Help pairing panel to fridge

[T-Willy]

I seek help with the following questions from those who understand off-grid power better than I. Thank you in advance for your assistance.

I am trying to determine the adequate wattage of 12V solar panel to power this fridge, which has a built-in lithium battery. The solar panel alone must be able to power the fridge continuously, absent any other input power.

Specs on the fridge and battery:

- 42 amp hour
- 65 watt total input power
- 1.1 amp hour average power use

Questions:

Will a 100 Watt, 12 volt solar panel at full power will exceed the charge that this battery can accept? Is there thus no charging benefit to using a panel with more wattage?

Am I also correct to understand that, given 12 volts at 1.1 amps per hour:

(a) the fridge would consume 13.2 watts per hour (12 volts x 1.1 amps per hour = 13.2 watts per hour), and

(b) under full charge of 65 watts, while operating, the fridge would have a net charge rate of about 51.8 watts per hour (65-13.2=51.8) or, in amps, 4.3 amps per hour (51.8 watts /12 volts = 4.3 amps), and

(c) it would require ten hours of continuous charging at 4.3 amps per hour during operation for this 42 amp hour fridge battery to fully charge (4.3 amps per hour x 10 hours = 43 amps)

Lastly, I understand that solar panels only produce maximum power under the most optimal conditions.

Assuming I were parked in mixed sun all day with a fixed roof panel, how large of a panel in terms of wattage would one need to reasonably ensure at least 65 watts of continuous output throughout the day?

 

[john61ct]

Personally I would go to 200W, minimum 150

but in a very sunny area closer to the equator

and not wanting reserve for 2-3 cloudy days

might get away with 100W.

Certainly not in winter away from the equator

or places where cloudy days are more than 100 days a year.

I also would want 200Ah of battery.

This assumes sitting off grid.

Just weekend runs, or driving every day is different.

Have you put a wattmeter between the fridge and battery?

Get an Ah/day average over a week in worst case conditions.

 

[T-Willy]

Personally I would go to 200W, minimum 150

but in a very sunny area closer to the equator

and not wanting reserve for 2-3 cloudy days

might get away with 100W.

Certainly not in winter away from the equator

or places where cloudy days are more than 100 days a year.

I also would want 200Ah of battery.

This assumes sitting off grid.

Just weekend runs, or driving every day is different.

Have you put a wattmeter between the fridge and battery?

Get an Ah/day average over a week in worst case conditions.

Thanks for the solar panel wattage suggestions.

In this case, the battery is built into the fridge. However, the more I research this, the more it seems as if a separate, larger battery that can accept a faster charge paired to a normal fridge (without a built-in battery) makes more sense from both price and electricity management standpoints.

 

[hour]

Wow that fridge is a lot of money for an imperfect solution. I'd buy a normal DC fridge and a $600 100ah lifepo4. Depending on how fancy($) you went with the fridge, you could easily have money leftover for the rest of the system - the solar panel(s), charge controller, wiring, and whatever power panel you prefer for charging phones or running inverters.

I have 200w in sunny Colorado and most of the time (weeks on end) their only job is to charge the battery, to power the fridge. I've never gotten behind in the summer but it does take til noon to top off the battery from the previous evening. Thing about summer is that it can still be hot as hell even if the sky isn't clear, and even if a storm rolls through and drops the outside temperature 20 degrees, the truck is still roasting.

 

[NatersXJ6]

I can see some advantage to an integrated fridge and battery, especially if you are constantly moving the fridge into a bear box or something, but on the whole that thing is going to be huge and heavy. I personally would separate the fridge and battery system into more manageable bits and then be able to independently select the best choices for me. I’m always frustrated by all-in-one tools as the inevitable compromises in design were evaluated by someone else, and I didn’t get to choose.

however, in central California, I have 200w flat, fixed solar, charging my starting battery and running a fridge. According to the Victron App, the battery rarely draws enough to have needed more than 100, but at about $1/watt I figured it was cheap insurance against low production or shading.

 

[67cj5]

A bigger panel would be good but if that 42A battery is a lead acid you need to have at leased twice that because a fridge can chew 45 to 50Ah per day in the hotter months plus you need a battery that can put out around 100Ah so you don't take it below the 50% safety margin,, 42Ah Lithium yes, 42Ah Lead Acid No.

 

[plh]

Yeah, I wouldn't buy that fridge. 200W solar, 100ah LiFePO4 and a different fridge (many around $400 on Amazon that work just fine) and move on.

