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Old 02-25-2021, 12:00 PM   #1
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Refrigerators, Start-up Surge and Inverters, oh my!

I've researched how to calculate my load and needs, but before I actually order my inverter/charger, I'd appreciate a little reassurance of my understanding of how all this is supposed to be calculated and/or correction of my math.

The inverter I'm considering is the Xantrex xc 2000 with a rating of 2000w continuous and a surge of 4000w for 2 seconds. I understand that it's a high frequency inverter and they don't handle surges as well or for as long, yet I'm thinking since this is a fairly small load that it's well under the demand of the inverter.

All my estimates are on the high, worst case scenario, side.

My loads are as follows:

4.5cuft 115v refrigerator running 24/7. It's rated at 330kwhr a year.
330kwhr / 365 days = .9kwh or 900w per day.
900w / 115v = 7.82a per day
900w / 12v = 75a a day (inversion multiplier of 1.2 =90a)

That's just the average, not sure what actual will be, but from what I read, figure a 33% duty cycle, yet I also read that newer refrigerators are trying to run the compressors up to 90% duty cycle to avoid start-up surge. I have no idea which duty cycle to use.

Here's where I get confused and hope I did this correctly.

900w per day / 24hrs = 37.5w per hour???

I'm assuming that since these are averages, actual running will be much lower and start-up will be much higher.

If the above is correct, and based on my research, surge for a refrigerator is somewhere between 1.5x and 10x (Xantrex support high estimate) running draws.

Worst case scenario is 10x, so 37.5w x 10 = 375w.

If it was just this, I know I would be fine, but, I also have these electrical draws to consider. Fortunately, they are all continuous, so that makes it a bit easier to compute.

Instant pot at 700w
Fan 28w
Computer and phone charging 70w

Realistically, these three appliances could be running at the same time when the refrigerator starts up.

Total of all appliances running at once (refrigerator at start-up draw) would equal just under 1200w.

How far off am I? Or, by some miracle, is my math correct?

Thanks!
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Old 02-25-2021, 12:12 PM   #2
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I love the math and how far you've dug into this...but I think you went a few miles past your exit! Energy, which is what you're looking at, is consumption over time. Power is draw at any given moment. So, for example, a 100W light bulb draws 100W of power when it's on. If it's on for one hour, that's 100Wh...ten hours would be 1000Wh or 10kWh. What you've done is (very nicely!) calculated energy usage, which you need for battery and charge capacities. It looks like you did this for the fridge, with the 375W figure, but you were right on track for the instant pot and others.

But for surge and inverter capacity, you need to look at power consumption of your appliance. If it's rated at, say, 500W when running then the surge might be 1000W when is starts.

Did I correctly read what you did? If not, redirect me.
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Old 02-25-2021, 01:48 PM   #3
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Quote:
Originally Posted by rossvtaylor View Post
I love the math and how far you've dug into this...but I think you went a few miles past your exit! Energy, which is what you're looking at, is consumption over time. Power is draw at any given moment. So, for example, a 100W light bulb draws 100W of power when it's on. If it's on for one hour, that's 100Wh...ten hours would be 1000Wh or 10kWh. What you've done is (very nicely!) calculated energy usage, which you need for battery and charge capacities. It looks like you did this for the fridge, with the 375W figure, but you were right on track for the instant pot and others.

But for surge and inverter capacity, you need to look at power consumption of your appliance. If it's rated at, say, 500W when running then the surge might be 1000W when is starts.

Did I correctly read what you did? If not, redirect me.
1000 Wh is 1 kWh.
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Old 02-25-2021, 02:19 PM   #4
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Good math. I had a similar dilemma sizing my fridge, the only 'big-draw' appliance. I bought a clamp-style meter with the ability to capture 'max' or 'inrush' current, and did some experimentation with my dorm-style fridge.

It drew a max of 8 amps for a second or two, then dropped down to about 1.5 amps while the motor ran.

