Total Noob wants to run a mini fridge

[DIY_And_Artsy_Guy]

Hello, All!
I'm completely new to solar setups, and at this point I am just researching, but I have something specifically in mind. If I sound like a total and complete dipstick, please explain how. I think I've got the general just of things from the research I've done the last couple of weeks, but I could be completely wrong since I've never talked with someone who's done this before.
I am planning on going completely off grid at some point this year. I understand that you can create large solar setups to run an entire camper, but I'm not comfortable doing something that large scale starting off. My goal is to run my 4.2cu foot single mini fridge. I've been contemplating using 2 6v batteries that I already have (bought by mistake at Christmas and never got around to returning them) rather than buying a single 12v to test it out. The question is would my two 6v LiFePO4 with 6Ah and 36Wh run my mini fridge the same as a 12v in a solar set up? Just the mini fridge. Would it be worth it to try, or am I likely to nuke my fridge if I try?

 

[sunshine_eggo]

If it's an absorption fridge capable of running on propane, forget it. They consume massive amounts of power. Don't bother.

If it's a compressor fridge, it will use about 0.4kWh/day or 400Wh/day. Your batteries only have 72Wh.

You would need about 200W of PV with sunrise to sunset exposure to guarantee you could generate the needed 400W/day, but without enough battery storage, it's a no-go.

 

[DIY_And_Artsy_Guy]

If it's an absorption fridge capable of running on propane, forget it. They consume massive amounts of power. Don't bother.

If it's a compressor fridge, it will use about 0.4kWh/day or 400Wh/day. Your batteries only have 72Wh.

You would need about 200W of PV with sunrise to sunset exposure to guarantee you could generate the needed 400W/day, but without enough battery storage, it's a no-go.

It's a regular dorm fridge. Thanks for the explanation. I understand that a 12v battery with 100Ah can run a mini fridge, but is there a formula to determine how many Ah your battery needs to have depending on what the wattage of the single appliance you're trying to run is? I think that's where I'm running into a mental roadblock on this whole idea thing.

 

[Mattb4]

W=VA
Wattage over time gives you watt hour.
Amp hours X Battery nominal voltage =wh

So to figure how long a battery will power an item you need to know running watts and the amount of time it runs. You also need to know starting surge of an item in order to size things like an inverter properly. For items that cycle on and off like a refrigerator it helps to have a Kill-A-Watt meter to plug it in for a day and get a reading of watt hours.

 

[Rednecktek]

W=VA
Wattage over time gives you watt hour.
Amp hours X Battery nominal voltage =wh

So to figure how long a battery will power an item you need to know running watts and the amount of time it runs. You also need to know starting surge of an item in order to size things like an inverter properly. For items that cycle on and off like a refrigerator it helps to have a Kill-A-Watt meter to plug it in for a day and get a reading of watt hours.

Once you have that it's all about the math. Let's do some napkin math using handy round numbers:

So you got your WallyWorld mini fridge and plug it into a Kill-A-Watt. After a couple days of running the Kill-A-Watt tells you that it takes 22 watts to run but spikes at 327 watts at the highest (for example numbers).

Well, since your fridge is 120v, you'll need an inverter. Since nobody makes a 327 watt inverter, you find the next size Pure Sine Wave inverter up and get yourself a little 500w unit. That's plenty of overhead for that motor to kick on.

OK, so now you've got your inverter, let's look at the battery. The Kill-A-Watt says it uses about 22 watts to run, and over 24 hours that's 22w * 24hr = 528Wh. A bog standard LFP 100Ah battery is good for about 1200Wh, so that would last 2 days. Perfect!

Lead Acid batteries are only good for discharge down to 50%, so if you're using lead you gotta get double the battery to power the same load.

Now you gotta refill that battery by harnessing the power of the sun, so you go grab some panels. You get about 4 hours of usable sun per day on average, and want to generate 1200 watts in that time, so 1200w / 4 hours = 300 watts of solar panel. Rule of thumb is you'll see about 80% of what the label says, so we'll factor that in to the math, 300w of panel * 120% = 360w of panel would be about perfect. You can always have more, but I really don't recommend less than 300w.