 

[Josie'slandcruiser]

You should never expect to get the electrical production (wattage) the solar panel sellers quote. Typically a 100 watt solar panel will produce 70 watts in a best case scenario. Where the sun is on the horizon, clouds, etc. also impact solar electrical production.

Then consider the electrical draw of the load (i.e. a 12V refrigerator.) The amperage draw will go up in tandem with ambient temperature.

In my Mojave desert camping truck I have an old "Norcold" brand 12V refrig, an Odyssey 92AH AGM battery, and 2 x 100 watt folding solar panels. Camped among Joshua Trees in 90 - 100 degree weather with direct sun exposure I am glad to have plenty of solar production. The two 100 watt panels in conditions like this will keep the battery completely charged despite the draw of the (not very energy efficient) old 12V refrigerator.

 

[plh]

What kind of fridge works like that ?
Please let us know so nobody unknowingly buys one.

apparently his Norcold

 

[john61ct]

What kind of fridge works like that ?
Please let us know so nobody unknowingly buys one.

every single one

impossible to not work that way.

Of course they meant duty cycle, Ah per 24 hour consumption.

But there are also modern designs with variable speed compression units, so the actual amps rate goes up as well compared to cooler ambient conditions.

 

[john61ct]

The distinction between "amperage" and Ah is a pedantic one, when anyone reading knows they simply meant energy consumption increases with ambient temp.

Sure if you want to walk them through the difference between the two units that is a useful educational service

but just tossing in a sarcastic comment is IMO not conducive to community

 

[broncobowsher]

That 1.1Ah average consumption is very suspect. If running the fridge in fairly cool (as need a light jacket) conditions, yes possible.
Really you should be basing it on better data points (Engle was publishing power consumption curves at different ambients and different interior temps at one time). My ancient ARB (rebadged Engle) has been pulling power off the starting battery (not recommended) with just a 70W rated panel and a PWM charge controller. Partial shade no less. Set to fridge, it can run for days and still start the engine. Been running it that way for nearly a decade. Really should be a dedicated house battery. 100 Ah is common and overkill. The PWM charge controller is fine, it is more affordable in this scale to use a PWM charge controller and a slightly larger solar panel. I would be using a 100W solar panel but packaging constraints limited size to only 70W. My backup plan was to switch to a MPPT charge controller if the setup wasn't sufficient. Modern panels today I could get a 100W panel now, but I don't need it. The early on math (using the published Engel charts, government solar collection data for my general location, and a lot of safety padding to the numbers said a 100W panel would be more than enough. 70W has proven to be a sufficient choice. I will run the fridge hard at times as well. Summer costco runs, in Arizona. Turn it down to freeze, 100% duty cycle, couple hours before going into the store. The solar is enough to run the freezer at 100% and still have enough excess to charge the battery. I have even done some testing and early morning in the winter with a very low sun angle, remove the battery, fridge will run on 100% solar.

 

[Josie'slandcruiser]

Yes, as it gets hotter outside (for example: from 60s*F in the morning to 100s*F range in the afternoon) the amount of electrical power necessary for a refrigeration unit to maintain it's internal temperature will increase.

If there is a 12V refrig brand that can maintain the same power draw at 4am and 4pm please share their contact info!

What ?
Despite ”amperage draw will go up in tandem with ambient temperature” was written, is not what was meant ?

 

[clydeps]

The specs on the fridge are suspect. It's said to draw "1.1 Amp per hour" which I assume is meant to mean that its average power draw is 1.1 Amps (there is no such thing as "amps per hour".) The battery is quoted as being 42Ah so 1.1 amps average it should last about 38 hours or so if you run the battery to zero. But it then says it will run 16 hours on a full charge, which even allowing for some reserve is nothing like 38 hours. So one or more of those three numbers is wrong.

I agree with the other suggestions to buy a separate LiFePO4 battery and fridge rather than that one. Buying highly integrated appliances is rarely a good move since if part of it fails, you may have to junk the whole lot.

 

[john61ct]

The specs on the fridge are suspect. It's said to draw "1.1 Amp per hour" which I assume is meant to mean that its average power draw is 1.1 Amps (there is no such thing as "amps per hour".)

I bet they meant 1.1Ah per hour in some spec'd test setup, so obviously on the low end for marketing purposes.

Under 30 Ah per 24hrs is in the ballpark for tiny cubic area with decent insulation.

Means in the southwest summers, no aircon double maybe.

I've seen some efficient units go under 15 Ah per day in cool shaded conditions, but that's not a realistic average.

And turning the thermostat between drinks only vs lowest possible, makes just as much if not more an impact