Inverters pull lots of Amps from Batteries

[WooferHound]

Many newbies are very surprised when they find out how much power is pulled from batteries with inverters. Here is an example.

You are running an inverter from a 12 volt battery and supplying 120volts to a TV that is pulling 120watts. So 120watts divided by 120volts equals 1 amp to power the TV. The inverter is putting out 120volts and the TV is using 1 amp of current. To go from 12volts up to 120volts the amps get multiplied by 10, so the 1 amp pulled by the TV at 120v will be 10 amps coming from the battery at 12volts. Any one battery will not last long when 10 amps is being drained out of it.

Many popular inverters are 2000watts and 3000watts. Lets look and see what they would pull from a 12volt battery if being used at full capacity.
2000watts divided by 120volts equals 16.7 amps times 10 equals 167 amps from the battery
3000watts divided by 120volts equals 25 amps times 10 equals 250 amps from the battery
This kind of power will kill a battery in a very short time and would need a substantial battery bank to supply those kinds of loads. Not to mention the big copper wires to carry all those amps to the inverter.

Now for the bad news, inverters need to use power too. They need some current to power the electronics that do the work of raising the voltage and converting it from DC to AC. This is in addition to the power needed to run your 120volt appliance. This is called the efficiency. Inverters run about 90% efficient, but under certain conditions can be as low as 80% efficient. It's been said that an inverter is the most efficient when it is operating at 1/3 of it's labeled capacity.

So we lose 10% of the power when it goes through an inverter, this changes the numbers a little bit. Instead of multiplying the 120volt amperage by 10, instead we multiply by 11 to account for the 10% inefficiency of the inverter.
The 1 amp TV is now pulling 11 amps from the battery
The fully loaded 2000watt inverter would go from 167 amps to 184 amps
The 3000watt inverter would go from 250 amps to 275 amps from the battery

So if you want to use an inverter, you will need to plan your usage carefully in order to take care of your battery properly.
If you already use an inverter regularly, then you will already know all of this.

Note - If using 24volt batteries, it would be half the amps from the batteries
If using 48volt batteries, it would be 1/4 of the amps from the batteries

[birdhouse]

very good info woof! 

the wire/fuse size between the battery bank and inverter is very important.  you see it all too often, especially on youtube, when some guy has a 1500w MSW inverter pulling off a 12v bank with #10 wires.  i think this is a huge factor when the inverter shuts down before its rated capacity. 

i run a 1800 PSW xantrax from a 24V bank.  used a 150 amp ANL type fuse with 3/0 copper between the battery bank and inverter.  works great! 

adam

[rossw]

To go from 12volts up to 120volts the amps get multiplied by 10, so the 1 amp pulled by the TV at 120v will be 10 amps coming from the battery at 12volts.

Nice post Woofer.

Another way of working it (that I personally find easier and more consistent) is to just think of watts throughout the calculation.

120 watts of load (in a perfect world) would need 120 watts of input power.
If the input is 12V, thats 120W / 12V = 10A
or 120W / 120V = 1A
or 120W / 48V = 2.5A

Like you say, inverters are not 100% efficient. If yours is (say) 90% efficient, that's 10% waste. To make 120W out, you need 120W + 10% input... so sticking with the "thinking watts" thing, it's   

120 + 12  = 132W / 12V = 11 amps

Whichever method you choose to use, remember to use some common sense. You can't make power from nothing. Nothing is more than 100% efficient. If you come up with an answer that (for example) means you calculated you need 5A at 12V (5*12 = 60W), and your load is a 1kW kettle, you KNOW you've done something wrong!

[ChrisOlson]

Note - If using 24volt batteries, it would be half the amps from the batteries
If using 48volt batteries, it would be 1/4 of the amps from the batteries

Like to point out something that most people overlook.  I'll leave inverter efficiency out of it to make it simple.

Let's say you have four batteries hooked in parallel on a 12 volt inverter.  The draw on the battery bank is 1200 watts at 12 volt nominal.  This means the batteries have to deliver 100 amps to make the 1200 watts.  Each battery delivers 25 amps of the total load.

Hook the same four batteries in series/parallel for 24 volt.  Apply the same 1200 watt load.  Total amp draw is 50 amps @ 24 volt nominal.  Each parallel group delivers 25 amps of the total load.  Each battery in each series string delivers 25 amps to meet the load.

Hook the same four batteries in series for 48 volt.  Apply the same 1200 watt load. Total amp draw is 25 amps @ 48 volt nominal.   Each battery in the series string has to deliver the full 25 amps of the load.

With any given battery bank, whether you have 12, 24, or 48 volt system, the amount of amps each battery has to deliver to meet a certain load is the same.

Edit to add another piece of information for people new to inverters - the efficiency of a power inverter is not constant.  It varies with load.  This is a scan of the efficiency curve for the inverters we have powering our house.  If you study it you will see that the inverter is least efficient at very light loads, and most efficient at 500-1500 watt load.

