How to determine how many amps my residential fridge takes?

So... I'm looking at solar installations and trying to determine how much I need. My rig is a new class c with an 18 cu ft residential fridge being the big electrical consumer. 2 12 volt batteries and an inverter that runs fridge/ tvs/ plugs/ etc.

I looked at the fridge and it says 378 kw per year. I'm sure that's an average consumption in a house... how do I determine how many amps it will pull for me in a motorhome?

thanks!

Ok I found another sticker on the fridge and it says "full load amps 6.5"......

so how do I figure out how many watts of solar I need to run the fridge?

doubravsky said:

so how do I figure out how many watts of solar I need to run the fridge?

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6.5amps AC x 120VAC = 780 watts.
That is a fair number of solar panels.

You also need the battery pack capacity to run overnight.
780 ? 12VDC = 65 amps DC.    12 hours of night x 65 amps = 780 amp-hours. 

780 amp hours is a real battery draw.  Hope your battery pack capacity is big ... really big


By the way, are you planning to run any other electrical loads  -- they will add to the requirements for panels and batteries.

Carl's calculations are fine for sizing your system for average power usage, but be careful with respect to sizing for peak instantaneous current draw.  I don't know what fridge you are using but the Samsung's are popular among RVers so I'll use those for my example.  Samsung makes it very clear on their FAQ pages that inverters need to be sized to accommodate ~11 amps peak load. 

A couple of years ago the FAQs were more "welcoming" of the use of Samsung fridges in RVs, but it seems as if there may have been issues with people undersizing the inverters they were using because they were just looking at continuous current draw, not peak instantaneous.

Your full amp load of 6.5 is when the unit first starts up and the compressor draws more when starting, once running it'll pull a lot less. Our Maytag French Door unit pulls only 1 amp after the compressor gets going. Just like an A/C there's a much bigger draw until the unit gets up to speed so to speak.

doubravsky said:

So... I'm looking at solar installations and trying to determine how much I need. My rig is a new class c with an 18 cu ft residential fridge being the big electrical consumer. 2 12 volt batteries and an inverter that runs fridge/ tvs/ plugs/ etc.

I looked at the fridge and it says 378 kw per year. I'm sure that's an average consumption in a house... how do I determine how many amps it will pull for me in a motorhome?

thanks!

Click to expand...

The peak power the refrigerator draws vs. the average power used over the course of 24 hours are two different things.  The size of the inverter and it's wiring have to be large enough to handle the starting surge and the power usage of the refrigerator when it's running, but the average amount of power it uses over the course of a day will be less than that, because the compressor doesn't run 100% of the time.

The power the refrigerator draws will be roughly the same whether it's in an RV or in a house, so you're looking at an average consumption of 378 Kwh (kilowatt-hours) per year.  Since there's 365 days in a year, that's a little over 1 Kwh per day.

Power = Voltage x Current, so 1 Kilowatt-hour at 12 volts is 83 amp-hours.  A good rule of thumb is to round up the 12 volt draw to 100 amp-hours per Kwh to allow a margin of error for inverter loss, etc.

100 amp-hours is the amount of energy you can store in a pair of 200 amp-hour 6 volt golf cart batteries, discharged to the 50% level.

You'll have to generate a like amount of power (100 amp-hours) to recharge the batteries each day.  An average day has about 6 hours of full sunshine, so your solar system will have to generate at least 16 amps of current to replace 100 amp-hours over a 6 hour solar day. 

When it comes to any appliance that has a 15 amp plug (2 flat parallel blades and an optional round pin) there is a super easy way to find out how much power it uses both at any given point in time (Such as when just plugged in) and over a period of time.

YOu buy about a thousand dollars worth of meters and plug them in.. Thankfully.. This does not cost quite a thousand dollars.. In fact it costs more like 20 bucks and sometimes you can pick them up for half that.

A Kill-a-Watt p3. 

