T&T: How Does an Inverter work

[Ken Bloomfield]

Hi Lori,
As already posted, the inverter draws from the 12 VDC battery bank and 
converts to 120 VAC.  This can be done in a number of ways, too much for 
this listing, but the main categories are (a) square wave output, 
(b)modified sinewave output, and (c) true sinewave output.  The last most 
resembles the 60 Hz waveform that shorepower gives, but is the most 
expensive.

Since you go from 12 to 120 volts, neglecting efficiency issues, you draw 10 
times as much amperage from the battery as it delivers on the 120 VAC side. 
This means that there is no magical energy produced, but rather a balance. 
For example, if you put on a coffee-maker rated at 1200 watts, then it needs 
10 amps at 120 VAC (watts = amps x volts), and the inverter will have to 
draw 100 amps from the battery to supply this.  (Again, 100 amps x 12 volts 
= 1200 watts).  Actually, it will need a bit more as it is not 100% 
efficient.

So, to your questions.

1.)  First of all, remember that it draws very little if nothing is being 
"asked" of it.  If there is little or no load, then there is little drain on 
the battery.  The no-load standby current from the battery is probably 1 amp 
or less.

2.) Whenever you want to run something or several things at once on it, the 
wattage of those items must not exceed the inverter rating.  Virtually every 
appliance is rated in wattage.  Try running more watts than the 2500 and it 
will normally "fold back" and cut out to preserve itself from damage. 
Example might be coffe-maker at 1200 watts and hair-dryer at 1200 watts, and 
you are about loaded up.  In fact, I would normally never run two BIG loads 
like that at once on mine, but rather do them in sequence.

3.)  When you know how many watts you are running, (i.e. my example of the 
coffeemaker with 1200 watts, hair-dryer is similar) then you can calculate 
the amperage that the inverter is delivering at 120 volts AC by simply 
dividing the wattage by 120 (the line voltage) so in this example 1200 watts 
div. by 120 volts = 10 amps.

4.) When you know how much it is delivering, it is close enough to just 
multiply that amperage by 10 to see in turn what your battery bank must 
deliver. (i.e. AC 10 amps x DC factor 10 = 100 amps).

5.) Your battery bank will be rated in amp/hours.  This is a bit complicated 
as it varies depending on the rate that the amperage is "taken", but for 
first approximation the following will be given as example.  I will assume 
you have 4 golf-cart batteries.  These I will assume are wired as 
series/parallel.  This will give you about 440 amp/hours total capability.

6.) Since you should never discharge your batteries below 50% of full charge 
(it hurts them) you really have 440/2 = 220 amp/hours available.  So, you 
could run the coffee-maker for 2.2 hours (actually considerably longer since 
when it gets to temp after brewing it cycles on/off) so rather lets say you 
could dry your hair for 2.2 hours (might have the frizzies, but what the 
heck).

This ought to give you an idea of how to predict how long the system can 
stay up.  Likely, if your system is like mine (I have a normal household 
fridge) then you will have about 3 to 4 amps at 120 VAC running (about 50% 
duty cycle, i.e. on/off) so there is 360 to 480 watts right off the bat.  Go 
to http://tinyurl.com/4j73s3 for a nice PDF explanation that is more 
in-depth.

Hope this is as clear as mud, and helps,
Ken.



> Hi all. I recently bought a 37ft Roughwater, my first boat, that I am 
> living on. I have lots o questions but esp how the inverter works? When I 
> am anchored I cannot quite figure out (its 2500watts) when and how it 
> drains? What load I can put on etc.........any help??
>
> Lori