# Microwave math check

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Banned Posts: 17,615 ✭✭✭
Let's see if I've got this right.

A microwave oven that uses 1200 Watts (not cooking power) would in 5 minutes use the equivalent of 100 Watt/hrs? (5 minutes being 1/12 of an hour, 1200/12 = 100). And it will consume just over 4 Amp/hrs on my 24 V system?

So using that microwave for 5 minutes is like running the computer set-up for 30 minutes (200 Watt/hrs).

Just checking the viability of it. Or lack thereof.

• Solar Expert Posts: 5,436 ✭✭✭✭
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Re: Microwave math check

Marc,

If you use 6 minutes you get to a nice decimal .1 hour. Helps save a bit of confusion.

1200 watts for .1 hour is 120 wh for the the microwave. Other than that,, I am not sure what you are asking, since you didn't provide computer info.

T
• Banned Posts: 17,615 ✭✭✭
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Re: Microwave math check

Tony;

Just trying to get a handle on the equivalent Watt/hrs a microwave would use given its high power consumption yet short duration of use.

The comp draws approximately 200 Watts when running full-out, as it were. Over-all, it's our biggest hourly consumer.
• Solar Expert Posts: 5,436 ✭✭✭✭
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Re: Microwave math check

Plug your kill-a-watt in, should give you real time data on both,

T
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: Microwave math check

marc,
your general line of thinking is correct.
in the title you had me somewhat curious if their was a new math.:roll::p:D
• Banned Posts: 17,615 ✭✭✭
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Re: Microwave math check

I'd use the meter, but the microwave I just bought utterly failed to function when I plugged it in. It had a good 'ratio' to: 1050 W in to 700 W cooking. The next choice uses 1200 Watts, so I'm trying to double-check my assessment and see how viable it is. Big power users all!
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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Re: Microwave math check
So using that microwave for 5 minutes is like running the computer set-up for 30 minutes (200 Watt/hrs)
Assuming perfect inverter efficiency, a 1200 W inverter running for 5 minutes will use 1200 W x 5 minutes / 60 min/hr = 100 Wh of energy from the battery bank. A computer that draws 200 W and operates for 30 minutes would use a comparable 200 W x 30 min / 60min/hr = 100 Wh.

However, microwave's instantaneous load on the 24 V battery bank (~56 A for 90% inverter efficiency) is different than the computer load (~9.3 A).

When a battery is first loaded, the battery voltage drops, and then recovers somewhat as the electrolyte starts to mix and fresh electrolyte contacts the battery plates. This voltage drop and recovery is called the coup de fouet, or "crack of the whip".

Depending on a combination of (relatively small) battery bank size, (low) battery SOC, (poor) battery health, and (low) battery temperature, a heavy load surge can cause a significant initial voltage drop. If the battery voltage drops below the inverter's low battery cut-off voltage, the inverter may shutdown, and/or the load may not be able to complete its start.

A smal load such as the conputer may not be "big enough" to cause this behavior.

HTH,
Jim / crewzer
• Banned Posts: 17,615 ✭✭✭
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Re: Microwave math check

Jim;

I understand what you're saying. This is often a problem with pumps, for instance, which demand a heavy, sudden load at start-up that the batteries can't always supply.

Fortunately in my case this shouldn't be a problem, as the water pump and sump pump both manage their start cycles, and they draw about as much as the microwave would I should think (the sump kicks in at 16 Amps AC, then settles back to 8).

It's the over-all consumption that I think will be the problem. As you keep adding loads, however small or large, the daily capacity goes down, down, down.

I can always fire the generator for a few minutes if necessary. What a pain! Of course first I have to get a microwave that actually works. I'm hoping it will provide further propane savings for re-heating things and time savings for defrosting.

Lots of people ask about running microwaves off solar panels. Consider this practical research.
• Solar Expert Posts: 814 ✭✭✭
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Re: Microwave math check

microwaves have a high start up current (inrush) about 3 times rated power.. it does not show up coorrectly on Kila a watt meter or similar as they only do 1 a second readings
Another thing to be aware of microwaves need to be on twice as long when used on modified sine wave invrerters as compared to grid or pure sine wave inverter..
A1200w microwave becomes a 600w on MSW inverter.
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Re: Microwave math check

A big help for you would be if you can use a laptop computer which will consume 20-60 watts average power, vs the 200 watts of a big system+monitor.

Also, checking other devices so that they are on only when needed (laser printers, etc.) can help too.

