We have discussed how photovoltaics produce power, and how to calculate power draw, but there is more than that to a solar system. Apart from the batteries (more on that next month), you will need a charge controller. This is because the brighter the sunlight, the more voltage the solar cells produce, and excessive voltage could damage the batteries. A charge controller is used to maintain the proper charging voltage on the batteries. As the input voltage from the solar array rises, the charge controller regulates the charge to the batteries, preventing any over-charging.
Maximum Power Point Tracking
Most multi-stage charge controllers are Pulse Width Modulation (PWM) types. I would recommend using one of these regulators. However, the newer Maximum Power Point Tracking (MPPT) controllers are even better. They match the output of the solar panels to the battery voltage to ensure maximum charge (amps).
For example, even though your solar panel is rated at 100 watts, you won’t get the full 100 watts unless the battery is at optimum voltage. The Power/Watts ratio is always equal to Volts times Amps or P = E x I. (See Ohm’s law for more info).
With a regular charge controller, if your batteries are low (at say 12.4 volts), then your 100-watt solar panel rated at 6 amps at 16.5 volts (6 amps times 16.5 volts = 100 watts) will only charge at 6 amps times 12.4 volts − or just 75 watts. You’ve just lost 20% of your capacity!
The MPPT controller compensates for the lower battery voltage by delivering closer to 8 amps into the 12.4 volt battery, thus maintaining the full power of the 100 watt solar panel. 100 watts = 12.4 volts times 8 amps = 100 (P = E x I).
The Charge Controller is installed between the solar panel array and the batteries, where it automatically maintains the charge on the batteries using the threestage charge cycle described on the next page. A power inverter can also charge the batteries if it is connected to the AC utility grid, or, in the case of a stand-alone system, your own AC generator.
If you are using four 75 to 80 Watt solar panels, your charge controller should be rated up to 40 amps. Even though the solar panels don’t normally produce that much current, there is an “edge of cloud effect”. Due to this phenomenon, I have seen my four 6 amp panels (4 x 6 = 24) pump out over 32 amps. This is well over their rated 24 amps maximum.
The Science of Watts & Power
In its simplest form, Ohm’s law states that the current in an electrical circuit is directly proportional to the applied voltage and the resistance of the circuit. The three most common mathematical expressions are:
- E = I x R
- I = E / R
- R = E / I
Also, the power can be expressed as P = E x I, and with a little simple maths, we can combine these expressions and derive P = E² / R.
So what does all this mean? Well, for one thing, it becomes clear that an appliance (load) that draws 1 amp (ampere) of current at 240 volts, will draw 20 times as much current at 12 volts (1/20 the voltage) or 20 amps. Since P = E x I, then 240 volts times 1 amp = 240 watts. Also, 12 volts times 20 amps = 240 watts. So, you can see that the power remains the same. As the Voltage goes down, the Amperage increases to maintain the power, which will be determined by the 3rd factor, resistance.
Okay, now let’s say you have a nice 1200 watt hairdryer. Well, that would work out to 5 amps at 240 volts. But, when your power inverter uses the 12 volts supplied from your batteries, the amperage goes up to 100 amps to produce the same 1200 watts (P = E x I). This means that even the very large cables connecting your batteries to the inverter will become warm. This is why it becomes impractical or impossible to run, say, a 4000 watt electric clothesdryer. Even if you had large enough wires to handle the required 333 or so amps, your batteries would not last long.
It is true that the cables would not get as warm if the current could be reduced by increasing the voltage by using a 24-volt battery system, or even a 48-volt battery system. This still would not change the amount of power that your batteries must supply.
You will primarily be interested in the formula P = E x I (watts = volts x amps). With this single formula, you can determine the wattage a device uses by multiplying the Voltage in Volts, times the Current in Amps.
For a solar-powered system, you will need to replace electric appliances that need large amounts of power with ones powered by gas (natural or LP) or other alternatives. This would usually be anything that uses 1500 watts or more. All appliances that are UL rated will have their power consumption in watts listed on a placard or label near the AC cord.
When you find the wattage listing, you can divide by 220V to get the number of amps the appliance will require. For a 12 volt system, multiply this number by 20 to determine the number of amps that will be drawn from the batteries. For a 24 volt system, multiply by 10. For a 48 volt system, multiply by 5.
The Multi-Stage Process
After the absorption time has passed, the voltage is lowered to float level (usually 13.4 to 13.7 volts) and the batteries draw a small maintenance current until the next cycle.