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Old 05-26-2017, 05:16 AM   #1
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2013 Interstate Coach
Waterloo , Iowa
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Running microwave from inverter

I have seen posts about upsizing the battery bank and Magnum invertor in order to use the microwave. My question is 'Do I need to?' We often fire up the microwave to use it for only a minute or two to heat up leftovers. We often spend more time waiting for the generator electrical system to come to life than we do actually using it. If we are only going to use the microwave for a couple of minutes, couldn't we do so with the standard 1000W Magnum and 2 group 24 batteries? Per the Cusinart OM the microwave draws 1500 W. Per the Interstate OM the Microwave is protected by a 20A fuse in the generator/shore power circuit and the Magnum output is also through a 20A fuse. So I am considering unplugging the microwave from the existing socket and plugging it into the invertor outlet that is above the stove. Long as I am careful about not drawing the Lifelines down too far and too often, I think it would work. I don't think that we have every used the microwave longer than it takes to pop a bag of popcorn - about 4 minutes. These 4 minutes would only use about 10AH out of the coach batteries. Did I do my math wrong, or am I missing something?
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Old 05-26-2017, 05:32 AM   #2
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Fuses protect the wiring, but I am not sure I would want to try to power a 1500 watt appliance form a 1000w inverter. I recently installed my Victron 3000, 2400 watt I/C and I did test it on the microwave, with a cup of water. I seem to recall a battery draw of about 80 amps. A 1000w inverter is really only good for entertainment system equipment and small appliances like an electric razor. Not hair dryers, not microwaves, not even most toasters.

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Old 05-26-2017, 06:01 AM   #3
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Hi Titus, our stock 1000W inverter ( with two group 24 batteries) shuts off immediately if we try to run anything with a draw above it's rated power, presumably to protect itself, so if yours is the same, it likely won't work with the microwave. Best, Joe
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Old 05-26-2017, 06:08 AM   #4
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I think you answered your own question when you asked if you could run a 1,500 watt appliance on a 1,000 watt inverter.
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Old 05-26-2017, 06:16 AM   #5
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On the question of "do I need it", our answer was yes, but our circumstances are not typical. I'm self-employed and spend some of my time working out of our van, so our cost-benefit analysis took into account the fact that with the extra work I could elect to do, I'd make the money back that was spent on inverter, lithium batteries, solar, etc., especially given that we had no labor costs.

Our DIY lithium job (B Van thread) included a 2,000 watt inverter which allows us to easily run the microwave, hair dryer, etc. I went from being ambivalent about the microwave (running it was solely my husband's idea) to not wanting to live without it. Over the past 5 years, the quality of certain frozen meal brands has risen dramatically. If I'm in a hurry, which I often am, there's nothing more convenient than tossing an Amy's Kitchen organic frozen meal in there. An instant well-balanced low-calorie meal! The internet is littered with stories of full-timers who added major poundage because it's so hard to maintain a healthy diet in a super-small space (one popular Airstreamer who shall remain nameless self-reported that he gained 45 pounds in 18 months).

So, no, I would not try to mate a microwave with an insufficient inverter. But I run ours off our 2,000 watt and I'm very grateful for it. It was worth the money and labor IMO.
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Old 05-26-2017, 09:05 AM   #6
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Being able to run the microwave off the inverter was the main reason I upgraded to a 2000 watt inverter and 440AH battery pack. Now I hardly ever use the generator. If we end up in a spot without 120VAC power I just use the microwave to steam veggies or warm milk for my morning coffee. I think the bigger inverter along with larger batteries and more solar (400W) makes the Interstate into a very flexible camping van.
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Old 05-26-2017, 11:07 AM   #7
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We too enjoy our upgraded 2,000 watt inverter that can power the microwave. I can measure its current draw next time I am in there if you are interested. From memory though, I recall it pulling near 100 amps which puts it at 1,200 watts.
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Old 05-26-2017, 12:03 PM   #8
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For those who do not want the hassle of upgrading your inverter, make friends with your stove -The rapid burner on our 3 ring stove top is quick to boil a small kettle of water or heat a cup of milk in the saucepan. We will often heat a kettle of water to wash dishes rather than turning on the water heater. Our oven heats up quickly to heat up leftovers, or bake from scratch. Popcorn can be easily made on the stovetop. We use silicone baking containers so when we have shore power they can double duty in the microwave. We dry camp a lot and adapt to our environment, cooking outside a great deal and indoors when necessary.
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Old 05-26-2017, 01:27 PM   #9
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Appreciate all the replies. We do use the stove for hearing water but reheating some things on the stove is difficult (like bacon). I was not sure of the 'margin of safety' associated with the 1500 W rating of the microwave. I have also toyed with the idea of running it on 30-50% power for the reheating that we do. The upgraded ME-RC remote with shunt kit should let me play around with microwave power settings to see if I can stay under 1000W. Just need to get of the couch and go out and play with it.

