Mnemonics and easy explantions
The "Vir" Triangle
First let's define the variables:
• V = Voltage, measured in volts
• I = Current, measured in Amps
• R = Resistance, measured in ohms
In the first triangle, we've colored the horizontal line GREEN, and the vertical line PURPLE. (the other two triangles have not been altered, the color coding is for clarity only, and is not an official part of the Triangle.)
If you think of the green line as a division bar (÷), then we can see that V / I = R, and V / R = I. Either way, you get an answer.
Similarly, think of the purple line as a multiplication symbol (x), thus I x R = V, and of course R x I = V is simply reversing the figures.
So we see that if you want to determine the current draw through a resistance (your coil) then you want to solve for R --> Looking at the triangle we see that R = V / I
(Note: Because we assume the green bar to be a division bar, I = V / R also.)
Calculating Power in Watts
Power is not in the Triangle and yet almost all modern devices display power in wattage these days (rather than variable voltage), here is how you calculate Power (measured in watts and signified to be P):
P = V x I
We see how calculating voltage (V) and amperage (I) is quite straightforward. Multiply them together to get Power. The units will sort themselves, since volts x amps is the definition of Watts (1W=1V•A). So if we have 4.2 volts (V), and 10 Amps (I), then you would have a Power (P) level of 42 watts.
Easy, right? Just always assume your 18650 is pushing 4.2 volts. Better to err on the side of caution, despite voltage drop and other technicalities which may lower this value in practice.