Yup,  The very popular among many of us Kill-A-Watt is an amazing device.

It will show current amps draw
Current watts draw
Current POWER FACTOR (can be important)
and it will show KWH  Which is what you need to find out for your fridge,, Total power used in a 24 hour (OR better yet 240) Hour period, OH, and it shows hours since it was plugged in too..  That way you know what you need to budget for.

Thanks for all of the information! My fridge is a Whirlpool. I have noticed that it cycles off and on every once in a while after it's all cooled down... and the inverter is an 1800 watt inverter that came OEM installed in the rig.

Adding to that... We realized after a couple of nights of draining the battery to an inverter warning beep at 3:30 am, that once we added 4-5 Walmart plastic ice thingees, froze them and put them in the fridge at bedtime, we could turn off the fridge and by 10:00 am, it was still around 40 degrees and freezer stuff was still frozen. So we plan to run the fridge probably 14 ~ hours per day. But I'd like to get to the point where I don't have to run the generator multiple times during the day when boondocking.

When I look at solar stuff... they talk about wattage and not amp hours.... what amount of solar panel wattage would I need to generate 100 amp hours?

Installing solar panels to overcome the draw of appliances in an RV is a balancing act - I know, because I'm right in the middle of it. It's easy to put too big or too small of a solar array on an RV. Too small and you'll be disappointed, because you're still drawing your batteries down - too big and you'll be unable to store all the power that your panels are capable of producing (That's the situation I'm in right now). Carl's math and the info about additional loads are important considerations.

Our coach has a lot of appliances, including a 20 square-foot Maytag residential fridge that's rated at 653 KWh per year. We have six 160 watt panels on the roof (960 watts), an Outback FM-80 MPPT solar controller and six AGM house-batteries that are rated at 400 amp hours. Even with all those batteries, we still don't have enough battery capacity to store all the power that's being generated by the panels. In direct sunlght, the panels are putting about 45 amps back into the battery-bank each hour, but when the solar controller detects that the batteries have reached 14.4 volts, it throttles back and goes from bulk-charging to absorbsion-charging. That happens even though there is plenty of daylight left for additional bulk-charging.

Basically, I'm wasting some of the charging capacity of the solar panels but the fix is pretty easy - install additional house-batteries, which I'm about to do. Then, of course, you need to know if your RV's battery charger is capable of putting out enough amperage to charge a substantially larger battery-bank in a reasonable amount of time. Some are, some aren't. Like I said, it's a balancing act.

Kev

After my last post I saw that you had just posted again, and you asked a question about how many panels you'd need to generate 100 amp hours in a day. On paper, it's going to seem better than in reality. I say that, because I know what the performance specs for my panels say, but I also know what I'm really getting out of them.

The specs for my 160 watt panels say that each panel's output is rated at 8.65 amps per hour. But remember, that's on a clear sunny day in direct sunlight with an ambient temperature of 75 degrees. Solar panels become less efficient as they get warmer. In reality, after running my panels' output through about 20 feet of 8 and 4 gauge wire, on a 92 degree day (today), my six panels are averaging 7.5 amps (each), each hour. If you factor in six hours of daily sunlight (I think five is more realistic but it's pretty subjective), that means three of my panels would generate 135 amps each day - under good conditions.

If you really wanted to get 100 amps each day out of a solar array, I wouldn't get anything less than that. By the time you factor in partial cloud-cover, parasitic loads, partial shading and other "real life" factors, that's what you're going to need to see 100 amps - and that might not even happen.

Kev

Kevin- thanks for the info. How do I determine whether my battery charger can put out the amps to charge additional batteries, and is that the inverter or the converter?

My inverter says it is a Xantrex Pro XM1800....and it says "MSW, WXFER 1800 watt" on a little page they gave me with serial numbers.

the converter says "converter, power, 55 AMP, 30 AMP AC, 11 DC Circuits" 

I don't know what those descriptors mean yet though

I was thinking in the 400-500 watt range would meet my needs.... which sounds like you described a 480 watt set up......

