Ohm's Law for DC circuits
Ohm's Law for DC circuits is very simple, E = IR . This expands into the diagram below.
Power "P"
"P" is power, power is measured in Watts. Watts is the measurement of total energy you will need or are using. Most electrical units will list the watts they use or will produce as in the case of an Alternator, generator, or solar panel. Power companies charge you per kilowatt hour. A kilowatt hour equals a thousand watts an hour, so if you have a hundred watt light bulb on for ten hours that would equal one kilowatt hour. Once you begin to understand how power is measured and how to relate that to cost, you can better understand the savings you will realize with the conservation efforts you are implementing and what works and what doesn't work.
Current "I"
"I" is current, it is measured in amperes(amps for short) which has a symbol "A". Most electrical units have their required Amperes listed. But this is where Watts comes in handy, many electrical units are not purely resistive meaning that they need to use Ohm's Law for AC circuits for their listing of amperes to have any meaning. But it can be very meaningful for power production. An Alternator or generator rated for, say, 12 volts, will also list it's max output for amperes or amps. Power(Watts) equals volts multiplied by amps. You then can calculate the maximum Watts your alternator will produce using Ohm's Law. Amps multiplied by volts equals Watts. You then add up the total Watts needed to power the things you are going to connect to your alternator, your alternator must be rated for more Watts then your going to need.
Voltage "E"
"E" is voltage, it is measured in volts. Voltage can be thought of as the pressure that's pushing the current through your circuit. You can have too much current because a circuit will only use the Amperes it needs but the voltage has to be right for the circuit. The most common voltages for running consumer products are 12 volts DC(direct current), 24 volts DC, 110 volts AC(alternating current), and 220 volts AC. The ohm's Law formulas above are for DC current or direct current. That means that one wire is positive and the other is negative and this does not change. AC current or alternating current is where the current is changing direction usually about 60 times a second. Your home uses 110 to 120 volts 60 cycles per second AC. Some products use 220 volts 60 cycle per second AC, like some washers and dryers. That means that the voltage and current is changing direction 60 times a second. 12 and 24 volt DC or direct current are the standards for batteries. Batteries also come in many other voltages.
Ohm's Law formulas for DC circuits are only good for direct current or purely resistive circuits. For circuits that use AC current and involve transformers or capacitors Ohm's law for AC circuits must be used because transformers and capacitors change the phase between the voltage and current as the voltage and current rises and falls as it changes direction.
Resistance "R"
Resistance is the opposition to current flow. Everything has resistance to some extent, the only exceptions are super conductors and they require special conditions to become virtually resistance free. Energy is lost, due to Resistance to current flow, to heat. Resistance is used in electronics along with other devices to control current flow to perform some work. But resistance also causes unwanted heat and loss that must be taken into account when dealing with electrical devices and networks. Power lines have huge losses due to the hundreds or thousands of miles of wire a national power grid may use. But working with DC "direct current" even a hundred feet may incurred unacceptable loses. This was one of the major differences between Thomas Alva Edison and Nikola Tesla, Edison was top dog when it came to generating DC power. Tesla developed AC (alternating current) that allowed for present day distribution networks that span thousands of miles that would have been impossible with DC (direct current) power generation.