Boat Forum / Cruising / December 2004

74 amps/hr? or 74 amps to start it?

I too have a 650 watt microwave and my modified sine wave Trace 1000w
inverter won't handle it. Oddly, it'll run my 600w toaster oven for
hours. There's a lot of 'gotchas when it comes to microwaves running
off inverters.

Microwaves are rated by their output power and require significantly
higher AC input power to drive them. For example, My "650" watt Sharp
R-190 requires 1100 watts household grade AC power when running,
according to the sticker on the back.

Microwaves have high power factors; they are very reactive loads. An
inverter drives reactive loads inefficiently, and microwaves have
trouble with a modified sine wave power supply, their transformers run
hot and noisy; guesstimate these losses at 30%, so now 1500 DC watts
are required.

Inverters are inherently inefficient, typically "90%" (per brochure)
efficiency maximum.  Well, that's *if* the inverter's loafing along at
about 1/2 its rated output; at or near its rated output that number may
drop to 80% or less depending on how hot it gets (which may depend on
where and how it's installed). So if you're near maximum inverter
rating running that microwave, 1800 DC watts are required.  Assume the
inverter's twice the required rating so this problem can be ignored for
our purposes here.

And, the dirty little secret many inverter manufacturers don't tell:
Inverters are rated at 13 volts input, only seen if the battery bank is
fully charged *and* the discharge rate of the bank is less than 10% or
so (for wet cell batteries) of the bank's total capacity. Even if the
wiring is properly sized, (wet cell) batteries themselves suffer from
voltage depression under high loads.  So, to deliver 125 amps(1500w) to
the inverter at 13v for any length of time, a bank capacity of 1250
amphours would be required. That's 5 or so 8D batteries!

Real world sized banks ("at least 20% of your inverter's rating in
amphours . . ." - West Marine) at around 300-600 ah capacity, are going
to suffer from voltage depression with the high loads a powered up
inverter demands, and the rating of the inverter *drops* the lower the
input voltage!  A "1500 watt" inverter may only be capable of
continuously delivering 1000 AC watts at 12.0 volts, which is what the
system voltage may be under this heavy load, and turns into a wimpy 500
watt inverter somewhere around 11.5 volts with the added burden of
partially discharged batteries and/or an undersized battery bank.
Understand that once the load is removed, the bank may jump back to
12.4-12.7 volts, it's load voltage depression, not no-load voltage
we're talking about here.  To deliver 1500w with a voltage depressed DC
supply, you might need to buy a "2000w" or even "3000w" inverter.

So I think your 2000 watt inverter isn't overkill, it's pretty much the
minimum specification to power your 700 watt microwave.

With all of the above gotcha's in play, it's a wonder any microwaves
are humming away on boats.  Here's the golden parachute concerning
inverter specifications --> a 1000w inverter, when cold, with a hefty
bank of fully charged batteries and properly sized cables, can deliver
up to 1 1/2 times its rating for a minute or two or maybe more, just
enough time to heat up a cup of java or warm a breakfast roll in a 700
watt microwave.

I actually ran one (650 watts or so) for about 5 yeaqrs off an inverter.
Drew a little over 100 amps.  Used to start an engine to keep the battery
draw under control.  Did not really have a bank big enough to sustain the
current.  Actually worked well.  Run one at fast idle for about 15 minutes
before breakfast and we got both microwaved stuff and the coffee pot.  Big
alternator on one engine.

Jim Donohue

I have taken to reheating my coffee in a "Sierra cup" (metal camp cup).
Best coffee maker is stainless/vacuum French Press type, water is heated in
teapot on stove & poured in.
Lee Haefele
Nauticat 33 Alesto