Tuesday, March 2, 2021

Basic Electrical Math

If you've been following the blog recently, you'll know that Erin got herself a CPAP machine, which got her interested in revisiting my homebrew battery box idea. Our conversation inspired some upgrades that I'll discuss later, as well as a bit of frustration on her end. I've done electrical math so much, and for so long, that I can pretty much do it in my head, and I forget that most folks can't. I promised her a primer on electrical math, and it seems fitting to share it with the whole BCP family, so here goes.

Most electrical math is based on Ohm's Law, which breaks everything into ratios. All of it is contained in a diagram we call Ohm's Wheel.

Watts are Power. Volts are Energy. Ohms are Resistance. Amps are... current.
No, we don't know why "I" was used to represent Current. 

The formulas to find Watts, Amps, Volts, and Ohms are given in the appropriate quadrant of the wheel, assuming you have any two other pieces of the data. The ones I use most are watts, volts, and amps. Amps are equal to watts divided by volts, as the wheel shows, so if I have a 100 watt light bulb on a 120 volt circuit in my house, it will draw 0.83 amps. I know the wattage of the bulb because it's printed on the glass, and I know the voltage in my house because all standard appliance circuits in US homes are 120 volts. Plug that in to the formula, and the result is 0.83! These formulas will work any time you know two of the four elements of the wheel.

The other big math question that arises, especially for preppers, involves amp-hour battery ratings. They are often presented as "How long will this battery run my equipment?" Once you know how many amps your device draws, divide that number by the amp-hour rating of your battery, and you'll know roughly how many hours you can run your item before recharging or changing batteries.

That covers the vast majority of electrical math that most people will encounter in their lives. If you have any other electrical math questions, please feel free to ask in the comments here or in the BCP Facebook or MeWe groups, and I'll happily answer them.


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