Calculating Solar Panel Requirements and Battery Usage
What does a Solar Panel System consist of?
1. The solar panel
2. the Solar Charge Controller (Regulator)
3. The Battery
The Solar Panel and Regulator
The Solar Panel is the panel or panels that will convert sunlight into power. Their output is rated in WATTS.
What is important is the average hours of sun that shines onto the panel per day and also the angle at which the solar panel is placed to maximize the sunlight onto it. South Africa has an average of 8 hours sunlight per day, but we can work on an average of say 5 hours of sun per day (to be safe) to calculate the requirements we need. The panel should be placed facing North at a 30 degree angle to optimize the output.
If you thus use a 60 Watt panel, it will generate an average of 300 Watt Hours (Wh) per day. That is 60W X 5 hrs = 300 Wh.
Usually a solar panel’s terminal voltage is rated between 17 and 22 Volt, hence it is advisable to use a charge controller (regulator) to regulate the charge to around 13 Volts. The reason for this is that the safe voltage for charging a battery is between 13 and 14 Volts.
The use of the regulator is thus to regulate the charging not to over charge the battery which is sensitive to overcharging.
The Regulator you choose must be able to handle the maximum current that the solar panel produces. To be safe, add 30% to the rated output current (amps) of the solar panel.
So, if you have a 60 Watt solar panel with a 3.6 amp current rating, you will need a 4.68 amp regulator. That is 3.6 amp + 30% = 4.68 amp.
When the Sun, via the Solar Panel, produces electrical power, it needs to be stored somewhere, which in this case, is the battery. Preferably, a deep cycle battery which is designed to be discharged over a long period of time and recharged over and over again. (compared to a car battery that is designed to provide a large amount of power, like when starting a car, in a short period of time. A car battery is also not designed for big fluctuations in voltage).
Batteries are rated in Ampere Hours (Ah), e.g. a 100Ah, or 90Ah, etc. A 100 Ah battery drawing 2.5 amps per hour will thus provide 40 hours of operation. That is 100Ah battery / 2.5amps per hour = 40 hours. However, when doing your calculations work on 80% of the battery rating i.e. if you have a 100Ah battery, work on 80% of this which is 80Ah. This is purely for a realistic calculation as one cannot expect a 100% performance all the time. The calculation for the above example will thus be: 100Ah battery X 80% = 80Ah / 2.5amps = 32 hours.
It is also not advisable to let your deep cycle battery discharge below 50% of its capacity, which will reduce the life span of it.
In summary, assuming you have a 60 Watt Solar Panel with the specifications as above (with a regulator) and a 100Ah battery and you have a National Luna Fridge drawing 2.5 amps per hour, then
THE QUESTION IS: ‘How long will my fridge operate in the bush with only the Solar Panel supplying the charging energy’?
• The Solar Panel (as described above) will provide 300 Watts per day i.e. 60 Watt X 5 hours of sun (per 24 hour day).
• Assuming your fridge runs 12 hours per day (out of the 24 hours) then your fridge will draw 30 amps per day i.e. 12 hours X 2.5 amps per hour = 30 amps. This will equate to 360Watts per day. The formula is: AMPS X VOLTS = WATTS.
• Therefore, your input into the Battery is 300 Watts per day while your fridge draws 360 Watts per day resulting in a net ‘loss’ of 60 Watts per day out of your battery.
• With the 100Ah battery you have, but only working on 80Ah as explained above, you will have 80Ah X 12 Volt = 960 Watts in your FULLY CHARGED battery.
• Drawing a net 60 Watts per day, you will thus operate your fridge for 960 Watts in the battery / 60 Watts per day = 16 days of Fridge operation.
The above calculations will vary according to the factors as assumed above, but these formulas can be used in calculating the numbers using one’s own equipment and usage patterns.