two coil springs, is a heavy hurricane rated door with bracing.

This inverter uses a binding screw to hold the cable.  This one had sheared off the head, and had to be removed with an Easy-Out.

A 1680 Watt heat gun was used to test the capacity of this inverter.

That's 160 Amps DC!  Make sure you use very heavy cables to carry this current.  I used the largest cable that would fit in the terminals on the back of this inverter.  In this case, it was #4 gauge.  I used #2 gauge on the batteries.

The actual waveform from this MSW inverter.

This was my previous setup.  Two 226 AH 6V golf cart batteries hooked in series to give me 12V.  I'm using a Xantrex 10 Amp (dual output) battery charger.  This charger is much faster than a standard battery charger.  It maintains the 10 Amp rate until the battery is almost charged, where regular chargers quickly taper the charge rate down.  It out performs larger chargers because of this.



My Latest Inverter, An 1800 Watt Prosine Inverter (PSW)

These PSW inverters are MUCH more expensive than a MSW inverter!  This one was a factory refurbished model from Ebay.  Got it at a fraction of the MSRP.  It has a removable front display that can either be turned to match the mounting angle, or remotely mounted.

Big lugs on back make for easy mounting of heavy power cables.  I used #2 gauge.

Four mounting holes down each side.

Very clear digital display.

A nice feature of this unit is the Power Saving Mode.  It draws less that 1.5 Watts in the standby mode.  It sends out a pulse every couple of seconds to determine if a load has been applied.  If so, the unit comes up to full power.  You can leave this inverter on full time, and with an automatic transfer switch, put the load on the inverter during an outage.

The pulse is a single sine wave.

A quick bench check to make sure the output is as expected.

The recommended fuse for this unit is 200 Amps.  One interesting thing I learned when researching these fuses is they don't blow at their rating!  They'll handle their rating for x-minutes before blowing.  It may be as much as 20% over this before the fuse blows instantly.  For this reason, I undersized my fuse a little.  I don't plan on running this inverter near capacity very often.  Keep a spare handy as these might be very hard to find otherwise.  I bought the fuse holder and fuses at my local West Marine boating supply store.

Rather clean pure sine wave from the inverter.  You can enlarge this image by clicking on it.  Do the same with the picture to the right.

Less clean power from the utility grid.



I have 4 inverters at home (3 MSW, 1 PSW).  All perform very well, despite the price.  Obviously the more expensive PSW inverter will work the best with equipment designed for sine wave power.  But you don't necessarily have to have such power. 

I'm working on having backup power in my home.  I want to be able to run large portions of my house on an inverter (no 240VAC items).  While a MSW inverter will do this very well, it is less efficient, and can cause a hum on ceiling fans.  I will be running the refrigerator, a number of lights throughout the house (all compact fluorescent), and the bedroom (TV, lights, ceiling fan, cable box, computers), all from the inverter during an outage.  During the day, I'll run everything from my quiet Yamaha EF2400iS generator.

Hurricane season starts in 2 weeks, are you ready?  I wasn't in the past.  My story at this link.  Be prepared, suffer little.  My wisdom came at a price.





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Last updated 05/14/07    All rights reserved.