The next bit is the charge controller that turns solar DC into battery DC. Assuming a 12v system (single battery) and 360w of panel, well 360w / 12v = 30a of charge controller.

So, to sum up our example system:

500w Pure Sine Inverter
100Ah LFP battery OR 200Ah of lead acid flavor
360w of solar panels
30a of charge controller


When you're looking at a small system like this, you have to either get a used solar panel from someone's old roof install and an MPPT controller, or you can get 100w panels off Amazon and use a cheaper PWM controller. With only a few 100w panels it's hard to beat Amazon. Used panels are usually MUCH better price per watt but would require a more expensive controller to work, so it's kind of a wash. Pro tip: If the controller has USB ports on it, it's NOT a real MPPT, it's just lying to you and laughing all the way to the bank. Fortunately you don't need anything expensive, you're just needing to turn the higher voltage from the panels into a lower voltage your battery can work with.

Does that all help make sense of things?

 

[sunshine_eggo]

It's a regular dorm fridge. Thanks for the explanation. I understand that a 12v battery with 100Ah can run a mini fridge,

Only if it's a 12V fridge. Regular dorm fridges run on 120VAC. So, the 12V/100Ah can't run it, but it can power an inverter that can power the fridge. That setup should power the system for up to 72 hours.

but is there a formula to determine how many Ah your battery needs to have depending on what the wattage of the single appliance you're trying to run is? I think that's where I'm running into a mental roadblock on this whole idea thing.

Does it have a yellow energy guide yellow label, or can you find that information for you model online? That gives you the annual kWh, which can be used to calculate the daily kWh and allow you to use the formulas given.

A 12.8V/100Ah LFP has 12.8V * 100Ah = 1280Wh of energy storage.

Using my estimated 400Wh/day:

1280Wh/400Wh/day = 3.2 days.

 

[wkyongae]

Don't want to be the bearer of bad news but that little battery won't work, not even for 24 hours. Reason, I have a dorm fridge without freezer. It says it draws 77watts per hour. That's 1848watts per 24 hours to account for cloudy days and I haven't added in the watts that little inverter in using just for the pleasure of being turned on. Oh, and it has a little 15watt light bulb in there too. The little plate inside the door says .8 amps I verified the amps and watts from my inverter panel and they lied it's 96watts so that makes it even worse.

 

[DIY_And_Artsy_Guy]

W=VA
Wattage over time gives you watt hour.
Amp hours X Battery nominal voltage =wh

So to figure how long a battery will power an item you need to know running watts and the amount of time it runs. You also need to know starting surge of an item in order to size things like an inverter properly. For items that cycle on and off like a refrigerator it helps to have a Kill-A-Watt meter to plug it in for a day and get a reading of watt hours.

Thank you!

 

[DIY_And_Artsy_Guy]

Only if it's a 12V fridge. Regular dorm fridges run on 120VAC. So, the 12V/100Ah can't run it, but it can power an inverter that can power the fridge. That setup should power the system for up to 72 hours.



Does it have a yellow energy guide yellow label, or can you find that information for you model online? That gives you the annual kWh, which can be used to calculate the daily kWh and allow you to use the formulas given.

A 12.8V/100Ah LFP has 12.8V * 100Ah = 1280Wh of energy storage.

Using my estimated 400Wh/day:

1280Wh/400Wh/day = 3.2 days.

Yeah it's got all the information listed, I just didn't know how to apply the information I had. Thank you!

 

[DIY_And_Artsy_Guy]

Once you have that it's all about the math. Let's do some napkin math using handy round numbers:

So you got your WallyWorld mini fridge and plug it into a Kill-A-Watt. After a couple days of running the Kill-A-Watt tells you that it takes 22 watts to run but spikes at 327 watts at the highest (for example numbers).

Well, since your fridge is 120v, you'll need an inverter. Since nobody makes a 327 watt inverter, you find the next size Pure Sine Wave inverter up and get yourself a little 500w unit. That's plenty of overhead for that motor to kick on.

OK, so now you've got your inverter, let's look at the battery. The Kill-A-Watt says it uses about 22 watts to run, and over 24 hours that's 22w * 24hr = 528Wh. A bog standard LFP 100Ah battery is good for about 1200Wh, so that would last 2 days. Perfect!

