On 23 Dec 2004 00:33:56 GMT, (Gammara2) wrote:

for a small microwave for a boat, what wattage inverter do u need and what will
it draw in terms of amps to start and run the microwave?

Small? Say 750 watts
Using a 1 kW plus inverter, it uses about 750/(12 X .85) = 74 amps
for an efficient (85%) inverter with a little idle current in there.

Brian W

On 23 Dec 2004 01:57:36 GMT, (Gammara2) wrote:

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

Oh dear, I hope this is not yet another pseudonym from the resident
lunatic fringe. It mentions one of his favorite units - the amp per
hour or amp/hr - a unit which is unknown to engineers and
electricians.

But let me give a straight answer to the question - if there is an
ensuing argument, my fears will be confirmed.

Typical kitchen microwaves apply the magnetron heater to pre-warm it ,
then activate the cathode to generate electrical heating in the
cavity: that's the moment when the fan may run a little slower, and
the oven light may dim a little. They do it this way to avoid
stripping the cathode - i.e. reducing its life.
In these types, the initial current draw is low, perhaps 8 or 10 amps
at 12 volts, then 74 amps or so when the magnetron is activated.

Brian W

Gammara2 wrote:
for a small microwave for a boat, what wattage inverter do u need and what will
it draw in terms of amps to start and run the microwave?
I picked the most efficient 700 Watt microwave I could find at the time.
Through the inverter it draws 84 Amps DC. A 1000 Watt inverter might
handle it, but it would be close. I have a 2000 Watt Heart
inverter/charger and its overkill.

Jeff Morris wrote:
I picked the most efficient 700 Watt microwave I could find at the
time.
Through the inverter it draws 84 Amps DC. A 1000 Watt inverter
might
handle it, but it would be close. I have a 2000 Watt Heart
inverter/charger and its overkill.

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.

On 23 Dec 2004 00:06:56 -0800, wrote:

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

==============================================

It's probably a bit over what is needed but not by much. My old boat
had a 3000 watt modified sine wave inverter with a 440 A-H battery
bank. It had no problem at all with a 1200 watt microwave plus a
refrigerator. Even if you never plan to draw 3000 watts, the
advantages are that it comes with a bigger battery charger and a
higher rating on the transfer switch.

I think the biggest problems with most inverter installations are too
small a battery bank and wiring that is too small. Either will lead
to large voltage drops under load causing shutdowns or inefficient
operation. I used 2/0 wire on the 12 volt side and it was no more
than just adequate. Far too many inverters end up being installed
with ready made battery cables which are usually 2 or 4 guage wire,
not nearly big enough for a steady 150 amps.

I took the microwave off the boat when I figured out what it would do to
my battery capacity while on the hook. I have a propane stove and oven
and have never missed the microwave.

Capt. Jeff

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

"Jeff Morris" wrote in message
...
Gammara2 wrote:
for a small microwave for a boat, what wattage inverter do u need and
what will
it draw in terms of amps to start and run the microwave?
I picked the most efficient 700 Watt microwave I could find at the time.
Through the inverter it draws 84 Amps DC. A 1000 Watt inverter might
handle it, but it would be close. I have a 2000 Watt Heart
inverter/charger and its overkill.

Brian- sorry i am a microwave neophyte-i appreciate everyones help. as i
underrstand it i need to get a 2000-3000 watt inverter, big wires, and beef up
my battery system to keep my wife happy- jim

Jim Donohue wrote:
I actually ran one (650 watts or so) for about 5 years 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.

Wise. The alternator delivers most of the needed amps and keeps
voltage depression in check. My 650w microwave will run w/engine at
fast idle, but not very well, as the inverter is only rated @ 1000
watts and is undoubtedly delivering a *heavily* modified sine wave, and
only for about 5 minutes before the inverter overtemp trips.
What was the rating of the inverter you were using?

"Jeff Morris" wrote in message
...
Gammara2 wrote:
for a small microwave for a boat, what wattage inverter do u need and
what will
it draw in terms of amps to start and run the microwave?
I picked the most efficient 700 Watt microwave I could find at the time.
Through the inverter it draws 84 Amps DC. A 1000 Watt inverter might
handle it, but it would be close. I have a 2000 Watt Heart
inverter/charger and its overkill.


I have a Heart Interface Feedom Jazz 1kw (2kw peak) inverter - It says right
on the outside of the box that it will run a 500 watt microwave. Have not
installed it yet - have not found a 500W microwave.