I doubt I'll upgrade to 2000 W invertor just to run the microwave for 2-4 minutes instead firing up the generator. Our upgraded 200W of solar and stock 2 group 24 AGM's serve us just fine once we started being anal about electrical usage. We are into camping and not glamping, so hair dryers and curling irons are not allowed.
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Old 05-26-2017, 01:28 PM   #10
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We don't have 3 ring stove tops in our Interstate vans. But you are correct the 2 burner LP stove top is very useful. I even make toast in the frying pan while frying my eggs and ham.
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Old 05-26-2017, 02:10 PM   #11
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Originally Posted by Titus View Post
I have also toyed with the idea of running it on 30-50% power for the reheating that we do. The upgraded ME-RC remote with shunt kit should let me play around with microwave power settings to see if I can stay under 1000W. Just need to get of the couch and go out and play with it.
By all accounts, this won't reduce the 1000W requirement, and therefore will still shut down your inverter. From what I read, changing the power setting on the microwave simply changes the duty cycle of the magnetron. For example, if you set it at 100% for 1 minute, the magnetron will run constantly for 1 minute. If you set it at 50% power for 1 minute, the magnetron will be on for a few seconds, then off for a few seconds during the 1 minute period. During the "on" time, it's still consuming 1000W, just like when it's set for 100%. Apparently there ARE units out there that actually modulate the power to the magnetron vs. this duty cycle business, but those appear to be rare.

This should be easy enough to test. While plugged into shore power, stick a bowl of water in the microwave and set it to 50% for 1 minute. If you hear the magnetron cycle on and off, then you've got your answer (and saved your Magnum the humiliation of shutting down, LOL).
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Old 05-26-2017, 08:50 PM   #12
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I would agree with FlyFishin on how microwaves work. When it comes to inverters you get what you pay for and the pricey pure sine wave ones are worth the cost. Basically I figure what ever the appliance current needs at 120V you need to pull 10 times that current out of the batteries plus about 10% more for efficiency loss. A 12A draw for the microwave sucks about 130A out of the batteries! That's why you need thick wires and good clean connections on the battery side!
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Old 05-26-2017, 09:10 PM   #13
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Originally Posted by Titus View Post
I have also toyed with the idea of running it on 30-50% power for the reheating that we do.
I can't speak for all microwaves but mine at 50% power means it runs at 100% power for about 50% of the time. On/off/on/off. The fan runs all the time.
To further complicate it, my 50% power runs for 10 seconds on and 8 seconds off. Repeat. I don't try to quantify it, but I don't believe 30% power means you're not using 1500 w.
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Old 05-27-2017, 12:02 AM   #14
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A simpler option might be to downgrade the microwave to one that uses less power.

You can find them at big retailers down around 700w and some RV supply shops allegedly have them as low 500w. I'm not sure how satisfying the performance would be but for heating a cup of water or warming up a box meal, it would likely be adequate.
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Old 05-27-2017, 05:40 AM   #15
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Now that I think about it, I believe you are right about the microwave cycling to achieve partial power. Interesting idea about downsizing the microwave, but not a compromise that I am willing to make. Seems to be no simple low-cost solution and guess I'll just continue to exercise the generator when I need to heat/reheat something.
Thanks again for all who have responded.
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Old 05-27-2017, 05:52 AM   #16
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"These 4 minutes would only use about 10AH out of the coach batteries. Did I do my math wrong, or am I missing something?"

The math is right on the face of it, but doesn't take into account Peukert's Law. In a nutshell, when you drain a lead-acid battery at a high rate, its nominal capacity is greatly reduced.

Another thing to keep in mind: microwave ovens are rated in output power, but they are only about 67% efficient. Thus a "1,000 watt" microwave will actually use about 1,500 watts of AC power. Factor in an inverter's 90%-95% conversion efficiency, and you can easily see 1,600-1,700 watts of power drawn from your batteries.
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Old 05-27-2017, 06:33 AM   #17
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What is involved with upgrading the inverter to 2000w? Is this something that can be a DIY or would I need to take it to the dealer?
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Old 05-27-2017, 07:06 AM   #18
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What is involved with upgrading the inverter to 2000w? Is this something that can be a DIY or would I need to take it to the dealer?

The 2000W Magnum is considerably larger and heavier than the original 1000W unit provided in Interstates. You would have to find a location where it would fit. In my case I moved the batteries in two racks under the van behind the rear wheels and put to larger inverter where the original batteries were located. It's doable as DIY project if you have the skills and tools.
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Old 05-27-2017, 09:44 AM   #19
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Originally Posted by Boxster1971 View Post
The 2000W Magnum is considerably larger and heavier than the original 1000W unit provided in Interstates. You would have to find a location where it would fit. In my case I moved the batteries in two racks under the van behind the rear wheels and put to larger inverter where the original batteries were located. It's doable as DIY project if you have the skills and tools.
Were the wires to/from the old Magnum long enough so they could reach the new one, or did you have to replace some to make it all work? I think the former would make this a pretty simply DIY, the latter might get a bit more involved, especially with the heavy gauge wire needed as you'd need to buy a special crimper for those. Eventually I'll be making this upgrade, so it's good to know ahead of time what I'm getting into.
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Old 05-27-2017, 02:21 PM   #20
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Inverter driven microwave ovens from Panasonic and a few other companies do not cycle full on and off. They will indeed provide proportional power. Alas, the sharp is not inverter based so won't help here.
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