Chapter Six - Inverters

The ability to operate power tools and appliances from household power can make life aboard a house truck or bus more pleasant, and allow you to accomplish tasks that would otherwise be impossible while traveling. An inverter is an electronic device which converts direct current battery power into alternating current very like that which flows from utility grid outlets.

Modern inverter technology is durable, efficient, and relatively inexpensive. Choosing the correct inverter for your requirements will help insure that you are getting the best value and satisfying your needs. Inverters are available in many output power ranges, with a variety of options, and are manufactured in several distinct types.

Selecting your battery voltage is the first step in choosing an inverter. Most installations will use 12 volt battery power, although inverters over 2,000 watts will benefit from a higher input voltage, perhaps 24 volts.

The output voltage and frequency of the inverter should match that of the appliances you intend to operate on it. In the US, this is 120 vac/ 60 Hz (Hertz). Nearly all manufacturers offer European specifications of 240 vac/ 50 Hz, and custom voltage and frequency inverters are often available by special order.

Specifying wattage can be a bit tricky. Buying the largest inverter you can afford might seem the best track, but remember that a large inverter running only a small load will consume a disproportionate amount of power from the batteries due to poor efficiency at low output. The same inverter may have better efficiency when operated between 25 and 75% of it's rated output.

Nearly all inverters will power a load larger than their rated output for a reduced period of time. The components and construction are such that the inverter must handle overloads as a matter of starting motors, etc. This may allow a 600 watt inverter to run a 1,000 watt load for several minutes without damage. The continuous-duty rating of the inverter remains 600 watts, but the surge rating might be 1,000 watts for 5 minutes. It is important to inquire about these ratings when purchasing an inverter, as you may find that you need only buy a smaller inverter for your daily usage, and that the surge rating will take care of occasional overloads due to unusual circumstances.

Two varieties of inverter output wave form are currently available, modified-square wave (sometimes called modified sine-wave), and true sine wave. True sine wave inverters produce power that is exactly like utility grid supplied power. They are superlative for powering all types of loads, and may be necessary on sensitive electronic instruments. Efficiencies of sine wave inverters are slightly less overall than modified square-wave.

Modified square-wave inverters (MSW for short) approximate the wave form of utility grid power, and the better ones use circuitry to adjust the wave form to best power the load at hand. The advantage of MSW inverters is that they are relatively inexpensive and quite easy to acquire. There can be serious disadvantages though. MSW inverters aren't a good choice for running induction (capacitor-start) motors, and some cordless tool battery charger systems malfunction and burn up when fed MSW. Additionally, MSW inverters are electrically "noisy" and induce "hash" or interference into audio equipment such as telephones and stereos. Having said this, I must be quick to say that I have both types of inverters in my system, and I find the MSW inverter's performance quite satisfactory. Just be aware of the issues at hand when purchasing.

As for options, most inverters these days will include a 'stand-by' option, which switches any loads that are connected over to the grid when utility power is available. Nearly every type of inverter these days has a 'sleep' feature, which turns the inverter's circuitry off when no appliance is being used, and then automatically turns the inverter on when a power demand is sensed. This saves quite a lot of power over running the inverter full time, and saves the user from having to manually turn the inverter on and off to operate loads from it. Built-in battery chargers, some which are quite sophisticated, are commonly standard equipment on inverters. Low battery cut-out, short circuit protection, thermal cut out, over current protection and other safeguards should also be standard on any inverter you seriously consider.

Some of the more expensive inverters use internal microprocessors to allow user-defined parameters to be set, and sine-wave inverters may have the option of back feeding the utility grid with excess power from your generating sources. This is called utility inertie.

Some inverters have communications ports which allow them to be controlled by and send data to a personal computer. Many manufacturers also offer digital metering systems which can be integrated into the setup to give readings and control the system remotely.

Even an inexpensive inverter the size of a box of kitchen matches can prove itself to be quite useful on the road. Be sure to include a place for an inverter of some description in the plans for your housetruck's electrical system.