Originally Posted by
RiaanJ
Let me try ... I will use a lot of assumptions, you will be able to change my calcs....
With their compressors running, each of your fridges will use some 2.7 Amps. That is 5.4 Amps total. But they will not run the whole time, clearly a freezer set at -10 degrees C will run more often / longer than a fridge set at +2 degrees C. This is also ambient temperature dependent.
So let's for argument say they will both run 70% of the time...that gives a total usage of 3.78Ah per hour (2.7 x 2 x 0.7) or 90.72Ah per 24 hour period.
Since your batteries is 85Ah each, and without any solar power to charge the batteries, you will consume just over 50% of your batteries - and unless your batteries are "deep cycle", this will damage them...
But you will be solar charging your batteries during the day. Let's assume the sun in winter shines for 8 hours, during this 8 hour period your must try to recharge the 90Ah used / to be used into the batteries. This is a charging rate of just over 11 Amps at say 14.5 volt (batteries in parallel) for the whole 8 hours. And this will be difficult, you will have to be able to move the solar panel to remain face on the the sun, no clouds, no bad weather .......Also, this theory makes no provision for "system losses" due to energy convertion / wire length / wire thickness etc.
Most solar panels will produce more than the required voltage, so it would be wise to invest in a small MPPT charge controller that will step the voltage down by increasing the charging amps from the panels. Cheaper (non MPPT) charge controllers will charge your batteries at the required voltage by limiting thru-production from your panels, lower charge rate, energy lost!
Onto Solar Panels: A typical (good quality) 100w panel will cost around R10 per watt, or R1000. And will produce around 18 volt / 5.5 Amps. A small (R1200) MPPT charge controller will change this into 14.5 volt / 6.8 Amps. (remember Watts=Volts*Amps) Flexible panels will be much more expensive, damage more easily but are lighter.....
So 2 of these panels in parallel will give you 14.5 volt / 13.6 Amps....This is a margin of 24% above what you needed above...and this is really not enough - only cloudy period per day and your batteries suffer..
My suggestion would be for 2 x 120w panels, joined with little s/steel hinges, in parallel, into a MPPT charge controller, minimum 4mm2 wire, no longer than 10 meter, all plugged together with Brad Harrison plugs. This should give you some 16.5 Amps ..... a 50% margin. Or put differently, instead of 8 hours for charging, you now only need 5.5 hours.
The total cost could be R2400 for panels, R1200 charge controller, R?? wire - plugs - hinges
Good luck!
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