Sine wave inverter

I’m restoring my motorhome and I want to replace my old generator with a sine wane inverter. This inverter will primarily be used to operate a 1100 watt microwave. I found a 2000 watt inverter and was wondering if that would be enough? The reason I’m doing this is because it will cost me around $4000 to replace the old generator and for under $2000 I can purchase a 2200 watt Honda generator and install the inverter. A big savings. I already got the generator and it powers the microwave, refrigerator and charges all my batteries just fine, so I’m thinking a 2000 watt inverter should be able to run the microwave. The inverter will be isolated and will only provide ac power to the motorhome. It will not charge any 12 volt batteries.

Any opinions and advice will be welcomed. Thanks.

If 1100W is the rating on the microwave spec plate, and you get a good quality inverter, I think 2KW will be fine. Are you planning for a solid state switching inverter (Samlex, GoPower, AIMS) or a heavy transformer model (MagnaSine)? Check both the peak/surge power and steady-state power for both your loads and inverter.

Be careful you can predict ALL the power load which may draw on the AC at the same time, to make sure you won’t overload things. Refrig or AC compressors, water heating, fans, plasma TV ;) are the biggest users. And make sure your batteries are big enough to store the power you need between generator running (I assume you have separate deep cycle batteries from your RV starter battery with a charge isolating system). The size and length of cables between the batteries and inverter are quite important, so don’t skimp on them or you will cripple your inverters ability to make high power.

Okay, I came to this thread right from the Picard Thread, and I feel like I haven’t really left.

Have you tried inverting the polarity?

Thanks for the info. I looked at the specs on a few and found a 2000 watt inverter that has a peek output of 4000 watts which should cover the surge from the microwave. The only other things that need ac is the air conditioner and refrigerator. The air conditioner is never turned on (wife doesn’t like it) and I only run the refrigerator when I’m using an ac hookup otherwise it’s using gas. I can run refrigerator for around 3 weeks on gas with no problems. My main concern about the 2000 watt inverter was the surge you get when the microwave is started. My only concern now is if this battery will be able to supply the inverter with enough power and how long it will last. Need it to last at least an hour before needing recharged. I’ll probably get a solid state because I have limited space (23" wide x 16" deep x 12" high). I could probably fit a transformer model in there but it would be tight and I have had no problems with the smaller solid state inverters I have used.

If you actually need to run a 1100W microwave for a full hour? that battery can probably do it… once or twice. But that is about the full capacity discharge of the battery at a high rate, and it will fail quickly.

If you want the battery to last, keep the cycling to <= 50% depth of discharge, so do some math assuming ~85% efficiency of an inverter… I think about 2 of those batteries would be the minimum you need. (Late night math not checked with diligence) The battery capacity and lifetime goes up a lot when you use lower current rates, so using 2 or 3 of those batteries in parallel will add more capacity than just 2x or 3x. (i.e. 1 battery at 1 hr rate is 122Ah, but 3 at 3 hr rate is 3x156=468Ah)

Thanks for the info. My problem is this, I have a very limited space to use and 2 of those batteries would weigh over 250 lbs and would most likely cause me to place the inverter outside of the old generator space. I’m thinking of replacing the current microwave with a 700 watt model. It would reduce the weight by 50 to 60 lbs and allow me to place the inverter inside the old generator space. The problem with this is that it would take more time to cook and would be more likely to reach the 1 hour threshold sooner, thus causing me to charge the batteries more often. Guess I’ll need to do more thinking and research on the matter.

In a sense that’s the petrofuel/renewable crisis in a nutshell… the energy storage density of petroleum is so hard to beat.

I like your thinking and it sounds like a fun project, so stay on it. I am an engineer and hobby in this tech area some but I am not a professional, so I would hate for you to overcorrect because of a nerd on the internet who can’t do math in his head properly or doesn’t know of a perfect cool new product in the area.

I did more research and I think I might have been looking at this wrong. I was thinking 2000 watt inverter running at capacity. Then I found this site and realized that I won’t be using the inverter at capacity but at 1550 watts 95% of the time. Which means, according to the site, I would only be using only 155 Ah. Why 1550 watts? Most 1100 watt microwaves need around 1550 watts in order to operate. Factoring in inefficiency The inverter would use about 1700 watts which becomes 170 Ah. Which in turn means I could run the microwave for about 50 minutes before I reached 50% charge using a 300 Ah battery pack. That is acceptable. Of course my figuring could be wrong. I chose a 2000 watt inverter because it gives my the headroom I need to run the microwave.

As a personal note I studied Mechanical Engineering in college before I had to drop out and get a job. Became a land surveyor instead.

Sounds like the right idea, but remember the Ah you get from the batts is dependent on the Amps rate – see how they list 3 different Ah ratings based on how many amps you are taking out. At very high amps out, the total capacity drops from 200Ah to 120Ah.

The voltage will also quickly drop with that high and long of a discharge which causes the inverter to draw more amps. Rinse and repeat.