With the inverter I have already, I'd only need the battery and a charge controller I think?

Watts (and amps too) are instantaneous measures of consumption.  You need to factor in time (hours) to determine how much power has to be stored, so the units you need are watt-hours or amp-hours.  Battery amp-hours are assumed to be at 12v, so an amp-hour on a 12v battery is one amp usage for one hour @ 12v.  Watts are equal to volts x amps, so that same one amp @ 12v represents 12 watts. That means that one 12v battery amp hour is 12 watts x 1 hour or 12 watt-hours.

So, if you need 60 watts for 8 hours, that is 480 watt-hours. Divide by 12 to get battery amp-hours for a 12v battery: 40 amp-hours

Just remember that the usable amp hours in a battery is only about 50% of the rated amp-hour capacity.

Thanks Gary- its starting to make better sense now....

including a 20 square-foot Maytag residential fridge  Kevin... don't you mean 20 cubic foot?

I was able to look up the inverter and determined it is an inverter and not inverter/charger....

Can anyone tell me what these numbers mean - for the converter? Will it be able to handle supplying power to charge an extra two batteries?

the converter says "converter, power, 55 AMP, 30 AMP AC, 11 DC Circuits" 


thanks!

wackymac said:

including a 20 square-foot Maytag residential fridge  Kevin... don't you mean 20 cubic foot?

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Yup. That's exactly what I meant. Typing faster than I'm thinking.  ;D

Kev

That sounds like a combination converter/charger and AC/DC load center. The converter/charger portion produces a max of 55 amps, with the first priority going to any DC loads and the balance available for battery charging.

Any charger will handle multiple batteries - it's just a matter of how much time to replace the amount of amp-hours that were lost through discharge. It's tempting to think that a 55 amp charger will replace 110 amp-hours in just two hours, but batteries don't work that way. As the state-of-charge  builds up, the rate at which charging occurs slows down, so you only get the full 55 amps for a short time while the battery is severely discharged. Then the rate falls off and finishes at something less than 6-8 amp. So your 55 amp (max) charger may come up a little short of ideal at first, but after an hour it probably makes no difference. And if the batteries were only partially discharged to begin with, it makes no difference at all.

Dave, before I bought and installed my system, I did a fair amount of research and I picked the brains of a lot of folks on this forum. When it comes to inverters/converters/chargers, however, I only know specifics about my system, because I read the manuals, but others here will be able to answer specific questions about your system better than I.

From what I understand, chargers are part of the converter and they're sized for the battery-bank that comes with the RV. If someone were to significantly increase the size of their battery-bank, the charger would still charge the batteries but it would take proportionately longer. And from what I've read, some factory-installed chargers might never be able to fully charge a battery-bank if it's too big. I'm quite sure there's a simple mathematic formula that would tell you how big your charger would need to be to charge whatever size battery-bank you have, but I don't know it.

I think your envisioned 400-500 watt solar array (with controller), would serve your needs well. And yes, other than wiring and a fuse, that's all you'd "need." I would recommend, however, that you also purchase a good battery monitor (not a simple volt-meter). A good battery monitor will let you monitor your battery-bank's condition easily and accurately (I like the Trimetric 2030 for RVs). Pay close attention to wire-size, wire-run length and where your panels are mounted. There are some seemingly small details about solar installations that can have a significant impact on the overall efficiency of the system. When you start shopping for controllers, I highly recommend buying a good quality MPPT controller. They're more expensive but they get more energy from an array that size than a PWM controller.

I'm certainly not the only person here who's done what you're doing, and I'm sure others will give you good advice. Keep asking questions so we can, hopefully, help you avoid some problems and save you some money.

Kev

Thanks Kevin! I'll keep researching once I get home. What does "MPPT" stand for when looking for a good controller?