Lead Acid batteries are only good for discharge down to 50%, so if you're using lead you gotta get double the battery to power the same load.

Now you gotta refill that battery by harnessing the power of the sun, so you go grab some panels. You get about 4 hours of usable sun per day on average, and want to generate 1200 watts in that time, so 1200w / 4 hours = 300 watts of solar panel. Rule of thumb is you'll see about 80% of what the label says, so we'll factor that in to the math, 300w of panel * 120% = 360w of panel would be about perfect. You can always have more, but I really don't recommend less than 300w.

The next bit is the charge controller that turns solar DC into battery DC. Assuming a 12v system (single battery) and 360w of panel, well 360w / 12v = 30a of charge controller.

So, to sum up our example system:

500w Pure Sine Inverter
100Ah LFP battery OR 200Ah of lead acid flavor
360w of solar panels
30a of charge controller


When you're looking at a small system like this, you have to either get a used solar panel from someone's old roof install and an MPPT controller, or you can get 100w panels off Amazon and use a cheaper PWM controller. With only a few 100w panels it's hard to beat Amazon. Used panels are usually MUCH better price per watt but would require a more expensive controller to work, so it's kind of a wash. Pro tip: If the controller has USB ports on it, it's NOT a real MPPT, it's just lying to you and laughing all the way to the bank. Fortunately you don't need anything expensive, you're just needing to turn the higher voltage from the panels into a lower voltage your battery can work with.

Does that all help make sense of things?

That is actually a very good explanation and I think I've got it now. Thank you so much! I'll be applying all this information to my planner.

 

[Skid]

Kinda real world here. I just built a small house in Nicaragua for a small family. They will use my Delta 2 and a couple panels for the time being. I tested it for a week running a generic mini fridge for two weeks, periodically charging it with AC.

From 4PM to 9AM it used 60-70% indicated of the D2, rated at 1024w but with about 900w usable from what I understand. I never ran it til it stopped, but it was over 24 hours. Kill-A-Watt says .8KwH for 24 hours. The house was finished and they moved in yesterday, with 3 LED lamps. The 400w of panels feed 360w to the D2 but I hope for a cloudy day to see if two more 200w panels in 2s2p are needed. Fridge going in as soon as I move another family into another house which will free up another fridge. (It's complicated...)

I'll let you know as soon as I know, within three days. Short answer, a 12/100 LFP, a Victron Phoenix 12/500 ($120 on eBay) using about 10w should keep you going for the 75% of the day without sun. With a SCC it should cost about $6-800? with panels.

 

[CaliSunHarvester]

Pro tip: If the controller has USB ports on it, it's NOT a real MPPT, it's just lying to you

I have read this statement in this forum before, possibly more than once. I wonder if it is true. I used to have an EpEver triron SCC that claimed to be MPPT. Yes, it had a USB outlet and also DC output. Would automatically support 12/24V batteries, IIRC. So if you hooked up a 24V battery to it, it better not be discharged to 14V

My very first steps were with a setup from Harbor freight. In hindsight, I think that really was a PWM controller. The maximum voltage was something like 24V (could have up to 5 panels in parallel only, nothing in series)

In contrast, the EpEver allowed up to 150V input and .. 20A? Not sure right now.

 

[Rednecktek]

My very first steps were with a setup from Harbor freight. In hindsight, I think that really was a PWM controller. The maximum voltage was something like 24V (could have up to 5 panels in parallel only, nothing in series)

It is, we don't sell any MPPT controllers at HF.

I have read this statement in this forum before, possibly more than once. I wonder if it is true. I used to have an EpEver triron SCC that claimed to be MPPT. Yes, it had a USB outlet and also DC output. Would automatically support 12/24V batteries, IIRC. So if you hooked up a 24V battery to it, it better not be discharged to 14V

There are a couple models of EPEver and a brand called Sunthsys that had usable USB ports on a proper MPPT, but those are the only ones I've ever seen that have that combination. There are a few models with USB ports on them that are used to program the controller, but not for powering your USB devices.

Never tried running the USB on a 24v based SCC, kinda afraid to find out but curious now.

