Conecting to 120 volts AC

If you want to get more precise, figure out everything in terms of power (watts).

Basic electrical rule 1, 2 and 3:

voltage x current = power

or re-arranged:

current = power divided by voltage

or re-arranged:

voltage = power divided by current


For example, 12V X 2 amps = 24 watts.

or another example, 400 watts divided by 120 Volts = 3.33 amps

A 55W headlight that uses 12V would draw 55 /12 = 4.6 amps @ 12V

A 55 watt light bulb in a lamp at home would draw 55 / 120 = 0.46 amps @ 120V


As the previous post mentioned, inverters are not perfect when convertering 12V into 120V. If the converter consumes 1000W from the 12V battery, then a 90% effecient converter would generate 900W of 120V AC power best case. The other 100W is lost primarily as heat.

The other thing that gets tricky is that these ratings and the formula above are used for resistive loads, like light bulbs or hair dryers. Anything with a motor or transformer is considered an inductive load and can get much more tricky to calculate.

Consequently you need to give your self a safety margin when figuring out how big an inverter you need.

How does work in a practical sense?

Lets say you want an inverter for TV, DVD and Sat. Receiver. Look at the back of TV or in the manual. It should say how many watts it consumes. Lets say it is 400W. The DVD might be 100W and the Sat. receiver 50W - just as an example.

400 + 100 + 50 = 550 Watts. (just as an example)

You might think, well no problem, I'll use a 600 Watt inverter and have 50 watts left over. Depending on your inverter, that 600W might really be 600 x 90% effecient = 540 Watts of AC, less a 20% margin of error for the inductive transformers in the electronic of the TV, DVD and Sat. receiver 540 - 20% = 432 Watts.

Now you can see your 600 Watt inverter isn't big enough to do the job.

If we really need 550 watts of AC, add 10% to make up the effiency loss, then add a safety margin for inductive loads.

550 + 10% = 605 + 20% = 726 Watts.

Sounds more like an 800W inverter fits the job.

What does that mean in terms of wiring the 12V batteries to the inverter?

from the formula above:

current = power divided by voltage

In our example, we have an 800W inverter that runs on 12V

The current would thererfore be:

current = power divided by voltage
current = 800 watts divided by 12V
current = 66 amps.

That is important info because you can not use light gauge wire to carry 66 amps worth of 12V to the inverter nor could you use a 20A fuse to protect your inverter.

Now that's a lot of science for a guy who just wants to run a toaster on an inverter right?

800W / 120V = 6.66 amps

Using garryp's ratio 11:1, 6.66 x 11 = 73 amps.

That is a good ratio with a good safety margin.

This is all just MHO and should not taken as solid technical advise. In other words, don't blame me if you blow yourself up.