If I get a 2000w inverter for my MH and use my 1100w microwave, what will be the draw off the house batteries? Using Ohm's Law I calculate about 9 amps draw at 120v. Then converting from 120v to 12v with a 90% efficiency, I calculate about 100amp draw. That's per hour, right? So if I run the microwave for about 15 minutes, I am using about 25amp hours. that's a lot of draw on just 2 group 24 batteries with about 85amp hr each. Are my figures correct?

Your calculations are basically correct. But, the faster the rate of discharge, the fewer amp-hrs a battery bank can put out.http://en.wikipedia.org/wiki/Peukert%27s_law

Hi,Rule of thumb is divide input watts by 10. An 1100 watt micro might have an input of 1500 watts, so demand may be on the order of 150 amps. Unfortunately there is another factor called a Peukert Calculation so apparent draw may be a heck of a lot larger.On a 170 amp-hour battery bank a 150 amp draw jumps up to 354 amps after the calculation. Your batteries will unlove you at that level of discharge.Basically you can probably get away with heating up a cup of coffee, but a 15 minute run time may take the battery bank down below 50% state of charge. It may be possible the batteries would be permanently damaged.

Use the "divide by 10" to get the inverter's amps draw from the 120v wattage of what is running. So 1500w would be 150 amps.Now look at the label on the microwave for its wattage rating. That will be about 1500w for a "1000w" microwave. MWs are named for cooking watts not for the watts they need to run them.Next, when running microwaves inverters are different when MSW or PSW. The MSW type (if it will run the MW at all--many do just fine, some don't) will run the MW at lower power and draw fewer amps as a result. So you can run a 1000w MW on MSW and it will draw 110amps say, instead of the full 150 amps that a PSW will draw. However the MW will run slower, take a little longer to cook/heat up something.Next is the battery bank to handle that big draw. First any battery bank will have an immediate voltage drop on that load, so when you turn on the MW, you want the battery voltage to stay above 11.0v, the inverter's alarm off. That means if the voltage drops 1.0v , you have to start off above 12v. Then voltage wil,l fall slowly while the load is on, so you need some room for that, so let's say you need 12.2v to run the MW for a couple minutes before the alarm goes off.If battery voltage is already down from the TV/DVD being on and the furnace while you watch a movie, and you decide to make some microwave popcorn, that means you need the batteries to be up even more at the start.So to get that ok, you need good wiring between the inverter and battery bank, and you need a big enough battery bank to hold up that voltage above 11v so you can get the job done.Think in terms of four Wet batteries of about 440AH and wire maybe two three foot lengths of #2. However, AGMs can hold up their voltage better under load, so you could get away with two of those, depending on just how big the load is and for how long. That means if you have a small rig with room for only two batts and you want to do microwaving, you should make those two batts AGMs.

160 amps to start and climb to 175 amps as the battery voltage drops.This is why you need four plus batteries. Small group 24 you should have six or more.Good luck running 15 minutes on the pair.

What we did here with our OFF-ROAD camping trips with our 255AH (3EA 85AH Interstate in parallel was to purchase a el-cheapo white faced manual knob controlled 600WATT microwave from WALMART. We like to use this just to quickly warm up coffee and maybe a few minutes to do a quick frozen TV dinner type snack. Baked POTATO with our steaks is a must have for us as well and this microwave works ok for a couple of baked potatoes with my 1500WATT pure sine wave Inverter.. We probably only use our microwave maybe 10 minutes total during one day/night run off the batteries.I tried running my big 1500WATT microwave and it eats up my battery power rather quick...The small microwave solved this problem for us...Roy Ken

12v inverter draw on batteries with microwave | Good Sam Community

37 Replies

BFL13
Explorer II
Dec 18, 2014
jrnymn7 wrote:
Piano,

I have the Xantrex PROwatt SW 1000.

Sorry, I can't say with any certainty, as to the 120v draw. I've been meaning to confirm with my kill-a-watt, but according to the display on the Xantrex, the 700w Danby pulls over 1200 watts. BFL has the same microwave, and he said that seems too high? Ours are both rated at 1080w, I think it is? What's worse, the Xantrex doesn't even register anything under 18w, so that could push the wattage up even higher.

As for 120 line voltage (drop), that too needs to be confirmed with my kill-a-watt. And if I remember correctly, the Trimetric's display says the microwave pulls in excess of 90a?

Vbatt drops to the low 11's, on say a 90% bank.

But I knew going in, I was taxing my system by using the microwave. At some point, it just makes more sense to run off gen power. For example, 5 minutes of microwave time can easily turn into 20-30 minutes of gen run time, to replenish the Ah's used. Same goes with any high power-hungry appliance. But running the gennie for 5 minutes is quite ridiculous, as well, so...

To clarify, my inverter is a Can Tire 1000w PSW made by Xantrex. It has a continuous rating of 1000w and it will run on overload for 5 minutes.

When I ran my Danby 700 the inverter red light was on for overload. The watts readout was 1050w approx. (from memory) I did not have the MW running more than 5 minutes for that test. Trimetric amps was close to 100 (high 90s ISTR) Inverter connection to batteries is two 16" (approx from memory )lengths of store-bought #4 starter cable.

Manual says after the 5 min of overload you have to wait 15 minutes before it will do that again.