So it might be worth buying or borrowing a clamp meter to get the actual inrush current your motor(s) will need. For safety, I decided to use this as the minimum rating of the inverter and not rely on the peak capacity of the inverter to handle startup. This ensures the inverter will have a long life and not be stressed and prematurely fail.

Similarly, to protect the fridge compressor, I decided to get a pure sine wave inverter, not a modified sine wave. This ensures minimal stress on the fridge motor, again, to minimize the risk of premature compressor failure.

You may end up making different choices in your system.

Regarding total operating time: it is largely dependent on the usage-how much stuff is put in, how frequently the door is opened, and what the ambient air temperature is. I used the annual usage information provided by the manufacturer to estimate this. I did buy a watt-meter and set up the fridge in the house, but I never followed through with the measurements because I decided it was data collection overkill. To mitigate, the other thing I did to minimize cooling cycles was to add insulation to the fridge-basically, I put the fridge in an enclosure that was insulated with rigid foam.
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Old 02-25-2021, 02:21 PM   #5
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Originally Posted by rossvtaylor View Post
But for surge and inverter capacity, you need to look at power consumption of your appliance. If it's rated at, say, 500W when running then the surge might be 1000W when is starts.

Did I correctly read what you did? If not, redirect me.
The websites I checked when I was doing this exercise said assume rated wattage times 8 or 10 as a rough approximation of inrush.

My little dorm fridge was about 4x operating wattage, though I'm guessing this will creep up as the fridge ages.
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Old 02-25-2021, 03:57 PM   #6
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If you're worried about surge load on startup popping fuses or kicking the inverter, you might look into something called Lightning Caps. These are commonly used to prevent voltage sags on high-wattage car stereo systems. I can see where they might help prevent voltage sags causing problems with an inverter as well.
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Old 02-25-2021, 04:22 PM   #7
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My little chest freezer converted to refrigerator has about a 3.5 - 4 x surge when it is cold in the bus 60 degrees and about 10x if it is hot. 100 degrees. about 40% more on the modified sinewave inverter.
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Old 02-25-2021, 07:49 PM   #8
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Quote:
Originally Posted by mcduffieja View Post
1000 Wh is 1 kWh.
Yes, my error. I know better...just a typo. But good to catch and correct so nobody follows my pre-coffee math.
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Old 02-25-2021, 07:53 PM   #9
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Originally Posted by Rucker View Post
The websites I checked when I was doing this exercise said assume rated wattage times 8 or 10 as a rough approximation of inrush.

My little dorm fridge was about 4x operating wattage, though I'm guessing this will creep up as the fridge ages.
That's very likely. I just used the 500W and 1000W figures as examples, for easy math.

The only way to be sure, really, is to measure the current draw. A clamp-on ammeter will do this and every bus owner should own one anyway.
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Old 02-25-2021, 08:15 PM   #10
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For the fridge, you should consider one of the newer inverter compressors, their inverters do slow start so no surge and don't need sine wave, as the motor is not run directly off the 120vac. See my Custom Fridge thread. You really need to actually measure the surge and the run amps.

Let me also add that most chest freezers use the outside walls of the chest as a radiator for heat, and thus can't be tucked away inside an enclosed space. Also true for some small fridges.
Really best to have a condenser coil with a fan so you can direct that heat rejection, like outside the bus.
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Old 02-25-2021, 10:03 PM   #11
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Residential fridge here too. I didn't calculate too precisely how much it will draw before to plug it in. Here are real world numbers: it draws around 12 amp when cooling, and needs around 70 to 100 Ah a day, depending of ambiant temperature, when we live in the bus. It is a 10.5 cu.ft from Insignia.
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Old 02-25-2021, 11:14 PM   #12
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Thanks for all the replies.

1) I looked up inverter compressor refrigerators. The ones I saw were all full sized and expensive. Do you have a lead on something around the 4.5 cuft size with an inverter compressor?

2) The electrical math and lingo, at least for me, is tied with my understanding of women. Lots of short circuits on both counts.

