Inverter and Battery Sizing.

[Orion]

Hoping that some of the more electrically literate members can help out with this as I’m trying to get a handle on the optimal combination of inverter and Lithium batteries to aim for to satisfy our (modest!) needs.

This is what we have installed currently and what we would like to possibly run off-grid:

There’s 340w of solar on the roof and no possibility of any more. Fridge is 12v compressor and we have LED lighting throughout. The existing 200 Ah of NDS Greenpower AGM batteries, (2x100 under passenger seat) will, even in winter with minimal solar input, give us a good 3-4 days running with no external power source.
There’s decent Nordelettronica charging and 40amp B2B components that can handle different charging profiles, as can the NDS solar controller.

What I’d like to do initially is install a Renology inverter that can allow us to run the Truma Aventa aircon unit when off-grid in summer abroad - this doesn’t need to be in full compressor mode as with just the fan running, in conjunction with the low-level fanned ducts in ventilation mode, it’s sufficient.

Consumption figures for the Aventa unit helpfully provided by a fellow forum member are:
Low fan 4 amps
High fan 9 amps
Compressor 45 amps (intermittent and for interest only)

So, bearing in mind our current equipment level, can we:
1) get away with a 2000w inverter (my preference due to space constraints) or is a 3000w unit advised?
2) what would be the advised total capacity of lithium batteries to be installed to make best use of the existing solar etc?

Many thanks in advance so I can hopefully take advantage of the pre- Black Friday deals!

 

[Cush]

Hopefully this will bring your post to the attention of someone who understands the demands of an a/c unit on start up. The impression I have is that they can eat the amps with a large demand on start up.

 

[oldiesontour]

If the currents you quote are 240v and correct then 4 amps will be drawing around 80ah from your 12v supply,9amp180ah both plus start up surges ..4amp apx 960w 9amp 2160watt. Fridge likely 50 ISH watt.Solar max output 26amp hardly if ever likely to be anywhere near this more lie 18/20amp on a good day ( appears start up current for AC is 28 amp,but only for mili seconds 6700w

 

[cyberyacht]

How many amps out can your batteries cope with? It seems only lithium can handle continuous heavy demands

 

[funflair]

If the currents you quote are 240v and correct then 4 amps will be drawing around 80ah from your 12v supply,9amp180ah both plus start up surges ..4amp apx 960w 9amp 2160watt. Fridge likely 50 ISH watt.Solar max output 26amp hardly if ever likely to be anywhere near this more lie 18/20amp on a good day ( appears start up current for AC is 28 amp,but only for mili seconds 6700w

The amp figures are going to be at 12v, the 45 amps for the compressor running is the clue and would equate to around 600watts which isn't a lot actually.

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[BillandHelen]

We ran our Dometic fresh light aircon through an NDS 2000 inverter and 200 amp lithium no problem at all offgrid in our previous van. We were using it differently from yourself, to cool the van down before bed, would think your usage would be straightforward.

 

[oldiesontour]

The amp figures are going to be at 12v, the 45 amps for the compressor running is the clue and would equate to around 600watts which isn't a lot actually.

As the op was asking about 2 or 3 KW inverter I assumed (probably wrongly) he was talking AC loads.

 

[funflair]

Hoping that some of the more electrically literate members can help out with this as I’m trying to get a handle on the optimal combination of inverter and Lithium batteries to aim for to satisfy our (modest!) needs.

This is what we have installed currently and what we would like to possibly run off-grid:

There’s 340w of solar on the roof and no possibility of any more. Fridge is 12v compressor and we have LED lighting throughout. The existing 200 Ah of NDS Greenpower AGM batteries, (2x100 under passenger seat) will, even in winter with minimal solar input, give us a good 3-4 days running with no external power source.
There’s decent Nordelettronica charging and 40amp B2B components that can handle different charging profiles, as can the NDS solar controller.

What I’d like to do initially is install a Renology inverter that can allow us to run the Truma Aventa aircon unit when off-grid in summer abroad - this doesn’t need to be in full compressor mode as with just the fan running, in conjunction with the low-level fanned ducts in ventilation mode, it’s sufficient.

Consumption figures for the Aventa unit helpfully provided by a fellow forum member are:
Low fan 4 amps
High fan 9 amps
Compressor 45 amps (intermittent and for interest only)

So, bearing in mind our current equipment level, can we:
1) get away with a 2000w inverter (my preference due to space constraints) or is a 3000w unit advised?
2) what would be the advised total capacity of lithium batteries to be installed to make best use of the existing solar etc?