As you can see--6 minutes of 1,200 watts is not much (ignoring the starting current) in the big scheme of things vs a computer drawing 200 watts 10+ hours per day.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 452 ✭✭
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Re: Microwave math check

Just returned from the first full weekend of use in my solar camper. Working great so far. Here are some of the daily estimates that I used to size the system.

1000W Microwave - 2.5 min (.04 hr) = ~83 WH
TV - 3hr = 177 WH
Propane Fridge - 24hr = 78 WH (thermostat/control)

We tested the microwave last weekend by making popcorn. Trimetric showed 118A settling to 104A.

I think a MW is fine for a couple of times a day at around 5 minutes.

Craig
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Re: Microwave math check

Craig,

And how much sun did you "Harvest"? You should be in the 500+ WH range on reasonably sunny days. May not have even collected that amount if the batteries ended up in float by 11am.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 452 ✭✭
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Re: Microwave math check

I'm still developing skill at interpreting the info from the Trimetric. But the first thing I did upon arriving Friday evening (after 5 days @ 128WH/day) was to check "day since charged." It was .14, meaning earlier that afternoon. I was concerned that the highest battery volatge recorded was 14.9V. That seemed high for an AGM. However, the temperature in the camper, when closed down can get quite high. For two 8AGC2's in series the TC corrected upper limit for bulk/absorb is 14.85V at 98F. Seems about right. The "low batt" value was 12.5V. About 10% Doc. Batt % read FUL

That was Monday through Friday of last week. Friday night it rained, we stayed inside, watched TV and ran the exhaust fan. Batt V never went below 12.6. Saturday morning it was cloudy until 11AM, then full sun. Highest V that I saw was 14.4. Very stormy, steamy night Saturday. TV on for 4 hrs to track tornadoes (they went south of us.) Ran the exhaust on high all night supplemented by a small 120VAC fan blowing over the bed for part of the night. Cloudy again Sunday morning but Batt V still @ 12.6 8). Sun came out around 1030 and Batt volts hung around 13.3.

Long story short, I'm not sure how to measure how much sun I harvested, but I was pleased to see that the lowest Batt volts was 12.6.

Contrary to the good advice on this forum, we are going to increase our usage! :roll: Just for testing.

Satisfiedly,

Craig
• Banned Posts: 17,615 ✭✭✭
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Re: Microwave math check

Okay, the working microwave is in place.

The power ratings on it are: 700 Watts 'cooking' out of 1050 Watts 'usage'.
The meter does some crazy dancing when this thing is on. Peak Wattage was 1085. When it's not actually 'energized' it drops down to 39 (running turntable, controls, internal light). In between it ramps up and down, usually topping at about 1058. What the PF is on this unit I don't know. I may try boiling a cup of water on 'Hi' just to get a solid V/A reading.

Over-all, it's not that bad. But it is like running the septic pump in terms of power usage, and it takes longer (the pump is about 1.5 minutes per day).

Any suggestion for other 'experiments' just so we get a complete data set?
• Solar Expert Posts: 814 ✭✭✭
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Re: Microwave math check

this was my #9 post.
microwaves have a high start up current (inrush) about 3 times rated power.. it does not show up coorrectly on Kila a watt meter or similar as they only do 1 a second readings
Another thing to be aware of microwaves need to be on twice as long when used on modified sine wave invrerters as compared to grid or pure sine wave inverter..
A1200w microwave becomes a 600w on MSW inverter.
what is causing the meter you using to give errors and "dancing " amp readings is the way a microwave oven works.. it really only gets its power from the peak of the sine wave but when powered from an inverter that does not happen as there really is no peak.. its a flat top wave.. so the magnetron ends up working in a way its not designed to .. its hard to explain it in simple terms..
But it does not seem to kill the ovens opperating like this.
• Banned Posts: 17,615 ✭✭✭
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Re: Microwave math check

Well it is a pure sine wave inverter, so no problem there.
But you're quite right: the meters don't react fast enough to 'see' precisely what's going on.

This is a real world experiment for all those who ask the question: "Can I run a microwave off an inverter?"
The answer is: yes, if you've got enough battery power to back it up.
• Banned Posts: 17,615 ✭✭✭
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Re: Microwave math check

Some final numbers from the experiments:

Voltage drops from 120 VAC to 110 VAC with the microwave on.
Power factor appears to be 0.88 (there's some guesswork here as the meter doesn't react fast and the numbers vary a lot even when on high).

Microwave: doable, but a big power consumer even for a short time.