3) I've heard of three ways to come up with start-up / surge wattage.

a) Rated Load Amperage (RLA) x 1.5. This seems reasonable. My home fridge is 4.5a x 1.5 = 6.75a x 115v = 776.25w start-up/surge

b) LRA - Locked Rotor Amperage (found on the compressor lable) x voltage. A small fridge with a 350btu compressor will have an LRA of ~8a. So, around 900w start-up/surge.

LRA - Locked Rotor Amps: The max current you can expect if the rotor is locked up.

c) 8 - 10 times the rated wattage. Xantrex support used 10 in their example. Online I find a mini-fridges run between 85-100w, an average house fridge is ~150w running with 800-1200 start-up / surge. Yet, at 8 - 10 times it would be more like 1200 - 1500w.

So, between the three ways, we've got 776.25w, 900w and 800-1500w.

My friend is a refrigeration contractor. He says what you want to know is the LRA. Multiply that by the voltage and that tells you the maximum your compressor will pull if the rotor is stuck (i.e., lockup). Your actual start-up surge will likely be a bit lower.

If you calculate your inverter wattage based on LRA amperage, you should be fine with your start-up wattage.

Rucker said his mini-fridge was pulling 8a at start-up and dropped to 1.5a when running. This would be ~3.5x.

Cadillac said 3.5 - 4x cold, 10x hot environment temp.

Ironically, I was searching a bit more as I was writing this and this question/answer came up on E-Trailers website: https://www.etrailer.com/question-442653.html

Question:
Can someone look at the nameplate on the Everchill 4.5 cubic ft 120V fridge, part # 324-000109, model BC-128B and tell me the LRA locked rotor amps? Am looking to size an inverter for this. I think the average current draw is 0.8 amps 96 watts and 24 hr running average is ballpark 0.69 Kwhrs about 30 watts so avg running power is 30 to 96 watts but locked rotor amps help determine the inverter size required. Thanks.

Expert Reply:
The Everchill RV Mini Refrigerator part # 324-000109 will pull around 8-9 amps on startup (LRA) and then around 1 amp while running.

LOL! I wish I had found this a few days ago. Lots of thinking and calculating could have been saved.

SUMMARY:
I think all three get you in a start-up surge ballpark.

I think the 1.5x method is maybe a bit on the low side. The LRA seems to be right about there for average environmental temperatures and the 8-10x is more worst case high environmental temp situations.

I guess if you're choosing an inverter, the worst case scenario is the one on which to base that decision.

Hope this helps someone else. I'm done!
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Old 02-26-2021, 02:14 AM   #13
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Yeah, the inverter compressors are only used on the full-size afaik, however, they can be used in a smaller fridge. I think the Samsung draws under 1 amp when running, and has no surge amps.

Always better to get a bigger inverter...going to add a microwave next, and a projection TV, and a hair dryer, and a blender, and a 3d printer, and a tool charger, and ..
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Old 02-26-2021, 08:58 AM   #14
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Quote:
Originally Posted by Simplicity View Post
4.5cuft 115v refrigerator running 24/7. It's rated at 330kwhr a year.
330kwhr / 365 days = .9kwh or 900w per day.

...

900w per day / 24hrs = 37.5w per hour???
Close! Correction:
330kwhr / 365 days = .9kwh or 900wh per day.

When doing conversion, remember that the equation:
ax = cbx
Can be reduced to:
a = cb

When you divide 900wh by 24h, you remove the "hour" or temporal component. What remains is the wattage.

900wh/24h = 37.5w

To add the temporal component back in, multiply by a value of time. 37.5w average load running for an hour consumes 37.5wh of energy:
37.5w * 1h = 37.5wh

37.5w is a measurement of power (instantaneous).
37.5wh is a measurement of energy (power over time).

Does that make sense?

