One Inverter or multi

[drew.ward]

I'm going through diagrams in my head just to get an idea of what my electrical system is going to look like. I plan on charging the batts from solar and wind (and the alternator when motoring or shore power when hooked in and needed).

But one thing I was thinking is when it comes to power usage I don't have so many things that need AC. I mean technically most things do.

But I will be running a small fridge and a 5 cub ft chest freezer, occasionally a microwave or coffee maker, and sometimes a TV and the computer.

What I'm wondering is would it be more logical since most everything else is going to be 12v DC to have a few of those smaller car/rv invertors where I need them (and matched to the appliance) versus having one big inverter and wiring up a separate AC system throughout.

My thinking is this:

1. Inverters use power when inverting, but not when disconnected. It's my understanding they still draw some power even when you aren't actively using the juice.

2. Isn't DC wiring more efficient in short distances like on a boat and has less current loss?


On a slightly different topic, I have also wondered if it's possible to condition the DC current so that some of my regular appliances can run straight off DC -- basically figure out what they need (usually 12v) and remove their AC to DC transformers and run straight DC to them.

It just seems like you'd be wasting electricity going from DC to AC then letting your appliance take that AC and make it DC internally again.

[David M]

Inverters that are idle as in not powering anything do draw some power. But, they can also be turned off completely so they do not draw any power.

AC is a more efficient carrier of power over great distances which is why high voltage wires are stepped up AC. DC is less efficient but for the distances found on a boat, irrelevant. The key to having an efficient DC system is to not under guage your wires. Underguaged wires produce more heat which of course means wasted energy and can also start fires under extreme circumstances.

You can have two inverters on a boat but you cannot run two inverters on the same circuit simultaneously. The phases for two different AC sources must match. As far as I know, there are no marine grade AC inverters for boats that can match phases although there are large industrial inverters and those inverters that can match phases such as those used on a house to match the phase of the grid.

There are a few books on 12VDC and 120AC systems specifically for boats. I think that would make a good investment in helping you design your AC and DC system. Look for books by Nigel Calder on Amazon.

You can power 12 volt DC devices from the 12 volt DC onboard your boat. Be cautious though because most power supplies that convert 120VAC to 12VDC also prevent voltage spikes (or what some call "power surges" which is sort of a misnomer because it has nothing to do with watts which is a measure of power). A boats 12 volt system is filled with devices that can cause voltage spikes that could wipe out one of those devices. So be cautious.

[AnchorageGuy]

We have two inverters on Sea Trek. One is wired to our main outlets and can power most anything on the boat we need to run from an outlet. But the TV, DVD recorder and Sat receiver is powered off of a 300 watt inverter that is dedicated to just these items. It works well and we can use the TV, etc while at anchor without energizing the whole boat.

Voyages of Sea Trek: Preparations Are Coming Along 4/14/08