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Old 11-19-2017, 08:57 AM   #1
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Question run 700 watt microwave: 10 minutes / Dual 6v

Hi all

We have two ETI installed solar panels on the roof of our 2017 21' Escape

we have the dual 6v. batteries.

If i run our new 700 watt (small) microwave for 15 minutes with the built in all plug inverter - how much of an impact will that have on our dual 6v batteries?

I dont know how to do the math.

Assume full sun, but fall winter in Southern California.

Might it be a good idea to think about avoiding microwave usage to cook a meal at dinnertime, so instead to lunch time meal so the batteries have afternoon sun (low angle even) to recharge somewhat?

thanks for any info.

John
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Old 11-19-2017, 10:19 AM   #2
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I have a similar setup. Running my microwave for about 6-8 minutes will draw down my batteries up to 10% of their charge. And if the battery charge gets below a certain level, around 60% I think, the inverter will shut down altogether.

So I think that running the microwave for fifteen minutes is pushing the limit of what your system will do, and even then your batteries will need to be fully charged or nearly so before you begin.

The charge and drain relationships don't seem to be linear so it's really a bit more complicated, but that's it in a nutshell.
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Old 11-19-2017, 10:25 AM   #3
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Quote:
Originally Posted by Losangeles View Post

Might it be a good idea to think about avoiding microwave usage to cook a meal at dinnertime, so instead to lunch time meal so the batteries have afternoon sun (low angle even) to recharge somewhat?
John
Yes, especially if you need to use your furnace at night, but keep in mind that you don't get much charge from the panels after midafternoon. You will adopt this and other strategies to manage your power. I have found this to be an interesting and educational experience.
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Old 11-19-2017, 10:47 AM   #4
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The math says you'll be fine, using about 20 amps. Should get at least 30-35a back from the 2 panels. But I have no practical experience with it.
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Old 11-19-2017, 11:13 AM   #5
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My experience is similar. If your batteries are below 75% it is questionable if the microwave will even startup. With a full charge I might have trouble with a full 15 minutes of microwave time. Age of batteries will certainly factor in.

However, if you ran for seven minutes, paused for a food change and then ran for seven more minutes you would have a better chance. The pause gives the batteries a chance to recover.
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Old 11-19-2017, 03:37 PM   #6
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Quote:
Originally Posted by Losangeles View Post
Hi all
....

If i run our new 700 watt (small) microwave for 15 minutes with the built in all plug inverter - how much of an impact will that have on our dual 6v batteries?

I dont know how to do the math.
...

thanks for any info.

John
Here's a quick overview of the math.

1) Assume some power is wasted in the wires and in the conversion from 12 to 120 volts. So the real drain on the batteries is roughly 800 watts.

2) Convert 800 watts to current by dividing by 12 volts, you get 800/12=66.6 amps. This is the current you would see if there was an amp meter in the battery wire.

3) Convert 66.6 "instant" amps to amp hours. 66.6 amps * 1/4 hour = 16.6 amp-hours.

4) Estimate how many amp-hours your dual-6's can provide. When new and fully charged you can pull 100 to 140 amp-hours before reaching the level that might degrade the batteries in the long term (long term = many repeated discharges to that level). (Actual value is subject to much debate.) So 16.6 amp-hours is not a terrible drain and should be replenished by a couple of hours of sunshine.

5) Important notes that previous posters have noted: The math above is assuming that everything is linear. Unfortunately, with batteries this is not the case in real life. For example, you can pull more amp-hours out of a given battery if you pull them at 1 amp rather than at 66.6 amps. The other consideration, power inverters will cut off if the voltage drops below a certain point - to prevent damage to themselves and to the batteries. That magic point depends on many variables such as battery age, wire diameter, battery terminal corrosion, brand of inverter, etc.

Bottom line: It won't hurt to try it out once or twice while the sun is shining, making note of the state of charge when the sun stops hitting the solar panels. Experiment and eat a hot meal while doing so.

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Old 11-19-2017, 03:45 PM   #7
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thanks everyone for the great info.

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Old 11-19-2017, 04:01 PM   #8
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Quote:
Originally Posted by alanmalk View Post
Bottom line: It won't hurt to try it out once or twice while the sun is shining, making note of the state of charge when the sun stops hitting the solar panels. Experiment and eat a hot meal while doing so.

