Joe,

There are several issues with inverter selection, only one of which is the inverter device.  Other significant issues include the battery bank that will power the inverter, and any alternate means you have/plan to have to charge the batteries (generator).

In general, about size.  Small size means small components; i.e., small transformer, small fans, small heat sinks for diodes, etc.  Heat is an extreme enemy of these devices.  That includes internally developed heat and ambient air temps of their surroundings.  Small componentry increases the chance that the device will experience heat related problems.  Also, low cost inverters are designed for use as stand-alone devices, not as components of a larger electrical system.  You'll need to assure that the device you buy meets ABYC codes for use on boats.  Most do not.  For example, there should be a relay inside the device that switches the safety ground onto the inverter when it is operating in 'invert' mode, and off the inverter when its operating in "passthru" mode.  Harbor Freight type devices are not generally compliant with marine safety codes, and it is your job to verify that whatever you buy carries a "UL Marine" or equivalent testing certification and to understand the standards to which the device was tested.  The Magnum unit that others have mentioned is a superb unit, and does meet the marine safety certifications.  It can be mounted in any orientation, so I have mine bolted to the overhead in the engine room, mounted up-side-down, where it is out of the way of everything else, but also near the batteries.

I'm sure you understand that the 120VAC power the inverter puts out comes from energy stored in the boat's battery bank.  A 1500 watt space heater cube will draw 158 amps from the batteries all by itself, so even run for a very short time, will chew up the amp hour capacity of the battery bank.  Likewise, with a microwave oven.  A small 900 watt microwave oven will draw 95 amps DC from the batteries.  Furthermore, because of something called Peukert's Law, high levels of discharge current significantly reduces the total amp hour capacity of a battery bank.  These devices (space heater cube, hair dryer, microwave) can easily use as much amp hour capacity by themselves in 15 minutes that the whole rest of the boat would use over 8 hours, so you need a fairly robust battery bank to support them.  A 500 aHr bank would be marginal; a 750 aHr bank would be desirable.  Bigger is better.  Do not discharge deep cycle batteries past the 50% level.  Doing that shortens battery life.  Actually, 40% max state-of-charge is better.

Aboard sanctuary, we have stopped using the inverter alone for these purposes.  When it's time to run a hair dryer, or the microwave, or to make space heat on a chilly morning/evening, then I run the genset for that period of time. It's much easier on the batteries.

You need to also plan for how - and how quickly - your battery bank can be recharged.  On boats, batteries typically recharge from either the on-engine alternator, from an onboard genset, or from shore power.  With 15 amps from a genset or shore power, the Magnum MS2012 will charge batteries at a rate of 100 amps; the MS2812 will charge at a rate of 125 amps.  It'll take 2 - 3 hours of genset time to replace the amp hours of a 50% discharged battery bank of 500 aHr.  Sanctuary is fit with a 110 amp HO Balmar alternator on the main engine.  It charges the batteries at the same rate as the MS2012, and likewise takes 3 hrs to charge a 50% discharged 500 aHr battery bank.

Finally, Sanctuary has a single battery bank that serves for both house and engine starting.  My engine starts fine with a 50% discharged bank.  But as Bill Donovan said, it essential to have a way to monitor bank State-of-Charge (SOC).  Magnum offers an add-on for that purpose that's only about $200.  If you have the DIY skills to install this system yourself (not hard, by the way) the cost of a Magnum platform will be 2 boat units.  If not, double that to include the electrician's labor and markup on installation materials.

So here are the messages:

1. Make sure the inverter you choose meets the ABYC Marine safety standards for use on boats;
2. Make sure your battery bank and re-charging systems can support the AC loads that will run on the inverter; and
3. Think of this as an integrated system, not just a component.

Do it right the first time, or be disappointed, experience a breakdown or two, and do it over.

Hope this is useful.

Jim

Peg and Jim Healy aboard Sanctuary
Currently lying Ipswich, MA
Monk 36 Hull #132
MMSI #367042570
AGLCA #3767
MTOA #3436

Jim Healy said
" Aboard sanctuary, we have stopped using the inverter alone for these
purposes.  When it's time to run a hair dryer, or the microwave, or to make
space heat on a chilly morning/evening, then I run the genset for that
period of time. It's much easier on the batteries.  "

Jim:
How do you assign an operating cost to the genset? I have heard many
theories on genset operational costs including estimates about life cycle.
I consider my genset operational costs to be "low" although I have not taken
the time to do a thorough analysis and calculation.

I have been running the genset with the following criteria -

1. batts need charging - gen set comes on.
2. Celeste wants to use the convection oven - genset comes on. The load on
   the batts is more than I want to put to them.
3. We desire heat from the air conditioner-heat pump in the aft cabin or
   from a heat element device (like a cube).
   Other instances of desired AC power use, I use the 3000 w inverter,
   including making coffee and microwaving my oatmeal or heating up coffee
   later in the day.

I have a condition in which I call "free electricity". That is, if I am
running the genset because the batts need to be charged or any other
"required" reason, then I try to perform another function simultaneously.
That is, I will also make the coffee, oatmeal, etc while charging the batts.
I consider it no extra operational cost to perform these functions since the
genset was running anyway...hence "free" (tongue-in-cheek).

R.

From: Ralph Yost ralph@alphacompservices.com
To: 'Jim Healy' gilwellbear@gmail.com; joroy40@hotmail.com
Cc: great-loop@lists.trawlering.com
Sent: Monday, August 20, 2012 5:25 AM
Subject: Re: GL: Inverter selection
Also the genset likes to be run at 50% or more load, Jon.
Jim Healy said
" Aboard sanctuary, we have stopped using the inverter alone for these
purposes.  When it's time to run a hair dryer, or the microwave, or to make
space heat on a chilly morning/evening, then I run the genset for that
period of time. It's much easier on the batteries.  "

Jim:
How do you assign an operating cost to the genset? I have heard many
theories on genset operational costs including estimates about life cycle.
I consider my genset operational costs to be "low" although I have not taken
the time to do a thorough analysis and calculation.

I have been running the genset with the following criteria -

1. batts need charging - gen set comes on.
2. Celeste wants to use the convection oven - genset comes on. The load on
   the batts is more than I want to put to them.
3. We desire heat from the air conditioner-heat pump in the aft cabin or
   from a heat element device (like a cube).
   Other instances of desired AC power use, I use the 3000 w inverter,
   including making coffee and microwaving my oatmeal or heating up coffee
   later in the day.

I have a condition in which I call "free electricity". That is, if I am
running the genset because the batts need to be charged or any other
"required" reason, then I try to perform another function simultaneously.
That is, I will also make the coffee, oatmeal, etc while charging the batts.
I consider it no extra operational cost to perform these functions since the
genset was running anyway...hence "free" (tongue-in-cheek).

R.

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