Inverter Detailed Selection and Sizing

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DETAILED SELECTION AND SIZING--The size and type of inverter depends on:

Continuous running wattage of all loads to be run at one time. List all items you'll run at one time. Add up Watts. (From labels as W or VA, or as measured with an ammeter, or calculated as Watts = Volts x Amps, so 120VAC x 5A = 600 Watts.) Examples: Computer 160W, Monitor 40W, Printer 110W could operate from a 300W inverter. But, if total is near the max rating of the inverter, look closely at inverter manufacturer's data sheet. Some will state, "200W for 25 minutes, 140W continuous" or similar. (Not bad if drilling a hole. Terrible if running a computer.) With microwaves, don't mistake cooking power for actual running power. Read the label. A 500W cooking-capacity microwave might use over 1,000 watts actual power.

Surge power needed to start heavy loads. Some tools need heavy jolts to get started, then less electricity after they're running. (Pressing a drill hard and then hitting the trigger, for example.) In the computer example above, items can be turned on one at a time. A large TV with built-in VCR can't. Everything comes on at once, but after the VCR motor settles down and the picture tube warms, all is well. If the inverter is just slightly undersized, often it can be turned on first, without a load. Then the TV/VCR can be turned on. If it goes off after a second, repeated attempts will gradually "warm" it. This is hard on the equipment. Better to buy a larger size inverter. Again, a good inverter's literature will state something like,"200 watts to 140 watts continuous." (Almost no inverters have enough surge power to start a fairly-good sized air compressor.)

Quality of power needed. Most of the new, sophisticated inverters provide a "quasi" sine wave that is remarkably close to perfect and suitable for almost any application except exotics like a laser printer. Cheap, square-wave inverters provide square TV pictures. (There ain't no such thing as a free lunch.) Still, many household appliances (blenders, sewing machines) will run from a square-wave inverter if that's all you need. On the other hand, don't be penny wise and pound foolish. That cheap $75 inverter is electrically inefficient and severely limited. It's going to waste a lot of valuable battery power just running itself. A $130, 200-watt, hi-tech inverter will do much more and do it efficiently.

Efficiency. A measurement of how small an amount of electricity an inverter uses just to run its own "innards." If you're just going to run a drill a few minutes, you don't care. If your TV, VCR, computer is going to run for hours, this is a prime consideration. Some of the quality inverters mentioned idle at less than one-tenth of an amp and are well over 90% efficient. You can leave them running 24 hours a day.

Bells and Whistles, Options, Safety Features. Quality inverters in medium to large sizes will have several features: Remote switching (some with monitors) so you don't have to go outside to turn them on. Low voltage warnings--to alert you it's going to shut itself off pretty soon. Circuit breakers. Monitor lights--so you don't leave it on and run down your battery. These are important. A few inverters have automatic load switch protection that keeps them from being energized if there is already 120VAC in the lines (from commercial power or generator). Plugging an inverter into a "hot" circuit will destroy the inverter in seconds (and they won't honor your warranty in such cases). Battery chargers built into the larger inverters are another worthwhile option. No, they don't use a battery to charge a battery. They simply use some of the existing circuits in the inverter (that are far superior to those in an RV converter) to operate a very sophisticated battery charger (that is also far superior to anything in a standard RV converter).

phred Tinseth © 1998-2000 Reproduction Permitted