A car battery is an excellent medium for storing electricity. An ordinary headlight connected to a fully charged 12 volt truck battery can yield many hours of light. A larger sized auto battery, for pickup trucks or the Expedition or the Suburban store in access of 35 amp-hours of energy at 12 volts.
A 100 watt 12 volt solar panel in bright sun gives us up to 8 amps of current. Four hours of charging should yield us 32 amp-hours of 12 volt energy.
Power cost per battery charge
At $.10 per kwh, or ten cents per kilo-watt hour, 100 watts for one hour would equal one cent off your electrical bill. So if five hours on a sunny day is a complete charge with solar panels, that would save you five cents. A $ 200.00 system would need to charge the battery 4,000 times to compete with the electrical cost from the power company.
Solar Power output on sunny and cloudy days
Battery charging could still occur on cloudy days, depending on the type of solar panel. Some solar panels are less efficient in bright sun but more efficient in cloudy weather (than bright sun panels). A solar panel optimized for bright sun may produce as little as ten to 25% of full power on a cloudy day. By aiming the solar panel at the sun, you could in practice charge two car batteries in a single bright sunny day. But cloudy weather makes pointing the panel directly at the sun less important because the photons are diffused. Still, it may take two to three days in cloudy weather to charge a 35 amp car battery.
Cold cranking amps
If you have a battery with 800 cold cranking amps, it means nothing important. You need to know the storage capacity, which is mostly something much different than cold cranking amps. Cold cranking amps are useful in a brief, high draw application, such as feeding a starter motor with electricity for two or three seconds. It is a measure of current flow, and not really important in solar applications. When storing electricity, we need to concentrate on the amount of energy, or ‘work’ stored in a battery.
Power as opposed to work
In physics, work is measured in Joules. To measure electricity in real world terms, we need to measure the amount of work in can accomplish. This happens to be done in the same way as which the power company charges us for electrical consumption, as power over time. KWH on your electrical bill stands for Kilo-watt hours. At 100 volts, a motor that draws 10 amps for one hour would consume one kwh of electricity. That motor, running 24 hours would consume 24 kwh, and for an entire 30 day month the consumption would be 24*30 or 720 kwh. At ten cents per kilowatt hour that cost would reach $ 72.00 per month.
two or three room air conditioners running 24 hours a day all month can add more than this amount to your electrical bill.
Running my air conditioner on a car battery
So, since a car battery might hold 35 amp-hours of ‘work’, and a room air conditioner uses 10 amps per hour, can a car battery run my air conditioner? NO! The problem is that your car battery has 35 amp-hours stored at 12 volts. In order to provide 120 volts of potential, the current varies by the inverse. 35 amp-hours need to first be converted to power to compare apples to apples.
35 x 12 = 420 watt-hours. In other words, that kilo-watt hour is more than twice as much energy as the car battery holds. So, in theory, you could run the window air conditioner for about 25 minutes on the energy stored in a car battery.
Back to the headlights
So this car battery can run a headlight for hours and hours, why? The answer comes when we consider the work that a headlight consumes producing 50 watts of light. 50 watts of power and 12 volts are roughly the same as 50 watts of power at 120 volts, the only thing that changes is the amount of current. So if a headlight bulb consumes 50 watts of power for one hour, it requires 50 watt-hours of work. In order to calculate the amount of work in a car battery, we use Ohm’s Law, power equals volts time amps. 12 volts times 35 amp-hours yields 420 watt hours, or roughly 8 hours of energy to power a 50 watt light.