Category Archives: Power Systems

How Much Battery Do I Need?

When attempting to power devices from a battery several factors should be considered. In the examples I will use, I will use a solar system as the charging mechanism for the batteries. I use this example since it will provide an example for a self-contained power system that needs nothing but the sun to keep running.

Batteries are rated in Amp-Hours. This rating is the theoretical time in which a battery can supply its energy. Because of factors in the real world that are difficult to exactly calculate, I will use examples that don’t take temperature, moisture, and other environmental factors that can decrease the rating of a battery in the real world.

A battery that is rated at 100 Amp-hours can last for 100 hours if the drain from the system connected to it is drawing 1 Amp. This is a linear calculation, meaning that the same 100 Amp-hour battery will last 20 hours if the power drain was 5 Amps on the system. Using this knowledge of how the ratings on batteries work, you can start to calculate how much storage power in batteries you need to run your system.

The next piece to place into the equation is the solar charging system to be used that can keep your batteries charged while they run the system. Solar panels are typically rated in Watts. Since batteries are typically rated in amp-hours a small conversion will be necessary. The following formula will accomplish this calculation.

W(Watts)=V(Voltage)*A(Amps)

A=W/V

I am going to base the rest of my calculations on a 12-volt system. If you have a solar panel rated at 48w you can use the formula in the following way.

A=48/12

A=4

So, what this means is that at full power this solar panel will supply 4 Amps of power into the system at 12 volts. That being said, in reality solar panels tend to operate somewhere between 14 and 17 volts, so they can be used to charge batteries.

A battery will only charge if the volts going into it are slightly higher. Most solar panels will come with a voltage regulator that is set up to charge batteries.

The next part is where it starts to get tricky. Solar panels only run when the sun is out. To properly set up a solar charging system you must take length of day and the amount of cloudy days usually occur in your area. If you live in a desert, like I do, that calculation isn’t as difficult as a person living in the Northwest.

The goal is for the solar panels to put more energy into the system during daylight hours than the system uses during an entire 24-hour period. In that case the solar panels will be able to completely charge the batteries during day and run the system. By the end of daylight hours, the entire system should be at 100 percent and ready for the dark.

In the next part of this series I will go into some more detailed calculations on determining exact numbers to setting up a solar power system.