Do I need a generator?
By Wolfe Rose
In my last column, I explained why there just isn’t enough power stored in your batteries to run a sizable inverter for long. The amperage draw is damagingly aggressive, and it’s not realistic to carry the pickup truck full of batteries necessary to replicate an off-grid-house-size battery bank. Since you’re not creating electricity, you’d still need to add windmills or huge solar arrays to recharge the batteries in any reasonable manner. Some long-term boondockers do exactly that, collecting “green” energy and then using it conservatively.
The only way to realistically charge batteries to sustain a large inverter is with a multi-KW generator, but that makes the inverter mostly moot because of running a generator. Helpful here is that you already have a large 12V generator – in the form of your alternator and running propulsion engine. It’s still a fueled generator, but you’re driving down the road anyway, so the alternator runs your 12V loads, recharges both the starter and house batteries, and powers the inverter “real time” to drive your 120V loads. But that still all stops when your engine stops if you don’t plug into shore power.
Traditional generators are really just a smaller dedicated version of your engine/alternator combination. An engine burning fuel (gas, diesel, LP) spins a dynamo, producing electricity. Traditional generators do this by spinning the dynamo to directly produce AC, burning even more fuel to maintain that speed when a heavy electrical load slows the dynamo – which can’t slow down without generating lower voltage and a slower AC frequency. While the smaller engine is more efficient than idling your 600hp propulsion engine, it’s still terribly inefficient because the engine maintains a fairly high constant speed. The engine can’t scale itself to the load, so we effectively have Atlas carrying a pebble around, waiting for a boulder.
Generators are literally 0% efficient when running with no load, but a lot of efficiency can be gained if they could at least run slower when less wattage is demanded, and buffer just enough extra power to get the engine back up to speed without browning out when it is demanded. “Inverter generators” (IGs) solve these issues by not trying to produce AC directly. Instead, they copy your alternator by generating DC power, and using that wattage to drive an inverter which can scale better. While this extra conversion is still inefficient, the engine can run much slower at lower loads, producing only slightly more power than the wattage demanded. A variable DC voltage is produced, but compensated by the inverter, which also creates the correct AC frequency. The engine can idle quietly sipping fuel until demand justifies throttling itself up. At full load/throttle, fuel and noise are comparable to non-IGs, but IGs don’t blindly stay there. Since electrical demand often stays under 50% for long periods, some people manually scale even farther by selectively running two smaller IGs.
With diligent shopping, the popular Champion 4KW traditional generator runs $250. An affordable 3.5KW Predator IG runs $750, about the same price as the popular Honda or Yamaha 2KW IGs. Since good inverters are expensive, IGs are harsh on the purchase budget, but your ears and fuel budget will thank you longer term.
[Editor: Here are portable RV generators at Amazon.com]