calculating energy usage

[native pepper]

Not sure if this is the right area, here my ignorance of maths comes to the fore. You'd think after so many decades working with low voltage electricity, wired everything from boats, homes, cars and buses, and having spent the same amount of time with some one with degrees in the subject, I'd be on top of it all. But the facts are the opposite, so how do you work out the amount of amps you are using from your energy pack when using an inverter.

We've decided to install split level a/c in the bus, instead of using one of the those portable ones which really doesn't do the job. Going to buy a split system 3.5kw domain inverter a/c, it uses 4.2amp at 910w x 240V. looked at a 5kw, which is only $50 extra, but uses 7amp, 1500w 240v. Need to work out what that relates to in 12v, as that's what the 1700w bus inverter uses. Making a rough guess, it seems ok, but will be running it from a 360amp x 12v gel pack driven by the bus alternator when moving and want to work out how long it will run on the gel after we stop and on lifpepo4 when camping and no access to the grid.

[Wilbor]

Watts are watts. Divide 910 by 12 for 12v amps + inverter losses

[BruceS]

One thing that you must remember is that once the temperature reaches the set level the amount of power used drops.
So if a 3.5Kw one has trouble firstly getting the temperature down (or up?) it will be drawing the 910w for almost non stop.
The 5Kw one will quickly reach the temperature & use minimal amount on the fan until next 'cycle'.
In actual fact it may only use a little bit more than the smaller one but won't be battling as hard.
That make sense? A pity this test between sizes hasn't been done for us?

[Peter and Sandra]

With split aircons the other thing to watch is that most manufacturers use the same sized indoor units for three or four different outdoor units.
So what you need to do is look at the efficiency of a range of split systems to get the best combination of inside and outside units.
I was looking at a range of systems and three units, 2kw, 2.5kw and 3kw all used the same indoor unit and the most efficient combo was the 2.5kw, had the best pull down time and the lowest power consumption. The same applied for the next group of larger sizes, there was a sweet spot where the best combo was.

[Dot]

Would the vehicle's alternator have /produce enough power to keep all batteries charged (house & vehicle) and run the air con? What would happen if it didn't?? (yes I know female Q but I would like to know)

[Dibsy]

I want to put a meter on my system so I can charge della for the power she uses lmdo

Dibs

[jon_d]

4.2amp at 910w x 240V.

42 amps at 910w at 24V
84 amps at 910w at 12 V plus inverter losses. (10%)


looked at a 5kw, which is only $50 extra, but uses 7amp, 1500w 240v.

7amp, 1500w 240v.
70 amps at 1500w at 24V
140 amps at 1500w at 12V plus inverter losses. (10%)


I have a 2.5kw in a 30ft insulated bus. Does the job..... hardest part was working out where to mount the parts.

[native pepper]

Thanks everyone, that makes it easier to work out. Dot, the gel batteries are all that gets charged from the alternator, the house runs on lifepo4 and 1000w solar. I can switch the bus to charge the lifepo4, but don't have the need and when we do, the a/c won't be working. Big difference between 76amp x12v for 3.5kw, compared to 125amp x 12v, 75amps is pushing it a bit I reckon, but will be fine and 125amps a bit over the top. Not even sure the gel will provide that amount constantly, but will find out very soon.

Our bus is 12m, a friend has one the same size and found a 2.5kw not quite good enough, but 3.5kw nicely adequate. He has a domain in his and it works really well for the $650 they are on ebay.

[JohnM]

When I converted the bus I left the factory air fitted, works great when travelling and the bus stays reasonably cool for a couple of hours when stopped, I originally fitted a roof top rattler as well, when I did the conversion, we mainly free camp and found it next to useless, I ended up pulling it out, and fitting a roof hatch in its place, more effective in my view.
I always meant to look at the "Cool Pack" units as used in the Road trains up north, the compressor normally runs from the motor, however there is a separate compressor that can be run from 240v shore power, generator, or in some cases direct drive from a small motor.
We have a full length awning and I fit a full length, shade cloth awning over the drivers side. I find that by shading both sides of the bus, the solar panels on the roof sort of work as a "tropical roof" and opening the windows works well enough when combined with a 12v fan. in most circumstances.

[triptracks]

I have a 5kW in a 29' fifth wheeler. It works great, pulls the temp down really fast and is super quiet. Once down to temp, I turn the fan to low and you hear nothing. It pulls 120A when getting temp down but sips 25-35A when down to temp. It doesn't use any more than a 3.5kW, probably less as it is idling more as a 3.5kW would be working harder.

I have a ceiling cassette, so air is pushed pretty evenly both ends. We often leave the door open and it still cools, just uses more power, so we don't do this on batteries.

I can keep up with my 1500W of solar and LiFePO4

I wouldn't go to a 3.5kW unit, especially if you have a wall unit and it is at one end. You will need the fan running high all the time to keep the flow up.