Quote:
Originally Posted by Simplicity View Post
The inverter I'm considering is the Xantrex xc 2000 with a rating of 2000w continuous and a surge of 4000w for 2 seconds. I understand that it's a high frequency inverter and they don't handle surges as well or for as long, yet I'm thinking since this is a fairly small load that it's well under the demand of the inverter.s!
I'll just say that having used both HF and LF inverters, I very much prefer LF inverters. Even small surges on a regular basis can reduce the lifespan of a HF inverter significantly. Not saying there aren't reasons to go with HF or that HF isn't the better solution for you.

IMO, if you want anything like a shore power experience, LF is the way to go.
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Old 02-26-2021, 01:58 PM   #15
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Quote:
Originally Posted by CHEESE_WAGON View Post
If you're worried about surge load on startup popping fuses or kicking the inverter, you might look into something called Lightning Caps. These are commonly used to prevent voltage sags on high-wattage car stereo systems. I can see where they might help prevent voltage sags causing problems with an inverter as well.
Thanks for the recommendation. Did a little research on this. The search term to use is 'soft start'. There are a couple of products around. I'm not entirely sure these will work in practice to trim the inrush. Some sites warn of premature device failure, others don't recommend use of these or air conditioners.

Sure would be great for someone to do a bit of a tutorial on them, best practice, manufacturers etc.

Anyone already engineer soft start for induction motors in their rigs? What off the shelf solutions exist for typical bus applications?
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Old 02-26-2021, 02:06 PM   #16
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Maybe I'm wrong, not an electrician, but I think if your system can't cope with the relatively small starting surge of a refrigerator, it could make more sense to just upgrade the system than to find a way to make it accepting this surge by using add-on components.
Just may thoughts, and once again, I may be wrong (I often am).
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Old 02-26-2021, 02:25 PM   #17
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You're not wrong at all. Simplest is just have a House system big enough.

My comments tend to be focused on minimal/optimal solutions, which aren't necessarily other people's focus, so good point. For my bus, which just needs like four days' juice, I wanted to minimize the equipment needs. My inverter is big enough to handle the fridge.

On the other hand, if I wanted to use the same system for other devices, that initial surge prevents any other loads for about three seconds so a soft start solution with trim the peak might allow me to use the inverter for more things, all else being equal, without a bigger inverter and battery bank. A basic 'soft start' brick is about sixty bucks.

The vampire load on my inverter is 1.5 A, so I shut it down between cycles. Bigger inverters, bigger vampire loads, so bigger battery bank, and the costs creep ever further up.

Here's another example of this kind of thinking: I'm researching portable battery packs designed to supplement laptop battery life. I'm imagining plugging this right into my 12V system as additional distributed battery capacity that I might pull out and take in the truck or car as needs require.
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Old 02-26-2021, 04:18 PM   #18
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Your inverter draws 1.5amps just being turned on? Seems excessive, does it have a big fan? I just checked my 300watt (no fan, it does have one, doesn't turn on until it gets hot), it shows .3 amps turned on.
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Old 02-26-2021, 05:59 PM   #19
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Rated for 1.5A idle. I've not measured it-so good point.
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Old 02-26-2021, 10:14 PM   #20
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Quote:
Originally Posted by kazetsukai View Post
Close! Correction:
330kwhr / 365 days = .9kwh or 900wh per day.

When doing conversion, remember that the equation:
ax = cbx
Can be reduced to:
a = cb

When you divide 900wh by 24h, you remove the "hour" or temporal component. What remains is the wattage.

900wh/24h = 37.5w

To add the temporal component back in, multiply by a value of time. 37.5w average load running for an hour consumes 37.5wh of energy:
37.5w * 1h = 37.5wh

37.5w is a measurement of power (instantaneous).
37.5wh is a measurement of energy (power over time).

Does that make sense?
.
Great feedback and education.

Yes, LF is the best for induction, especially when it will be running all day.

I just ran across this Cotek 3000 LF inverter charger https://www.solar-electric.com/cotek...-inverter.html.

I think I remember seeing this brand name mentioned, but I've not heard must about them.

Any input would be appreciated.
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