Inverters, do you know...

[dpe.ahoy]

I'm pretty dense when it comes to electric stuff, just know enough not to stick a knife in a wall socket.    If I use a 1000 watt inverter but only plug in 200 watts worth of draw, will it only pull that much out of the battery bank, or will it pull more out than is needed, say 5-600 watts of juice to power the inverter?   DP

[JustaShooter]

The inverter should come with an efficiency rating, say 80% or 90%.  So, for a 90% efficient unit you would expect to pull 1000 watts for 900 watts load.  Unfortunately, those efficiency ratings are not constant, it is a curve that starts low and goes up to rated efficiency quickly (say around 20-30% or rated load) then levels off for a bit and then declines slowly.  http://www.solar-energy-at-home.com/solar-power-inverter.html has a nice typical efficiency curve.

So, for a 90% efficient inverter at 20% load, you might lose another 5%, making your 200 watts draw pull something like 235 watts out of the battery.

Hope this helps.

Just a Shooter (and occasional electronics enthusiast)

[Lloyd Smale]

just a shooter hit it on the head. Inverters are very inefficient. Lots of loss just running one. Just consider that your running the cooling fan in one constantly and how long your battery would run one with even no load. check invertors when you buy them and if you dont understand buy one from a local electrical supply company where theres someone who does. there can be big differnces in effecientcy between brands. The cheap 50 dollar ones are about junk.

[BBF]

The other big difference in price depends on how true of a since curve you want or need.
Beg borrow or abscond with a WEST MARINE catalog and read up on inverters.

[vacek]

As far as I am concerned inverters are at their best running off a car battery while the car is running.  At my job we have a demo trailer for Lunch and Learns at various industrial plants.  We have an inverter off the truck battery to run some lights and equipment.  In this case I'm not overly worried about energy waste.
 
As to the original question, I'm not sure if you are pulling 200 watts on a 1000 inverter if indeed then you are only doing a partial load.  Interesting question.

[efremtags]

All inverters require some power called housekeeping power just to control themselves and keep cooling systems active. This power is proportional to the inverter size, figure 5W per 200W of inverter rating, so a 1000W inverter will require 25W of power to do nothing. This is part of the reason the efficiency at light load is so bad.

decent inverters will run 90% efficiency at full power, and will run close to that down to about 40% of its rating. So at 200W, (for a 1000W inverter) an inverter might only run 75% efficient. That means the battery draw will be 250W.

modified sine inverters are fine for most things. I would avoid runing large appliances on them like a fridge, it could damage the motor on startup. Most other appliances don't care on bit

And never use a car battery with an invetrer unless the engine is running. The battery could be over discharged an damaged easily, as starter batteries are not designed to be severely discharged. 

[dpe.ahoy]

Thanks for the answers guys.  I want to put one of the small friges in the cabin to use while I'm there.  Will be hooking up 90 watt of solar and thinking of a small wind generator made with treadmill motors as discussed in this section.  I need to get with Matt and pick his brain again and see if he is still doing the "brains" for that setup.  DP

[BBF]

I know there are small 12 Volt DC freezers on the market. I would be surprised if you couldn't find a  12 Volt fridge
 
I had a very small ( 500 watt) unit that plugs into a cigarette outlet on my boat to power a laptop and printer. They did not require 500 W to run together however the Inverter kicked out every time the printer started presumeably because of the motor start up surge.

[vacek]

Geez ... If its a fridge you are trying to run, then go to a used RV place ( there is a huge one in Denver called Cherokee) and get a 12V/propane fridge from a wrecked RV.  Run it on propane most of the time.  An inverter off of a deep cell to run a fridge just doesn't really add up with the other options.  Running a fridge means 24/7 while you are at site.  Go with propane.

[Lloyd Smale]

good advice. A 12v freezer or fridge would be a heck of a load for batterys and an invertor. Buy a gas one and just haul a 100 lb cylinder or two out there.

Geez ... If its a fridge you are trying to run, then go to a used RV place ( there is a huge one in Denver called Cherokee) and get a 12V/propane fridge from a wrecked RV.  Run it on propane most of the time.  An inverter off of a deep cell to run a fridge just doesn't really add up with the other options.  Running a fridge means 24/7 while you are at site.  Go with propane.

[dpe.ahoy]

That was my first thought, but around here the RV dealers all buy em up for the used units that go out they have on their lot.    I have propane there already, that was why the first choice was an RV unit.  Guess I'll keep looking, they are way over priced on used units, 2/3-3/4 the price of a new one.    I can't find out how many watts a compact frig uses, the ratings are in amps on the tag.  Have seen some close to one amp, others just a bit over.  I'll get on line and look a little harder, I may just try to do everything in propane.  Just gets pretty warm in the summer running propane lights,helps in the winter.  The propane frig will put some heat in the cabin as well.  I know it needs vented to the outside, but am sure some heat will be retained inside.  Combination of 12 volt and propane is the best route.  Maybe I'll get it finished before I die, maybe I won't .  Still my get away , better than my wall tent.    DP