 

[CaliSunHarvester]

Never tried running the USB on a 24v based SCC, kinda afraid to find out but curious now.

It definitely worked fine, I had a USB light fixture plugged in back then. Sometimes plugged a phone in as well.

To clarify: the EpEver was from Amazon.

The Harbor Freight product didn't have a brand name printed on it. The unit was smaller than a box of cigarettes with no settings. Just 2 LEDs.

 

[JWLV]

From what I’ve seen, most people underestimate how much power is required to run a refrigerator.

If you decide on a 12v 100a battery, I foresee that you’ll add at least another 100a to your system soon afterwards.

 

[Skid]

JWLV is right on. Update on the Delta 2 I always try the least possible and work up from there. I hooked up my Delta 2 and two 200w panels to power a small tropical house, really only a mini fridge, a phone charger, floor fan and 5 15w LED lights. The lights take 75w, (85w with the losses) the fridge about 30 watts per hour. Total power use from 4PM to 9AM (approximate, I'm not in the house) comes to 1.1KwH. I have what I need to connect a 24v100ah battery, SCC etc but I'm traveling soon for a year. If there were a problem there is no one with knowledge anywhere near.

Thankfully I saw a post about refurbs and a Delta 2 battery is $338. So the D2 including 2KW of battery and 4 200w panels will be a total of $1300 including $125 ocean shipping to Nicaragua. And total plug and play simplicity for a family that cannot read and would have no idea about component solar. They will have to juggle usage for 3 weeks until the extra battery gets here but then it will be sufficient.

 

[Skid]

An update. The house is now wired with 6 LEDs, 9w is totally sufficient. 8AM they sent a photo with 30% power remaining. The last post was an estimate and happily I was a little pessimistic on the use.

This comes to about 600wh per night. Your idea won't keep your food cold for the night.

 

[AstroBob]

Maybe just buy an efficient 12v compressor cooler fridge? a tiny power station or battery and a 100 watt panel will run them almost forever. No need to complicate things. Simple problem. This is what one of mine consumes in 24 hours. Easy peasey.

 

[AstroBob]

If it's an absorption fridge capable of running on propane, forget it. They consume massive amounts of power. Don't bother.

If it's a compressor fridge, it will use about 0.4kWh/day or 400Wh/day. Your batteries only have 72Wh.

You would need about 200W of PV with sunrise to sunset exposure to guarantee you could generate the needed 400W/day, but without enough battery storage, it's a no-go.

Just curious, have you ever owned an absorption fridge? If so, have you measured the power draw in AC mode? Yes, they are power hogs running on ac, everybody knows that. I did measure mine yesterday, just for fun. 325 watts. However, I can get 2 weeks on a 20lb propane bottle, with no electric draw at all. They do have their place.

 

[sunshine_eggo]

Just curious, have you ever owned an absorption fridge?

Yes. I've owned five. I've had three fail. I recently replaced a 12cu-ft unit and a 7.6cu-ft unit with compressor fridges.

If so, have you measured the power draw in AC mode?

Yes. Depending on the size of the unit, they burn 300-400W with a 60% duty cycle, but that's variable on ambient conditions and usage. My 7.6 ran at 310W and my 12 ran at 360W.

I've measured the consumption on the two with VRM showing when they turned on and off and their minute/daily/monthly usage, etc. Unfortunately, detailed VRM data is lost after 6 months, so I no longer have access to it.

My 12cu-ft nevercold would consume as much as 9-11kWh/day when we were on site in warm weather using AC full time. Since the site is remote, and it was a substantial portion of my consumption, I had them connected to a wifi ac plug with energy monitoring that would turn off based on time of day so that they would run on propane overnight. The 12cu-ft unit would gobble 6kWh of AC energy during the day while being powered on propane after sun hours.

Yes, they are power hogs running on ac, everybody knows that.

Strong statement. I've disclosed that fact to many on and off this site that had absolutely no clue that absorption fridges use 5-6X the AC energy as a same-size residential (compressor) unit.

I did measure mine yesterday, just for fun. 325 watts. However, I can get 2 weeks on a 20lb propane bottle, with no electric draw at all. They do have their place.

Given that this thread is about powering a fridge with electricity, I can't agree. They have no place here.