For this comparison we are compare an American style fridge / freezer and a Steca Fridge Freezer
If you’re thinking that’s not a fair comparison you’re right. My excuse is it’s what we have in our house and therefore easier for me to compare / meaure. Anyway this is more about the feasibility of running them on solar rather than an A vs B comparison. Also one of the main disadvantages of upright fridge/freezers is all the cold air “falls out” when you open the door (open the door with nothing on your feet) The Steca PF166 is a chest fridge / freezer so it doesn’t suffer from this problem. And neither would a mains powered fridge freezer for that matter
The American style fridge freezer consumes approx 850 Watt hours over a 24 hour period which is 0.85 of a unit of electricity. Although that’s quite efficient for mains appliance that’s still a lot of power that needs to be generated by the solar panels and then stored in batteries
Battery sizes for the grid powered fridge/freezer
A medium sized leisure / solar battery can store about 1400 Watt hours of power. It’s never a good idea to drain batteries excessively so we need to divide that by 2 so we always leave the battery about 50% full. That gives us 720 Watt hours of usable power which is much less than the 850 Wh (Watt hours) required to run it. Factor in the fact that it’s not sunny all day, every day so at least three to four days worth of reserve power must be available
Calculate the battery size or capacity
850 Watt hours per day x 4 days of no sunshine = 3400 Watt hours
Doubling battery size gives a better battery life so 3400 x 2 = 6800 Watt hours.
Volts multiplied by Amps equals Watts so a single 12Volt 200Ah (Amp hour) battery has the capacity to deliver 12V x 200Amps = 2400 Watt hours. So approx three batteries will be required
It’s generally not a good idea to connect too many batteries in parallel. Which is what we would have to do to get 12 Volts from three batteries. Batteries charge/discharge more efficiently when connected in series so connecting two 6 Volt solar batteries to get 12Volts is better.
The more powerful the battery the heavier it gets (there’s lead in them) so higher power 6 Volt and 2 Volt off grid batteries are easier to source, move and install
Two 280 Amp hour 6 Volt batteries will give us 2 x 280 Ah x 6Volt = 3360 Watt hours
Connecting another two in parallel is ok and will give us 6720 Watt hours to run our fridge/freezer. Even better would be to connect the four 6V batteries in series to give 24 Volts
Four 280 Amp hour 6 volt batteries in series gives us 4 x 280 x 6 = 6720 Watt hours
Exactly the same as having two pairs in parallel but much better for the batteries. To learn more about connecting solar batteries and how to connect them visit our help pages on this subject
Number of solar panels required for the mains fridge / freezer
We have already calculated the power consumption and how big the batteries need to be so we know that after 4 days of little sunshine 3600 Watt hours of power need to be put back into the batteries. A 100 Watt solar panel will produce about 75 Watts in good conditions. If the panel is in full sunshine for 5 hours it will produce 5 Hours x 75 Watts = 375 Watt hours of power. So that’s 10 solar panels? Well no not quite because some of the time the sunshine will be running the fridge as well as charging the batteries. So probably 6-8 will suffice. That’s still a considerable number of solar panels just to run a fridge freezer
Battery sizing for the Steca Solar Fridge
The Steca PF166 , when used in Freezer mode (-18C) consumes about 300 Watt hours per day and when it’s in fridge mode (+3C) it consumes an incredible 72 Watt hours. So if you had two of them one as a fridge and the other as a freezer total power consumption is less than half the mains powered fridge. In fact in most circumstances the PF166 will run off two 80 Watt panels and a small 90 Amp hour battery
Battery v Grid
Choosing the power source for a solar powered fridge / freezer is an interesting dilemma. There are essentially two choices:-
- Run the fridge / freezer on batteries
- Plug it in
If you have a grid connected solar system then just plugging it in is probably the best option as the energy from your solar panels will be offsetting your overall power consumption. But if you don’t have battery storage that’s only when the sun is shinning.
Running the fridge / freezer on batteries has the upfront cost of the batteries and other solar power equipment but it does mean that the fridge / freezer is using solar power stored in the batteries 24/7. You will also have top factor in the the cost of replacing the solar batteries because they will eventually wear out. If you are running a freezer on solar power you should probably add a battery charger to your setup so you can charge the batteries if the are really flat. The freezer won’t freeze if the flat batteries can’t run it!