AC battery requirements, and 120v vs. 12v systems

ReluctantTraveler

ReluctantTraveler

Well-known member
Just to clarify here: the problem with off-grid is capacity, not flow.

An alternator can easily send power to a 120v AC system, but you'll need a much larger battery bank to meet the electricity needs of the system.

I linked to the Mabru above specifically because it's a 12k BTU AC, similarish output to the Mach 3 Plus (13.5k BTU). The Dometic 12v that folks love has similar consumption, but at only 6-7k BTU.
 
  • Like
Reactions: plh
rruff

rruff

Explorer
Just to be clear and get the units right, energy is in W-hr (not W, power is in W) and it draws 2.7x W (not W/hr) at max... but it will also be providing more cooling when on... yes? Why would the efficiency be so different between the two?

I'd expect them to be within 10% or so, but we have PnM claiming the 120v ones are actually better, and the manufacturer data you have seems very much in the opposite direction... ?? Definitely need some measured cooling power vs input power data.

EDIT: Maybe this is the main reason for the price difference... most 120v buyers don't care much about efficiency.
 
Last edited:
ReluctantTraveler

ReluctantTraveler

Well-known member
rruff said:
Just to be clear and get the units right, energy is in W-hr (not W, power is in W) and it draws 2.7x W (not W/hr) at max... but it will also be providing more cooling when on... yes? Why would the efficiency be so different between the two?

I'd expect them to be within 10% or so, but we have PnM claiming the 120v ones are actually better, and the manufacturer data you have seems very much in the opposite direction... ?? Definitely need some measured cooling power vs input power data.
Click to expand...

I'm just doing math based on how much power they draw when operating. The cooling output is about the same, but AC units consume more watts to run.

There's a reason why so many builds use 12v AC systems even if they have an alternator.
 
rruff

rruff

Explorer
The Dometic RTX 2000 supplies 2000 W of cooling and draws 696 W based on their specs. That's a COP of 2.87 which isn't too bad.

This 120v mini-split supplies 9,000 BTU/hr (2640 W) of cooling and draws 720W, for a COP of 3.67 which is 28% better. Plus it's listed at only 25 lbs (?), vs 84 for the Dometic. Cost is $750 vs $2400. And you can put it on the wall.

I don't know if any of these manufacturer numbers can be trusted. There is the SEER rating which is standardized, and the mini-split is rated at 20.5 which is very good. I haven't seen this listed for any 12v systems though. Maybe it's only required for residential systems?

I guess Amazon links don't work? Google Senville LETO Series Mini Split Air Conditioner Heat Pump, 9000 BTU 110/120V
 
KiwiMtnClmbr

KiwiMtnClmbr

Member
ReluctantTraveler said:
Just to clarify here: the problem with off-grid is capacity, not flow.

An alternator can easily send power to a 120v AC system, but you'll need a much larger battery bank to meet the electricity needs of the system.

I linked to the Mabru above specifically because it's a 12k BTU AC, similarish output to the Mach 3 Plus (13.5k BTU). The Dometic 12v that folks love has similar consumption, but at only 6-7k BTU.
Click to expand...
It's worth noting that the 55amp draw of the Mabru is at max operation. Catalog (not tested :-/) draw for the low fan setting on Eco cool is 22.5amps and a low fan setting on Normal (think medium) cooling is 40amps. I have a very small camper in the design stages, like 8'x6'x6' so will not need much fan action and can use computer fans much more efficiently than the rooftop unit. My fervent hope is to get 9 hours a night at 40 amps which would take something like 370ah out of my batteries. If it's less than 400ah I'll be happy.

I would have liked to make a split unit work and I'm still keeping an eye on them. I just had a purchasing/shipping deadline to make and the Mabru was the most affordable option that would actually cool a camper enough to sleep when the humidity is 95% and the temps are well over 100deg even at night.
 
ReluctantTraveler

ReluctantTraveler

Well-known member
KiwiMtnClmbr said:
It's worth noting that the 55amp draw of the Mabru is at max operation. Catalog (not tested :-/) draw for the low fan setting on Eco cool is 22.5amps and a low fan setting on Normal (think medium) cooling is 40amps. I have a very small camper in the design stages, like 8'x6'x6' so will not need much fan action and can use computer fans much more efficiently than the rooftop unit. My fervent hope is to get 9 hours a night at 40 amps which would take something like 370ah out of my batteries. If it's less than 400ah I'll be happy.

I would have liked to make a split unit work and I'm still keeping an eye on them. I just had a purchasing/shipping deadline to make and the Mabru was the most affordable option that would actually cool a camper enough to sleep when the humidity is 95% and the temps are well over 100deg even at night.
Click to expand...

for sure! I was doing worst case math.
 
ReluctantTraveler

ReluctantTraveler

Well-known member
rruff said:
The Dometic RTX 2000 supplies 2000 W of cooling and draws 696 W based on their specs. That's a COP of 2.87 which isn't too bad.

This 120v mini-split supplies 9,000 BTU/hr (2640 W) of cooling and draws 720W, for a COP of 3.67 which is 28% better. Plus it's listed at only 25 lbs (?), vs 84 for the Dometic. Cost is $750 vs $2400. And you can put it on the wall.

I don't know if any of these manufacturer numbers can be trusted. There is the SEER rating which is standardized, and the mini-split is rated at 20.5 which is very good. I haven't seen this listed for any 12v systems though. Maybe it's only required for residential systems?

I guess Amazon links don't work? Google Senville LETO Series Mini Split Air Conditioner Heat Pump, 9000 BTU 110/120V
Click to expand...

when you need 3x as much battery AH to run them overnight, the savings on 120v units disappears and 12v becomes the more affordable option.
 
rruff

rruff

Explorer
ReluctantTraveler said:
I'm not seeing any details on the electrical draw? Should I be looking somewhere specific? I tend to miss stuff right in front of me sometimes!
Click to expand...
I gave calcs on the Senville. The spec page on the Aircon model is here: https://d11fdyfhxcs9cr.cloudfront.n...i/air-con_blue_series-iii_submittal_sheet.pdf

COP on that one is 3.88 so it's better than the Senville (3.67) as expected. And the Dometic is only 2.87 based on their input and cooling numbers. COP is the amount of cooling energy you get divided by input energy.

For residential AC, SEER rating is the common measure of efficiency, and it's standardized and tested. 22.5 is the best I've seen. Unfortunately it doesn't appear that RV systems use any standard efficiency measurements; just max power draw and max cooling numbers... that they could just pull out of anywhere.
 
ReluctantTraveler

ReluctantTraveler

Well-known member
rruff said:
I gave calcs on the Senville. The spec page on the Aircon model is here: https://d11fdyfhxcs9cr.cloudfront.n...i/air-con_blue_series-iii_submittal_sheet.pdf

COP on that one is 3.88 so it's better than the Senville (3.67) as expected. And the Dometic is only 2.87 based on their input and cooling numbers. COP is the amount of cooling energy you get divided by input energy.

For residential AC, SEER rating is the common measure of efficiency, and it's standardized and tested. 22.5 is the best I've seen. Unfortunately it doesn't appear that RV systems use any standard efficiency measurements; just max power draw and max cooling numbers... that they could just pull out of anywhere.
Click to expand...

Interesting... 6.3 amps is shockingly low. This is the first unit I've seen that does not provide a range with both min and max draw.
 
You must log in or register to reply here.

Forum statistics

Threads
188,154
Messages
2,902,882
Members
229,582
Latest member
JSKepler

Members online

Total: 437 (members: 10, guests: 427)
Top