Inverters For Your Truck
Updated August 4, 2006
I just would like to add that the free lunch has in fact yet to be invented to quote a noted economist recently ! Inverters have an approx. efficiency of 90% for the *best* designed ones (Cheap imported ones are less-forget the vaporware claims on the ads) operating at at least 66% load. At lighter percentages of load the efficiency drops markedly from there. When I hear of these stratospherically large inverters like 3000 watts or 2000 watts I smile. Divide 3000 by 10 roughly and you will come out with the max load on your batteries. Do you know how big a cable you need to carry 300 Amps properly. It looks like a garden hose. If you use smaller cable like 3 0 or so you will introduce more losses in the cables at that current. The cables would have to be not longer than around 6 feet to be efficient. The losses can be read in the Nat. Elec. Code Handbook. To imagine a simple example of this load picture cranking your engine with the starter continuously. How long will that go for.
The point of all this is that your truck electrical system is good for around 1200 watts max if everything in the truck is shut down and all power is provided to a power inverter. Any larger inverter than that is a waste of money and effort unless you are in need of high starting load capacity, such as an electric impact gun which runs very short duration at perhaps 2000 watts max startup. Then the large size inverter may be justified, but take great care in installing it with short very heavy cables. This will prevent extra waste due to poor efficiency. If you need 2 or 3 KW continuous you should consider a small generator of one of the Aura Humvee mounted power generator systems the sell to the military. Just remember the power formula of volts times amps equals watts, and the Hummer only has around 120 amps with the engine running at 1200 to 1500 RPMS at 13 volts average.
Wattage is a measure of power, regardless of voltage. The electrical formula is W=I*E, or "Wattage = Current (in amps) times Voltage".
12 volts at 100 amps = 1200 watts
120 volts at 10 amps = 1200 watts
Put another way, a theoretically 100% efficient invertor (which doesn't exist) that could put out 1200 watts at 120 volts would require an input of 100 AMPS at 12 Volts. That same theoretical invertor would require 200 AMPS at 12 volts to deliver 2400 watts of output. Now remember that this is with a perfect, 100% efficient invertor.
To deliver 3000 watts of continuous power with a 90% efficient invertor would require a CONTINUOUS supply of 12 volts at almost 280 AMPS!!!! That's more than double what the alternator can provide, so you'd be drawing down the batteries every time you used it at full power. If you were to provide this same wattage from a 24volt system you would reduce the current by half - to about 140 AMPS. The only problem with this is that the military alternators only put out about 65 amps. You're still drawing down the batteries with each use - and they won't last long at that draw.