DIY Question about Marine A/C Wattage Draw & DC Inverter Power


I Recently received an email from a gentleman named Tyler...

Tyler was inquiring about a DIY job installing Marine A/C on a 30 foot sailboat, and then running the Marine Air Conditioner on an inverter when he was away from shore power.

His question asked about wattage draw of a 8,000 BTU Cruisair self contained Turbo AC unit & if I thought the 8,000 BTU marine ac unit would be large enough for his 30' Irwin.

Since this "Air Conditioning on Inverter" question is being asked more & more often now days, I thought I would post my answer here for everyone's review...

Hi Tyler...Thanks for asking~

It's not really as simple an answer as you might expect...But I'll try to keep it somewhat simple...So here goes...

  • The 8K Turbo unit is rated to draw 5.5 amps which using the conversion formula of 5.5 amps X 115 volts = 632.5 watts in cool mode (slightly more in heat)
  • The 10K Turbo unit draws 6.7 amps X 115 volts = 770.5 watts in cool (slightly more in heat)
  • Depending on your location (seawater & air temps) 10K is likely better suited for a 30' sailboat...At least down here in central Florida...

Cruisair Stowaway Turbo Unit (SMXII) W/R410A 10,000 BTU Marine AC

OR

Marine Airrr Vector Turbo Self Contained A/C 10,000 Btu W/410A

Yes they can be run on an inverter, but starting amps can be many times the run load, and that can depend on several factors such as the inverter's quality & the voltage it maintains during that surge...

The inverter's Surge rating is as important as it's continuous wattage rating...And as with anything electronic... Over sizing is usually best for the life & performance of the product...Don't plan on just the minimum needed.

Also...A Dometic "SmartStart" can be added to any A/C unit to reduce it's starting amperage draw/surge by as much as 65%...In my experience they really do live up to the claims...

Once started...How long your battery bank can run the A/C depends on a few things too...

  • Size of the bank in amp-hours...Meaning size of each battery & how many batteries connected in parallel (Two 100 amp hour 12 volt batteries connected together in parallel provide for a 200 amp hour bank...4 would be 400 amp hours etc..)
  • Length Of, and Wire Size, run...DC Voltage to the inverter tends to drop pretty quickly (length of run is measured as a total of both to & from batteries)
  • The Longer the DC Voltage Run...The Larger the wire needs to be to limit voltage drop & overheating...(It's why Tesla won out with his AC voltage we now use over Einstein's DC voltage back in the day)
  • Duty Cycle of the A/C (actual compressor run time to pull down to & maintain a given set point temp)
  • Duty Cycle is usually figured as a compressor run percentage of overall time on...
  • Then also figure the small draw of the fan & pump...Which may, or may not cycle with the compressor (Now days with the digital t-stats most pumps cycle with the compressor but the fan may not, or is selectable)

Most battery manufactures recommend not running batteries below 50% charge state before re-charging (so figure battery amp hour rating 50% less than actual rating)

As a general rule of thumb...It's a 10 x conversion for DC to AC amperage...10 amps of AC amp draw (uninterrupted) per hour is around 100 amp hours of DC removed from the batteries...

So taking the 6.7 amp figure from the 10K unit above...Figure 67-70 DC amps being removed from the batteries (actually inverters are never 100% efficient and differ in ratings so YMMV but 10x is easy to figure)

Basically, It takes a pretty large battery bank to get a full nights sleep...Then you must figure recharge time to replace that amperage used & the charging system...Alternator or Charger output rating determines that factor...But other questions are...

  • How long do you have to run the engine to replace that energy ?
  • Or how long do you typically run the engine a day, and is it enough ?

To run that 10 K Dometic Turbo unit 9 hours (one hour pull down, 8 hrs sleeping) and I don't think you can find a similar sized unit that draws less...figure say...

  • 7.7 amps running (6.7 unit amps plus 1 amp for the pump) 7.7 x 10 (basic DC to AC conversion factor) 77 amps x 9 hours = 693 amp hours used...
  • 693 x .80 (80% duty cycle) equals 554 amp hours without loss thru wire, inverter efficiency, etc...
  • Then double that for the 50% discharge factor the battery manufacturers recommend & you would need a battery bank capacity of 1,108.8 amp hours...
  • That's 11 or more 100 amp hour 12v batteries...
  • Or you could use six pairs (12 batteries @ 225 Ah per pair) of 6 volt golf cart batteries for a capacity of 1350 amp hours...This should keep you above the 50% discharge for a full night of sleep in slightly less space than twelve 100 Ah 12 volt batteries that would only total 1200 Ah...

If you want to keep the cabin cool 24/7...

  • Does the alternator have enough output to keep up with the draw without the battery bank loosing percentage of charge & are you taxing that alternator doing so ? (at maximum output all the time)
  • Or can the alternator "Catch Up" After some engine off time while still running A/C underway ?
  • Meaning do I need to consider a larger alternator to do what I want without it overheating causing shorter life ?
  • Typically the standard alternators on small diesels or sailboat auxiliary engines range from 35 to 65 amp output...

There are trade offs...Yes it can be done, and is...

You can play with the figures for differing usage, but knowing all this info going in provides for fairly realistic operational & pocketbook expense expectations...

Hope that helps...Any questions please ask...

For folks reading this in Marketplace...We are linking this post to the "Installation" section of the Forum here on Marine-AC.com so that it can be discussed further if needed...

 

Steve~