Is a 3000w inverter too big?
 

I Got this inverter on sale for $175. I know that 3000w requires almost a 300amp amp alternator to get 3000w but will it damage anything if I run it of a weaker electrical system and just watch how many watts I draw from it? I only have a 90A alt and 3 regular car batteries, not deep cycle :( I already have a 1000w inverter that is a good size for my alt but it wont run many of my tools. The most I will ever run off it would be a 15amp chop saw for a few second at a time.

No such thing as a power inverter being too big.

so i wont damage anything? just wont get its full potential?

Right, if you load up that 3,000 watt inverter beyond the capacity of the alternator, the battery voltage will lower and so the inverter will be getting power from both the alternator and battery. At some point the battery voltage will get too low and the inverter will stop itself, and the lead-acid battery would incur some wear. Make sure to charge it up afterwards.
One other thing is that alternators often get too hot if they are trying to keep voltage up at the battery as hard as they can ("Full Field") unless you rev up the RPMs so the alternator can cool itself.

Invest in a cigarette lighter volt meter so you can figure it out

Lead acid starting batteries don't like to be discharged past about 20%.
Once you kill your current plan on buying the biggest deepcycle marine starting battery you can fit or afford.

Hmmm...have you considered letting the alternator generate a higher voltage, and then stepping that down to run the car's electrical system? It would be a pretty involved project, but you'd be able to get a lot more power out of the alternator.

Xantrex Motomaster 3000 6000 Watt Power Inverter AC DC Recharge Booster | eBay is more or less the same inverter only they give more specs, shut down voltage is 10.5v, so if your voltage sags to that point it will shut down to protect the battery, you will see that voltage under load if your battery bank is to small or if the batteries are drained to far.

You are getting a 6000 watt surge and 3000 watt continues, but only 15 amps from each outlet so to get it to 3000 watts you are going to have to have a few things running at the same time.

I agree that you will be hard pressed to find to big of an inverter, most of them are going to burn out pretty quick if you run them at 90% load all the time so this one is over sized just enough to work for pretty much anything you could want to do outside of running your whole house.

Your alternator is not going to keep up with a 3000 watt load but unless like I said, you are running your whole house off this thing you will never have that issue, your battery and alternator will see whatever load you put on it, plug a 3 watt LED in and the alternator will see a very small load, turn on an electric saw and the alternator will see a much higher load and the battery will act as a buffer, you of course will want to see how many amp hours your batteries have and I agree on not using much more then the top 20% of the battery, you can drain it down to 50% if you have to every now and then but making a habit of it is not a good idea because as your batteries age you creep closer and closer to 100% and then you kill your batteries, also, lead acid batteries don't give out as much energy if you are pulling it out quickly.
Important thing in not killing your batteries as well is making sure that you charge them up all the way, as they get full they take a charge much slower, so putting them on a regular battery charger over night is a good idea, the deeper you discharge them the more often you should do this.

thanks! I was starting to worry I would have to stick to the 1000w. I wont use it that often but I like to be prepared :thumbup:

It takes energy just to run the inverter, so running a few watt load like an LED bulb off of it is not practical because the inverter is going to have a base load, but the higher the load the more cooling it is going to need and the less efficient some of the electronics are going to operate at, that inverter is made by Xantrax (bought up Trace a few years back) so they might have specs online some place to tell you how efficient it is at different loads or for different amounts of time.

I have use of a heavier duty inverter that is designed for powering a cabin who's rated output is around 3,500 watts and I think the main difference is that it's designed for a sustained 3,500 watt load so it has better heat sinks, better fans and is more sensitive along with being programmable, a lot of these inverters are also designed to not switch on until they detect a load of a set amount, so don't be surprised if you plug something like a cell phone charger in and nothing happens, I think the one I have can be adjusted to come on with a 5-20 watt base load.
My plan is to build a back up power supply that is built like a small hand cart so it can be wheeled around to where you need power or if the power goes out at home I can plug the fridge or freezer in and power them for about 2 days off the battery pack that I have for my electric push lawn mower.

The larger inverter may have a higher stand by load.

Also if you go with a deep cycle or marine battery you should get a deep cycle marine battery charger and use it to give the batteries a good charge at least once a week.
The Deep cycle marine batteries like to see up to 14.8v when they charge.
Then every month or 2 they should be equalized and have water added, equalization charge goes up to like 16 or 18 volts.