Dear Boat Doctor, 

I am taking my boat to Grenada and would really like to be able run an air conditioner. I have read about the new efficient air conditioners that you can run from an inverter. I have a 2,500-watt inverter and 600-amp-hour battery. Could I use that?

I’ll be staying in a marina for a portion of the time and would really love to avoid paying for electrical service.  I have several solar panels and they have allowed me to keep the batteries up without running the engine.

Thom Biehl

Norfolk, Virginia

Dear Thom,

Self-sufficiency with solar is a wonderful thing, but it has its limits. You can easily keep up with LED lights and efficient refrigeration at anchor, and can come close to keeping up with an autopilot underway, but running air conditioning or any large load with an inverter is just too much.

Traditionally air conditioners required about twice the running current to start them, so a unit with a 12-amp running current could take 24 amps to start the compressor. This high starting current made air conditioners hard to run from small generators and even on docks with significant voltage drops. The new more efficient units have far lower starting currents.

An inverter allows you to run a 120-volt load from a 12-volt battery. It doesn’t do this for free. To be able to provide the 120-volt power it must draw 10 to 11 times more current from the battery.  A midsized air conditioner will draw 10 amps when it’s running, plus another amp for the water pump. This theoretical 11-amp load will require roughly 120 amps from your batteries.

To sum it all up, your inverter and batteries would likely be able to start and run a new efficient air conditioner, but not for very long. Your 600-amp-hour bank can efficiently supply maybe 400 amps of power.  Drawing current at 120 amps, your charge will last a little over three hours. Replacing that 400 amps will require you to run your engine for hours, and this is with the air conditioner turned off.

Your best bet for air conditioning in a marina is to plug in and underway you will need a generator. Enjoy your time in Grenada and look forward to cooler weather in fall up island.