The manual does say the wattage it can do varies with temperature but not by how much. Rating is for 77F.
jrnymn7
Explorer
Dec 18, 2014
If only Tesla was still around...
pianotuna
Nomad III
Dec 18, 2014
so....get an electric remote start for the genny? LOL

When I was urban docking in October with night time temperatures well below freezing, I would run the genny for 15 minutes to warm the bedroom up, repeating in about six hours. I refuse to sleep with the generator running. Last spring, after the warranty expired, I added a remote start to the generator. It is VERY nice to be able to shut down the generator when I'm in my bed.

I'm considering (toying with the idea) of adding a small inverter for just the fridge. It would not need to be psw but I should probably have a remote control for it.
jrnymn7
Explorer
Dec 18, 2014
Piano,

I have the Xantrex PROwatt SW 1000.

Sorry, I can't say with any certainty, as to the 120v draw. I've been meaning to confirm with my kill-a-watt, but according to the display on the Xantrex, the 700w Danby pulls over 1200 watts. BFL has the same microwave, and he said that seems too high? Ours are both rated at 1080w, I think it is? What's worse, the Xantrex doesn't even register anything under 18w, so that could push the wattage up even higher.

As for 120 line voltage (drop), that too needs to be confirmed with my kill-a-watt. And if I remember correctly, the Trimetric's display says the microwave pulls in excess of 90a?

Vbatt drops to the low 11's, on say a 90% bank.

But I knew going in, I was taxing my system by using the microwave. At some point, it just makes more sense to run off gen power. For example, 5 minutes of microwave time can easily turn into 20-30 minutes of gen run time, to replenish the Ah's used. Same goes with any high power-hungry appliance. But running the gennie for 5 minutes is quite ridiculous, as well, so...
pianotuna
Nomad III
Dec 18, 2014
Hi jrynmn,

Which 1000 watt psw do you have?

What happens to the 120 side voltage under the load of the microwave?

What happens to the DC voltage under the load of the microwave?
jrnymn7
Explorer
Dec 18, 2014
Another thing to consider is the inverter's 120v amp rating. My 1000w psw is rated at 8.3a continuous. My 700w (output) Danby easily exceeds that. And on a near full 430Ah bank, it takes longer than expected to nuke scrambled eggs.

I use one of these little guys for coffee. It requires no paper filters.

http://www.walmart.ca/en/ip/salton-space-saving-1-cup-coffeemaker/6000016950358
Hurricaner
Explorer
Dec 18, 2014
One of the best investments I have made this year is a quality PRESSURE COOKER. It does things no microwave can ever do. I like stew and I like beans, both of which cook in 1/5th the time they do with conventional cooking.
X2

Sam
MEXICOWANDERER
Explorer
Dec 17, 2014
One of the best investments I have made this year is a quality PRESSURE COOKER. It does things no microwave can ever do. I like stew and I like beans, both of which cook in 1/5th the time they do with conventional cooking.

TIP: COOKING RICE!

RINSE rice first. Let it soak in lukewarm water for 15 minutes, swirl the water and rice around which helps to suck out excess starch. Drain.

No matter what kind of vessel you use, flatten the raw rice out in the bottom of the pot. Add water.

The depth of the water to the TOP of the rice should be one cut short thumb nail's worth from the base of your nail. I add a smidgen more for extra soft rice. But it's barely more than the thickness of a hair more water.

Light off the stove. Bring water and rice to a boil. Reduce flame to absolute minimum. Cover pot. Set a timer.

FIFTEEN MINUTES LATER. Chop the heat to the rice. Do not peek. Let the rice sit unmolested for 10 minutes more.

At 18,000 feet the cooking time may be a half hour. Experiment.
DAS26miles
Explorer II
Dec 17, 2014
Thanks Sam. Will plan 24 hrs in advance instead of 8-10 hours.
Never use it for coffee. We use a metal coffee pot to boil the water on the stove. Then we use Melita individual little brewers (a plastic cone shape devise with filter and coffee grounds) atop our cups. I add extra grounds to each and pour in boiling water. When the cups reach full, I move the brewers over to a large measuring cup and continue pouring in rest of boiling water for extra cups. If we need to reheat, I pour the coffee back into the pot and reheat on stove top.
Hurricaner
Explorer
Dec 17, 2014
Dave, You just don't have enough battery to run a full size microwave. The smaller 600 watt units will probably work. We do the same thing but we take the food out and put it in the refrigerator at least 24hrs ahead of time. Here is an article that might help. Note that you can leave it thawed in the refrigerator for at least 2 days and even refreeze it if you change your mind.

Sam

Stick it in the ’fridge!
This is the best method and requires that you plan your meals ahead, which is also a good practice. Small amounts of frozen meat—a pound of ground meat or boneless chicken breasts—require a full day in the refigerator to thaw. Larger frozen meat items—roasts, hams, turkeys—require at least 24 hours for every 5 pounds of meat. When thawing foods in the refrigerator, the temperature of the regrigerator matters. Variations in refrigerator temperature can add to thawing time.

Items such as ground meat, stew meat, poultry, seafood, remain safe up to 2 days after thawing in the refrigerator. Red meat cuts—beef, pork or lamb roasts, chops, steaks—remain safe up to 3 days. Meats thawed in the refrigerator can be refrozen without cooking.