Neon John wrote:

Depending on the length of the run, that #2 wire may be a
bit small.

According to the chart in the West Marine catalog, it has a resistance
of 0.157 ohms per 1000'. At roughly 100 amps draw (1000 watts or so
output) that means about a tenth of a volt drop for every three feet
of cable run (two conductors) between inverter and battery. Under
peak current that would get larger. Depending on the length of the
run, it could have a significant effect.

To find out, check your battery voltage at the inverter terminals
while turning on the microwave. This will also check the resistance
of the connections. If you made the cables up using untinned auto
cable rather than tinned marine cable, it might be more prone to
corrosion and resistance in the lugs.

If the voltage at the inverter is OK, then it's probably the
difference between modified and pure sine wave.

Does that model microwave lower its power levels by running less than
100% duty cycle, so if you're running at less than full power you keep
hearing it cycle on and off? I ask because this chart
http://www.sailorsams.com/mall/tm5030_power_levels.asp seems to imply
that it actually lowers the peak power consumption. If so, try
running at 90% power. (It could also be that the chart shows average
rather than instantaneous consumption, or that the guy who made it was
using assumptions rather than knowledge.)

- Walt Bilofsky