sine inverter

[beau]

I went in to buy a 2,500 watt modified sine inverter, and was told it would NOT work with a microwave. Obviously trying to sell me a more expensive pure sine inverter.
What is your experience, is there a particular type of microwave I should buy.
I planned to use the modified sine for fridge/freezer and occasional air conditioning and have a much smaller pure sine inverter for the computer etc . However microwave use a fair bit of power.
Any ideas please.

[Rick]

All of the modified sine wave inverters have been successful at driving microwave ovens since the late '80's. With effecient inverter design and a good battery bank and proper charging system many people leave the inverters on all of the time for convenience. When doing this the microwave oven clock will not be accurate unless the inverter is clocked with an internal crystal reference that is accurate. Most modified sine wave inverters don't have accurate internal clocks while most sine wave inverters do.

Related to the clock; if you don't plan to always use the inverter then make sure that your microwave oven does not require you to reset the clock before it allows you to microwave...some will not allow it and that is quite frustrating having to always set even a bogus time entry before using the oven. This is true regardless of whether or not you opt to buy a sine wave inverter.

Microwave ovens extract their energy from the peaks of their input waveform and with modified sine wave inverters the cook time is directly proportional to the battery voltage regardless that the inverter output voltage is equal to the average sine wave voltage of normal sine waves. As a result, cook times are longer than normal when the oven loads the battery bank voltage below 12.0 Volts and is quite close to "normal" when the battery loaded voltage is above 12.0 Volts. You should not have this issue with quality sine wave inverters.

When buying an inverter consider that most people truly do not need more than a 1500 Watt continuous rated inverter because the 5 minute and 30 minute power ratings exceed that value....examine carefully the specs. Note that a 1500W continuous rated inverter will drive any microwave oven on the market. Better to consider what charger rating you get with that inverter/charger. Many people buy 2500W inverter/chargers in order to get around 130A rated battery charging capability.

When shopping consider the newer Magnum Energy sine wave (or modified sine wave) inverter/chargers. Outback Systems (Energy?) also makes sine units. Note that with inverter/chargers one gets more for the money than buying two separate units with the same relative specs.

[senormechanico]

A microwave will work with a modified sine wave, but not as efficiently.
It draws full power, but doesn't convert some of that power to microwaves, just internal machine heat.
If you want to use a microwave, get a sine wave inverter; they don't cost THAT much more.

I speak from experience before sine inverters were available at reasonable cost.


Steve B.

[camaraderie]

If you plan to leave the inverter on standby pretty much full time, you should look at the standby amp draw of the various models as well. Pure sine units draw a lot more. We used a Heart2500 modified unit successfully with 2 different microwaves and had no problems whatsoever and noticed no difference in performance compared to on shore microwaves. (We just used for re-heating/veggies so for real cooking Rick may very well be right about extra time).

[Rick]

Steve, are you sure that you aren't confusing efficiency with cook times? I've made calorimetric and kilo-Watt-input power tests with various inverters and the sine wave inverters are not quite as efficient as the modified sine, in general when running a microwave oven. This is how I came up with the cook time caveat that the input voltage needs to be 12.0 Volts or greater with modified sine units.

Even so, with lower input voltage the calorimetric results are almost the same until 11.0 Volts is reached and, then, the efficiency of the microwave oven drops drastically on the modified waveforms.

[irwinsailor]

I bought a heart 3000 inverter / charger last year and it runs everything that I want it to and it does it very well.

[Alan Wheeler]

There is one aspect that you guys have missed on. The start up current of the Microwave requires an inverter capable of supplying three times the microwaves power consumption. Although not truely accurate, if you take the microwave power (say it is 600W) then you need an inverter capable of supplying 1800W to start the microwave. Inverters have a Peak surge capability. but Depending on the quality of the inverter, that surge capablility may still not be enough. I imagine the 12V supply has a fair amount to do with that as well. Today, 600W microwaves are actualy quit small. 750-800W are the norm and we actually have an 1100W unit. Our 3KW inverter will not start it. We have to power it with the Genset. Besides, I would hate to imagine how fast it would suck the batteries dry if it did run on 12V. But on Genset it runs just fine. But you sure here the geny grunt for a split second when the Microwave kicks in.

[Cruisingdad]

We have a xantrex prosine invert/chg 2.0 and it works perfectly fine with the microwave, hair dryer, toaster, coffee maker... even the water heater (though that puts a strain on my batts). I give the xantrex a good reccomendation.

One thing I did not see mentioned that should be (for the record): Inverters require very large battery cables at 12v. I am not sure the 24v size awg (beau, since I think that is what your systems runs), but I had to run 4/0 (not 4 awg, 8 sizes bigger than that), but it was reccomended that I run 250 or 350 mcm. Anything over 25 foot r/t (as I recall) required the larger cabelling. Some inverters may be slightly different, but I doubt there is that much of a difference. When you price out and plan out your inverter, be prepared for a really high cost (and pain in the butt) running large cableing. The current cost of 4/0 runs from about $7-8/foot (if you can buy direct from mfg, like cobra for instance) to $20/foot if you buy through West Marine. Now add in all the 4/0 lugs (plan on close to 20 at $8-10/piece if you do not screw up any crimps), renting the crimpers, buying heavy duty cable cutters (or finding a buddy with them), 4/0 heat shrink, heavy duty 600 amp dissconnect, heavy duty fuse, etc... and your inverter just cost you a mint. You may find the inverter itself is the cheapest part of putting in an inverter.

Just FYI so you are not caught blind-sided.

- CD

[senormechanico]

Rick,

My previous boat to which I was referring had 2 Prevailer 8D's in parallel with short primary wire in good condition, a Cruising Systems Quadcycle battery monitor, and a Vigil Shorepower 1500 modified sine wave inverter. The microwave was a 1kw model. We monitored the battery current and voltage while running the microwave and it continuously drew 100 amps and never got below 12+ volts.

The cook time was noticeably longer on batteries than when running on shorepower.
I never actually documented the difference such as timing a cup of boiling water, but it seemed to be around +20%.

Come to think of it, I have a 2500 watt modified sine wave inverter in my house power backup system. It has 1000 aH of AGM's for power. One of these days I'll do a test with our microwave and post the results.

Steve B.