Many thanks in advance so I can hopefully take advantage of the pre- Black Friday deals!

Even if you don't want to run the Truma off the inverter once you have one you may well find other 240v that you want to run, remember short periods of high amps take out very little power compared to continuous running of the likes of air con, if you are going to go 200ah lithium I would go for 3kw inverter.

 

[funflair]

As the op was asking about 2 or 3 KW inverter I assumed (probably wrongly) he was talking AC loads.

It is easy to get confused by amps but when I saw 45 I thought no that must be at 12v.

 

[funflair]

I think the ideal total capacity of your lithium batteries depends on how long you are stationary and what you are using more than your solar capacity, if you start to make full use of lithium (kettle, microwave, induction, hair dryer etc etc), we can use 100ah per day if we are hammering it and that's without running the air con, on a good day that will go back in with our 520w solar but in the UK winter we have to rely on the battery capacity which is 640ah and will see us stand for 6 days without dropping below 40%.

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[AdrianChen]

I think the ideal total capacity of your lithium batteries depends on how long you are stationary and what you are using more than your solar capacity, if you start to make full use of lithium (kettle, microwave, induction, hair dryer etc etc), we can use 100ah per day if we are hammering it and that's without running the air con, on a good day that will go back in with our 520w solar but in the UK winter we have to rely on the battery capacity which is 640ah and will see us stand for 6 days without dropping below 40%.

On our last 3 day trip we averaged around 80AH per day. Solar contributed a bit, but trees/clouds limited that, so the balance came from EHU. Our batteries would have managed 4 days ish with no solar. We planned on a four day cycle (just because we did that on our boat, so we ran it's generator every 3rd day and made a bit of fresh water if needed), actually I would have liked a bit more battery but in the pandemic we bought what we could get hold of....

We have a Truma AC rooftop unit, and once in the summer I decided to test it (it's rarely switched on). The Multiplus II 3000wh inverter ran it without issue, but it is near it's max output when cooling. The newer so called "inverter" type units with brushless motors etc seem to be more efficient and have no inrush current when starting. Frigomar do them, but aimed at boats. We do use the kitchen roof fan to get a bit of air through the van, I think that uses less power.

 

[autorouter]

Your 340W of solar will give you about 140Ah over a reasonably good summer day. That's about 140 x 12 = 1680 Wh. If your A/C takes 600W while running, then that energy will run it for about 2.8 hours. Of course that's time when the compressor is actually running - it will cycle on and off, so the total time will be extended depending on how hot it is and how well insulated the MH is.

Being A/C, if you get a spell of bad weather you probably won't be using the A/C much anyway, so not as much a concern as if you were using it for heating. I would imagine that having two days worth of energy stored in the lithium batteries (ie 2 x 140 - 280Ah) would be OK. Maybe aim for a nominal 350Ah of lithium so that you don't get too near the full and empty limits.

That will also give you an extra in winter as a bonus.

 

[funflair]

On our last 3 day trip we averaged around 80AH per day. Solar contributed a bit, but trees/clouds limited that, so the balance came from EHU. Our batteries would have managed 4 days ish with no solar. We planned on a four day cycle (just because we did that on our boat, so we ran it's generator every 3rd day and made a bit of fresh water if needed), actually I would have liked a bit more battery but in the pandemic we bought what we could get hold of....

We have a Truma AC rooftop unit, and once in the summer I decided to test it (it's rarely switched on). The Multiplus II 3000wh inverter ran it without issue, but it is near it's max output when cooling. The newer so called "inverter" type units with brushless motors etc seem to be more efficient and have no inrush current when starting. Frigomar do them, but aimed at boats. We do use the kitchen roof fan to get a bit of air through the van, I think that uses less power.

We have the Dometic 3000 "inverter drive" air-con, it works well off our 3kw inverter and is super smooth and quiet, with it's heat pump technology it gives a quick boost of heat as well as the Alde first thing in the morning, just when we are on hook up though, the advantage of the air-con coming via the inverter is that the hook can be running the Alde and the air-con comes off batteries which recharge when the other demands reduce.

 

[Orion]

Many thanks to all who replied with information and personal experiences - it’s given me a good steer on what to aim for.

To reiterate, we are quite economical in our off-grid power usage at present and the current setup of 340w solar plus 2x100 AH AGM batteries serve us well, even in winter, as we have a plethora of USB sockets for charging devices, and the TV (when we do go use it) is 19v running from a laptop charger.

It’s the summertime running of the AC fans on the vent setting only (not the compressor) that I really want to have the inverter for, so 4 & 9 amps draw will suffice.