# How many hours could a 100 W mini-fridge run, per day, given a 120 W solar panel?

Joined Mar 1, 2016
5
...attached to a 40-60 Ah battery? (whatever makes the math simple)

I see 4-6 hours of peak sunlight, but it's still absorbing rays for the other hours of the day. (1) I don't get why additional diminished hours aren't also factored into the power calculation.

5hrs*120W = 600 W-hrs
600W-hrs/100W = 6 hours of time the fridge could run? Then (2), is that a reasonable time to expect a mini-fridge to run per day?

TMI
I have a truck and mini-fridge. I'm about to order a 120 W solar panel. I'm sure I could find a 40-60 Ah battery for \$50-100.
The mini-fridge runs like a freezer. (Anything I put in there gets frozen.) I want to put it in the back of my truck with a battery and the solar panel. I'm pretty sure I also need an inverter. I wonder if a charge controller is needed too...

#### Bernard

Joined Aug 7, 2008
5,788
Does the fridge ever cycle on & off ? ; Rated input current & V ? Here in Tucson today I would expect about 8 hr. run time.
@ 12" X 12" panel =4W, then need about 30 sq. ft. ?

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Joined Mar 1, 2016
5
Does the fridge ever cycle on & off ?
It just automatically runs throughout the day. I can kind of adjust the temperature. But it either acts as a fridge or a freezer. There is no in between.

Joined Mar 1, 2016
5

1. This illustrates why 4-6 hours of peak sunlight is meaningful. It seems like there might be another hour or two from the diminished sunlight hours.

2. The mini-fridge will probably run at least 8 hours per 24 hours. That's appears to be an optimistic estimate. So 12 hours might be at the other end...

I still have qualms with a cheap battery and an inverter not being accounted for in the calculation.

#### ronsimpson

Joined Oct 7, 2019
2,176
Bad news: Your 120W panel probably can not do 120W. Maybe at noon on a clear day with 0% humidity, maybe if you lived on the top of a mountain, etc.

Some where there is a graph of time of day verses power for sun power. (do the panels turn to face the Sun?)
It is complicated to calculate power for every hour of the day but maybe 5 hours of full power is close to what the panels will do. Too much math to say; at 6am=1 watt, 7am=10 watts …….. 12 = 100 watts …… (maybe simple math 5 house of 100 watts)

If you leave the fridge door open and it runs 24 hours then NO way. You need to find out how much it runs.
The panel can make the equivalent of 100 watts for 1/4 of the day.
The fridge can not run for more than 1/4 of the time. (assuming 100% efficiency) There is power loss in every box including the battery.

#### ronsimpson

Joined Oct 7, 2019
2,176
merely needs 110 watt PV system!
Assuming you never have a cloudy day. cloudy week. There is no room for margin. I have lived in places where you almost can't see the sun because of all the junk and water in the air. I think the energy can easy be 50%.
I also have lived in places in the winter the sun just peaks over the land for one hour and drops back down. But in the summer it is light 24 hours a day. In parts of Canada the kitchen is colder than the fridge. So the fridge does not run. lol

#### WBahn

Joined Mar 31, 2012
26,527
There are a number of things that need to be taken into account, some of which you haven't considered (here, anyway).

You seem to be basing your analysis on the assumption that your system only has to balance one day's energy input with one day's energy expenditure. What if it's cloudy all day today? Or all week? How long do you need the system to operate from the batteries alone before you have to resort to something else? Let's say it's three days. So after a couple of cloudy days, your batteries need more three days of charge pumped into them. How long are you willing to let that take? Do you want the batteries to always be chargeable in a single good, sunny day, or are you willing to rely on a string of good sunny days after a period of cloudy/rainy days?

How often do you stock your fridge with a lot of room temperature things that need to then be cooled down? That alone might result in the fridge running constantly for an abnormally long period of time. What kind of reserve and recovery capacity to you need to allow for that?

Then there are all of the inefficiencies along the way. At your location, how much energy will actually fall on your panels on a good sunny day? Do you want to design your system to perform adequately even in the depth of winter, or just on average over the course of a year (with a willingness to make up the energy with another source as needed)?

So what is it that you are really trying to do -- what is really important?

Each element has it's own inefficiency. Converting the output from the panel into energy stored in the battery has one, getting energy stored in the battery and into the inverter has one, and getting energy that goes into the inverter and get it out of the inverter has one.

#### Alec_t

Joined Sep 17, 2013
12,907
it's still absorbing rays for the other hours of the day. (1) I don't get why additional diminished hours aren't also factored into the power calculation.
I think you are confusing power with energy. Power is instantaneous (which is what a fridge needs), so diminihed hours mean less power. Energy (accumulated power) is more relevant to battery charging/capacity.

#### WBahn

Joined Mar 31, 2012
26,527
I think you are confusing power with energy. Power is instantaneous (which is what a fridge needs), so diminihed hours mean less power. Energy (accumulated power) is more relevant to battery charging/capacity.
Note that the TS is talking about using the solar panel to charge a battery and the battery to power the fridge, so as long as the battery can deliver the power (when adequately charged), talking about the energy balance is quite reasonable.