Tips for selecting your solar devices
- How much solar power will you need
To determine your home’s average energy requirements look at past utility bills. You can calculate how many solar panels you need by dividing your household’s monthly units by 30(amount of days in a month) to get your daily energy usage, then divide your household’s daily energy requirement by 5(average peak sunlight hours for South Africa) and dividing that by a panel’s wattage.
It’s important to note that solar panels don’t operate at maximum efficiency at all times. Weather conditions, for example, can temporarily reduce your system’s efficiency. Therefore, experts recommend adding a 25 percent “cushion” to your target daily average to ensure you can generate all the clean energy you need.
- Inverter size
To determine the inverter size we must find the peak load or maximum wattage of your home. This is found by adding up the wattage of the appliances and devices that could be run at the same time. Include everything from microwaves and lights to computers and clocks. The sum will tell you which inverter size you need.Example: A room has two 60 watt light bulb and a 300 watt desktop computer. The inverter size is 60 x 2 + 300 = 420 watts
- Battery bank capacity
Finally we can calculate the minimum battery AH capacity. Take the daily energy usage in kwh and divide them by 24 to get an average hourly consumption. Then multiply the hourly consumption with the number hours you want the batteries to last without charging. This should represent a 50% of your battery bank. Therefore multiply by 2 and convert the kwh result into amp hours (AH). This is done by dividing by the battery voltage.Example: You want the battery bank to last ten hours without recharging and that you use 0.8 kwh per hour. As 0.8 x 10 x 2 = 16.0kwh, this is the energy we need from the batteries. Converting this to AH we have to divide by the voltage of your system. This can be 12, 24 or 48 for commercial application. If we choose to use 48V, the minimum AH capacity is then 16 000/48 = 333.33 AH. Now if you divide by your battery’s rating you find the number of batteries you must use for example 2 x 180a battery banks will do. SInce you have a 48v system you will need 4 batteries for each bank, so 8 x 180a batteries will be needed.
The most important job of all solar charge controllers is to properly charge the batteries and to give them as long a life as possible. There are two types of charge controllers:
Pulse width modulation (PWM)
Maximum power point tracking (MPPT)
The difference between these two types of controllers is that the PWM is not as efficient the MPPT. The MPPT is the most common these days and can gain you up to 30% more power than the PWM controllers. The MPPT controllers also allow the strings of panels to be connected in series for higher voltages, keeping the amperage lower and the wire size smaller, especially for long-wire runs to the PV array.
The following information will be needed to manually figure out the amperage of the controller needed
The wattage of the solar array
The battery-bank voltage (12, 24, or 48). Typical bank voltage because inverters are offered in these voltages.
Now Ohm’s Law comes into play: Amps x Volts = Watts
Example: 3,000watt array/48volt battery bank = 62.5 amps, so you would need a controller capable of 62.5 amps. Most controllers out there are either 60, 80 or 100 amps so you would pick the controller with the next higher rating. In this case, it would be the 80 amp controller.
Now if you know the amperage of the controller, and you would like to figure out how the maximum solar array wattage that can go into the controller, you would also use Ohm’s law:
Example: 80 amp controller x 48 volt battery bank = 3,840 watts of solar panels.
The next thing that you must ensure is that we do not exceed the input voltage the controller can take. Again the manufacturer will dictate what the input voltage should be included in the design. Temperature and open-circuit voltages have to be considered. Since PV open-circuit voltage (Voc) goes higher as temperature drops, you will need to make sure the controller’s input voltage ratings can handle this in the cold of winter.