Since our typical "normal" loads average 2-3 kW in our house, we have dual inverters "stacked" in a 120/240 split phase arrangement with a load balancing autotransformer that balances the load on the split phase service so each inverter is delivering identical power to the loads.

It is important when you purchase an inverter to power your home to know what its power efficiency curve is.  Bigger might seem better.  But if the inverter is too big and you operate it on the lowest point of its efficiency curve from no-load up to, in this example, 250 watts most of the time, you are wasting a lot of battery power operating your inverter at very low efficiency.  You would be better off with two smaller inverters "stacked" in a master/slave configuration where the primary (master) inverter carries the load, operating at it's peak efficiency curve most of the time.  When the load increases to where the master inverter starts on the downside of its efficiency curve, it "wakes up" the slave to help out so both of them can meet the heavier load with both operating at or near peak efficiency.


--
Chris

[Wolvenar]

@ChrisOlson
This is an interesting addition that I have not heard a lot about.
I am curious how well this setup works for you,
it might be something I consider in the future.
 
What is the loss of the "autotransformer" in your system?
Do you have figures?
Is your load in a sweet spot area enough of the time to make up for the
losses from this additional hardware, or are the losses so low its negatable in almost all installations?

 

[ChrisOlson]

@ChrisOlson
This is an interesting addition that I have not heard a lot about.
I am curious how well this setup works for you

The only way I can describe how it works is "awesome".

What is the loss of the "autotransformer" in your system?

It's a Xantrex T240 and I don't know the exact figures.  But I think it's pretty negligible.  Our house is totally electric - electric range, electric clothes dryer, electric 240 volt water heaters.  We have everything electric because my wife has a phobia about having any sort of gas line coming into the house.  She won't have it.  The 240 volt loads are always leg balanced so those are taken off ahead of the transformer.  The inverters put out their sine waves 180° from one another for the 240 split phase power.  The transformer is used in "stepdown" configuration where the the 240 output of the inverters is stepped down to 120 VAC and this keeps the loads on the inverters perfectly balanced at all times.

I got the master's Peak Load Management programmed to bring the gen online if the load on the inverters goes over 20 amps (4.8 kW) for more than 2 minutes.  They'll normally run the clothes dryer, or the electric range without the gen coming online.  But if my wife turns on more than one thing, or I'm doing welding in the shop continuously, then the gen will start, the inverters switch the load over to the gen and the inverters "help out" with any load above what the gen can deliver.  So, basically, we have a 6 kW gen and 8 kW of inverter power, so we have a 14 kW service in our house.  When the load goes below 20 amps the master watches it and if the load stays below 20 for five minutes it then looks at bank status and if the bank needs charging it does a maintenance charge on the bank.  If the bank status is still good it just shuts it off without charging the bank.

So I dunno.  We like it.  Haven't had a single problem with it since we put the new system in last year.

I theory, you could do it without a load balancing transformer.  But then the master inverter might be carrying more load than the slave.  And if the gen comes online, the legs on your gen are also unbalanced without a transformer and you would never be able to get full capacity out of your system when needed.  If the water heater is going and pulling 4,500 watts, plus my wife plugs in her confounded infrared cooker thing on 120 volt that draws like 1.5 kW, then you got unbalanced load with one inverter delivering 3.75 kW and the other one at 2.25 kW.  Now - my wife goes all out and plugs in a roaster or slow cooker on 120 volt, plus the 'fridge starts and the well pump starts.  Your master inverter kicks out due to overload.  With the transformer, the load stays balanced and the inverters never kick out.

Personally, I wouldn't be without it.

Edit:  the other option for 120/240 split phase is to get a Xantrex XW, which has load balancing built in.  But we wanted 14 kW service (with the gen) and got the SW Plus inverters brand new, plus the T240, generator start and aux load modules, remote control module for the master, AC Conduit boxes, and the whole shebang for a better price than two new XW's with all the bells and whistles.  The XW's are a little over $4 Grand each with the AC panels and gen start.  We got our setup for $6,500, and the SW-series inverters have a reputation for being damned near indestructible.
--
Chris

[Watt]

Here is a link on Outbacks transformer and wiring.  Might help understanding a bit about how they work for some of us. 

http://www.outbackpower.com/pdf/archives/fw-x240_rev_b.pdf

I can't get the thing to finish loading but had it saved on my favorites. 

[DBCollen]

I thought the problem was with my internet connection. I have spent the last 3 days trying to download the flexmax 60 and Mate manuals from outback, no luck, gets part way through the download and stops with an IO error. Finally downloaded them from other sources just fine. Outback must be having trouble with thier servers.

[RichHagen]

I downloaded the flexmax manual in pdf format from their site to review for possible use in a project I am involved with a few days back without incident. (except for the 150VDC (- deregulation) open circuit voltage limit in the manual) Rich