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I have about USD $3,000 that I can use to invest in solar-powered alternatives at my apartment. Since I live in Latin America, my small rooftop receives tons of sunlight 12 hours/day but is not big enough to hold a bunch of panels.

The appliances that consume the most electricity now are the fridge (which is always on), the 18K inverter A/C (which is on about 8-10 hrs/day), and the washer/dryer.

Since the fridge is the only appliance that's always on, I guess my question is: what's the most inexpensive way to constantly power this appliance with solar energy?

I saw this Costco ad for a portable power station and thought that maybe something like this would help.

On a related note, a local group that deals with solar power alternatives in the Caribbean posted this battery array alternative. Would this be a possible solution for me?

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  • Wouldn't it make more sense to run the A/C since that's mostly on when the sun is up? I run a mini off grid system with 750W of panels + 1kWh LFP battery and put about 3kWh/day through it in summer.... because it mostly powers my daytime loads. . . Most "solar power stations" are built the other way round - lots of battery and a token solar panel so they can say it's possible to charge it from solar. The washer/dryer is likely too much load, but if you have enough panels to power the A/C plus enough battery+inverter to start it that seems more reasonable.
    – Móż
    Oct 3, 2023 at 0:36
  • There are lots of "DIY solar power station" guides around, and one of those might save you money but would mostly let you get exactly the system you want. So if you decide on a 3kW inverter with 1kWh of battery and 3kW of PV... a DIY system can be built to do that. If you buy an all-in one it looks prettier but you get what they sell.
    – Móż
    Oct 3, 2023 at 0:37

1 Answer 1

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what's the most inexpensive way to constantly power this appliance with solar energy?

I think you are mistaken in your strategy. You don't want to power this appliance with solar power. You want to power some cooling appliance with solar power.

I suspect your fridge has been optimized for usage on grid power. Grid power is incredibly cheap. Thus, appliances that run on grid power are optimized for low manufacturing cost, compact external size and large internal size. Very little insulating material there.

For example, I have two fridge-freezers that consume approximately 60 watts each (average over a long period). They are large, though.

To run this on solar power, you need an inverter, ideally a sine wave inverter, that is capable of starting a full-size fridge compressor. Those have about 88% efficiency plus about 10 watts of constant idle power usage. Therefore, your 60 watts turns into 60/0.88+10 = 78 watts. That's about 1900 watt-hours per day. The inverter would cost about ~350 euros/dollars if you select the cheapest Chinese model you can find.

To supply 1900 watt-hours per day using solar power considering cloudy days too, you should expect to regularly use 1900*3 = 5700 watt-hours of battery charge since you generally aim to have 3 days of energy storage. Lead-acid doesn't like discharging below 25% state of charge, so with lead-acid you need 7600 watt-hours. With lithium iron phosphate, though, 5700 watt-hours would be enough (so that's about seven and half Ecoflow River 2 stations which would probably cost too much). Expect to put around ~2000 euros/dollars into the batteries alone if you choose lead-acid, and those batteries need to be replaced quite often since no battery has an infinite shelf life.

Then the panels. Solar power has a capacity factor of around 15% near the equator. So to provide 78 watts, you need 520 watts of panels not considering losses. However, MPPT charge controllers are generally only about 95% efficient so 550 watts is a more reasonable figure. Expect to put around ~130 euros/dollars into a charge controller and around 550 euros/dollars into the panels. Wiring costs too, and installing the panels has a cost too.

By far, the largest cost is the battery (2000 USD), the second largest the panels (550 USD), the third largest the inverter (350 USD), the fourth largest the charge controller (130 USD). Together, this makes 3030 USD so your budget of 3000 USD may be met if you do the installation work yourself and don't need long wiring.

However, I have a better option. Select a 12V/24V compressor coolbox. For example Dometic makes those. Avoid Peltier models since they use huge amounts of energy.

My Dometic MCF32 uses about 4 watts DC if the indoor temperature is about 20 degrees Celsius and it cools the inside to 5 degrees Celsius.

So, to use Dometic MCF32, you need 384 watt-hours of lead-acid batteries (in practice the smallest marine deep cycle AGM 12V battery you can find, about 180 USD, is much larger than what you need here), the cheapest charge controller you can find (for such a small usage PWM wins, about 50 USD), and a 50 watt solar panel is plentiful enough even considering the inefficiency of PWM charge controllers (about 50 USD). Plus also the Dometic MCF32 appliance itself, about 300 USD. Only 600 USD budget.

Of course, a compressor coolbox is small. But it's so darn efficient that with your 3000 USD budget, you can buy the largest compressor coolbox you can find.

The only reason why the situation is so favorable for you is that you live in Latin America, i.e. near the equator, so solar power has practically no seasonal variation. Areas far from the equator suffer severely during the winter, making the whole project infeasible.

Of course, the question is do you want to do this project? Is there any reason for using solar power and not grid power? For me, the justification is that I want to have cool storage even during extended grid outages. If you don't have this same justification, the simplest way would be to simply continue using grid power. Grid power is an extraordinarily good deal, very cheap.

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    Thanks for your answer. I don't know much about this but I'm willing to learn if it means that I won't have to depend on the electric company. The refrigerator is a regular whirlpool fridge. "Grid power is incredibly cheap": not in my country. The electric company here is particularly expensive and famously unstable.
    – rbhat
    Oct 3, 2023 at 15:12

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