Size of the Inverter Needed

SuzieQ52 · Member

Size of the Inverter Needed

Plendo · Guru

Suzie, I am afraid your question is a bit like the length of a piece of string, there are just too many variables to give you a definitive answer.

Traditionally you start from the other end of the puzzle, and work backwards. So you work out what is the biggest thing you want to run (lets say it is a 1,200w hair dryer), so this then dictates the size of the inverter you would use (in this example you would need at least 1,500W).

Now you do not mention the size or type of the batteries, but it would be reasonable to assume that they are at least 100Ah, and a lead acid derivative (probably AGM) thus two batteries gives you a total of 200Ah, which will give you 100 usable amp hours.

Assuming the fridge is on gas, I believe you would need to allow 10ah to power everything else, so you would have 90ah available.

When you are running your hair dryer it will draw 5 amps at 240v (1200/240 = 5), thus the inverter needs to produce 5 amps at 240v, in a perfect world this would take 100amps (5*240/12) from the battery, however inverters are not 100% efficient so best to allow an extra 10% so assume 110 amps.

Based on the above your batteries could power your hair dryer for 0.8 hours (90/110) which is 49 minutes.

Now it is likely that you would only run the hair dryer for 5 minutes, this would consume 9.2 of your available amp hours, leaving 80 for other things. You may then decide to plug in your laptop charger that draws 1amp at 240v thus 22 amps from your battery, and leave it plugged in for an hour, thus consuming another 22ah from your battery leaving about 60 amp hours for other things.

Now the above is absolutely full of assumptions, and is intended as a guide only.

I would encourage you to spend a little time working out what you need to run from an inverter, and how long you need to run them for. Then i would work backwards to size the inverter, then check you have enough battery.

Of course the easier solution is to look for alternatives you can power from 12V, that then makes life much less complicated.

I hope it helps.

Phil C · Guru

I like Plendo's answer.. BTW welcome to the forum SuzieQ.

We decided on a large 2500W Inverter. Now we have solar charging our battery bank (2 x 110AH AGMs) and a generator for dark days.

I fitted the 2500W inverter as a "just in case" measure, most times our inverter powers our electric blankets at night around 25 amps on full from the battery. MOST days we will get to 8am and the low input voltage alarm will go on the inverter and shut it down (very handy for AGM batteries).

Occasionally I will charge my laptop or handheld CBs OR get that morning coffee from the nespresso machine (lots of amps) Now, the big deal with this is re charging the battery bank. I have 120W solar that should produce about 9 or 10 amps on a bright warm day. If we assume 10 hours of sunlight the bank should achieve close to full charge.

If I see around 11 to 12 volts, and still charging at 4 pm (my cut off time) I will run the genny till about 7pm, that should be enough for another night. We are rather lucky in that our phones, ipad all charge from USB and have very little effect on the batteries.

The big deal with all of this is making sure your investment in the inverter will do all you expect of it. As Dave has said, think about what you will be your biggest wattage item during free camps (like I have the coffee machine) and the I like to add another 25% to accommodate losses and anything that I havnt thought about (like my battery tool charger). My coffee machine is 2000W thus 2500W is perfect for me. ALSO use the inverter during the day, that way your battery bank has a better chance to provide the necessary charge. ALSO, only have one appliance at a time on the inverter and remember it is 240 volts and will not trip your caravan safety switch if you get hooked up to it. Take all precautions when connecting anything to these machines. Finally NEVER EVER hard wire it into the electrical system of your caravan, unless its done by an electrician and connected to a proper RVD safety switch, all sorts of bad stuff can happen.

Now all that may be a load of bollocks, for some, but it works for me.

Enjoy the great outdoors.

Reed · Senior Member

Agree with Phil and Plendo, I would think that a 2.0 to 2.5 kW PSWI should satisfy almost all requirements. We had a Magnum 2.5 kW PSWI on first rigs and it ran A/C, microwave or fridge nicely. PSWI are more expensive than others but your electronics will not fail.

We use our micro-wave a lot and it takes about 1500 W. Our son put in a Magnum 4.0 kW PSWI in our current rig as absolute overkill since we do run the air conditioner off battery suite/solar at times and that takes 1750 to 2000 W (at startup). Phil suggested 25% margin and this should be quite sufficient.

We have trialed system running a/c (1750 kW after start), water heater (1.5 kW), and fridge on AC and were drawing 3.4 kW for over half an hour. This is not something we would ever do in a boondocking situation but just to prove that the battery suite and inverter did work as designed.