--
Alan
I agree. The best way is to simply start using it. You'll very quickly get feedback on what's a reasonable amount of usage. I have to admit that the first few times I used mine and the meter showed amperage draw in the 60's I did a big gulp. But when all is said and done after a few minutes the battery hardly shows the inverter use for things like heating up a bowl of stew.

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Old 11-19-2017, 04:06 PM   #9
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Originally Posted by Losangeles View Post
If i run our new 700 watt (small) microwave for 15 minutes with the built in all plug inverter - how much of an impact will that have on our dual 6v batteries?

I dont know how to do the math.
700 watts is the output power of the microwave; they are always marketed by output power, unlike most appliances. The label on the appliance will provide the input power, but it will likely be about 1000 watts.

Power is the product of current and voltage, so 1000 watts at 12 volts would be about 83 amps from the battery to the inverter... or a bit more, due to inefficiency in the inverter. Of course, it would be less if your microwave uses less.

As Alan said, charge from the battery is current (in amps) multiplied by the time (usually in hours for this purpose). That gives you about 21 amp-hours from the batteries.

Quote:
Originally Posted by Losangeles View Post
... we have the dual 6v. batteries.
The battery capacity is rated at 225 amp-hours. Although the batteries should never be discharged completely (and 50% discharge is a common recommendation for routine use), 20 amp-hours is no problem, as Alan noted.

Quote:
Originally Posted by Losangeles View Post
We have two ETI installed solar panels on the roof of our 2017 21' Escape
...
Assume full sun, but fall winter in Southern California.
Sorry, I don't recall what your panel ratings are - the current option list for the 21' only shows a single 160 watt panel.

The panel rating is under completely ideal conditions. In those conditions, the panel power divided by the operating voltage (which is likely about 17 volts) gives the ideal current. For instance, a 160 watt panel might be able to produce 9.4 amps. There are lots of factors which reduce this output, including the angle between the panel and the sun (which will always have a substantial effect on a horizontal panel, especially in winter, and even in southern California), so this panel can realistically produce a few amps, for a few hours a day. Multiply the current and time together and this panel could put as much back into the battery as the 30 to 35 amp-hours which Bob suggested... or maybe barely cover the microwave use.

Lots of people put in the optional 1500 watt inverter, and some of them use it to run a microwave oven. This is obviously possible, but the details depend on other electrical power consumption, specific solar power conditions, and other sources of power (such as charging from the tug while driving, or occasional use of serviced campsites).
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Old 11-19-2017, 04:26 PM   #10
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A Microwave is one of the biggest power hogs in the trailer if you boondock. It's one of the reasons we didn't get it, and we use other methods for cooking. Can't say we miss it.

Having said that, with two panels and slightly angular full sun for at least a few hours a day, you should recover quickly.
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Old 11-19-2017, 04:43 PM   #11
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I've read you should figure on losing 2/3 of the max power generated by your panels come winter for the flat mounts. I recovered 50A on a perfect summer day here in MA. If this is so I'd get 15+ amps from my single 160 watt in winter, hence 30 - 35 amps for 2 panels. Don't know how this would scale for southern CA.
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Old 11-19-2017, 04:50 PM   #12
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I've read you should figure on losing 2/3 of the max power generated by your panels come winter for the flat mounts. I recovered 50A on a perfect summer day here in MA. If this is so I'd get 15+ amps from my single 160 watt in winter, hence 30 - 35 amps for 2 panels. Don't know how this would scale for southern CA.
Of course all of the "amps" in this are actually amp-hours of charge, not amps of current.
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Old 11-19-2017, 08:32 PM   #13
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When I run my microwave, the battery monitor shows a drain of around 100 amps, sometimes a bit more. The first time I saw that I stood next to the trailer door in case I needed to flee. I have a 700W microwave and the dual 6V batteries as well.
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Old 11-19-2017, 09:12 PM   #14
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We have used our microwave while boondocking, but we use it for seconds maybe a minute. Cheese & crackers or nachos stuff like that. Even at home we do the same thing. Heat up leftovers and the like. If we want to cook we use the barbi. My wife wanted a microwave so we got it. Actually she let me decide on most everything else so what could I say.
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