The air pump takes about 1.5 kW as well.

Reed and Elaine

PeterD · Guru

Susie, I will not commit myself to give you advice until I know what you expect to be able to power from your batteries. Can you put your expectations down in a reply so we can see whether your expectations can be met.

The suggestions above are understated. The rule of thumb for inverters is to divide the AC power output by 10 to obtain the current requirement to drive the inverter. Then if your load is not purely resistive you may need to allow a little more. Also if the current you are drawing from your batteries is more than the 20 hour test current (C/20,) you will get less power from your batteries. To work out how much less power you will get from your battery you need to apply Peukert's law. Most of the high current draw figures above will need to be modified using the appropriate equations.

Baz421 · Guru

PeterD wrote:
Susie, I will not commit myself to give you advice until I know what you expect to be able to power from your batteries. Can you put your expectations down in a reply so we can see whether your expectations can be met.

The suggestions above are understated. The rule of thumb for inverters is to divide the AC power output by 10 to obtain the current requirement to drive the inverter. Then if your load is not purely resistive you may need to allow a little more. Also if the current you are drawing from your batteries is more than the 20 hour test current (C/20,) you will get less power from your batteries. To work out how much less power you will get from your battery you need to apply Peukert's law. Most of the high current draw figures above will need to be modified using the appropriate equations.

Good advice from Peter here Susie,,,,, plus the thing to remember is what battery life you are aiming for?????? This is a big question.

You have a lot of theory already given here to think about BUT I'll tell you about ours from a PRACTICAL situation.

We have a 1000W pure sine wave inverter, top quality unit, that has the following MEASURED DC current draw from the batteries -

500w LIGHT the current draw on batteries is 40AMPS

1000W (2 lights) the current draw on batteries is 80AMPS.

Now remember this is not theory it is an in service application.

So the question you need to ask is what will this do to my batteries IF I use it for 2/5/10/30/60 minutes???? Obviously depends on battery capacity AND the maximum current your battery is rated to DELIVER as it easy to overload each individual or individual batteries in a bank.

I hope this helps.

Ask others with 2500-3000W inverters their DC current draw on their batteries and for how long they use the inverter and you will have a better guide.

PS We have a small inverter for VAST receiver and charging laptops/phone/cameras etc if required, we hardly ever use the 1000W although it was used often to run an AC battery charger to truck before I put a solar panel on the truck.

SuzieQ52 · Member

Hi Peter,
We plan to do as much free camping as possible with fishing and golfing the main criteria. The main reason for the inverter was to run the a/c only relatively hot humid days/nights. We have since the first post now upgraded our solar situation to 3 x 150 watt panels and 3 x 120 a/h batteries to help on the inverter size and usage. Probably the the laptop/ TV and maybe phone/ iPad charged up .
I remember reading some where in these forums of a really good place to buy the inverter from some thing like Bits'n Pieces (not sure of the name if some one knows would be helpful.
Cheers Keep Safe

The Doo crew · Senior Member

Suzie

Unless your running an inverter type air conditioner that's uses minimal power you will struggle to achieve your task no matter how many panels & battery's you have.

Roof top air-cons just suck far too much juice!!!

We have a Daiken "L" series inverter split in our motorhome that pulls a maximum of 520watts at start up @ 240volts with a 24v 3000w inverter (far too big for what we need but suitable 24v inverters were harder to find) were running 500w of 24v panels & only just keep up with charging the battery's to an acceptable level only using the air-con when really needed, an hour before bedtime & days that are a real stinker. We mostly free camp & our "house" system also charges when driving from a large alternator.

Everything we run from 12 or 24 volts including laptop, phone, tablet chargers so the 240 volts is just for the air-con. Once the temp & humidity has been reduced the split often drops consumption down to 50-80 watts a very manageable number.

It took months of internet research & then practical testing at home to get our motorhome to a reliable point so we didn't have to break out the generator on a regular basis, in saying that the generator has only 0.8Kva output & can run the air-con & battery charger at the same time with ease if needed on those cloudy or stormy days but I hate the noise & smell when were in a peaceful bush location.

Were located in the NT so this was the most trying environment to test our system, it worked a treat last year in the Pilbara all day long, everyday as they have long sunny days with low humidity, so do the research before you commit to something that will leave you poorer & disappointed.