These are the basic concepts you need to understand to appreciate what your solar power system is doing. I’m summarizing on an intuitive level, so physics professors please avert your eyes…

Voltage

Voltage is the key to everything. Electricity is often likened to water flowing through a pipe, except electrons can flow through solid metal, but not air. This is a good analogy because the physics of what’s happening is very similar. A better analogy might be the old steam engines used in factories. A boiler creates a nearly constant pressure (voltage) which passes through the pipes. Using a control valve, a piston engine accepts as much steam as it needs to go at the speed it wants. The boiler controls the pressure, but the equipment (called “load”) controls the flow. Using this analogy, voltage is a measure of the pressure on the electrons in the wire. Amperage is a measure of the flow. (See more about amperage below.)

Voltage actually is pressure

Remember that the electrons in each atom have a negative charge and protons in the center of the atoms have the exact same positive charge and opposite charges attract; like charges repel. As long as there are the same number of electrons as protons in a wire, the charges cancel and everybody’s happy. But add just a few extra electrons and the excess charges repel each other, pressurizing the wire. Those extra electrons want to get out of there to make the wire neutral again. Voltage in reality is a pressure measurement, and that pressure is what makes the electricity flow and does all the work for us. The electrons never get used up or fall on the ground, but return in a closed path back to the generator or battery. The equipment is just using the difference in pressure to do work. That pressure is used to turn motors, heat things like light bulb filaments or flip bits in your computer.

Virtually every electric or electronic device out there is made to work with a tightly controlled supply voltage. This applies to light bulbs, vacuum cleaners and computers alike. A power supply’s job is to accurately provide that specified voltage no matter how much current is being used. A power supply might mean the wall socket you plug into, or the power brick that provides 19V DC to your computer or perhaps a battery. The power supply doesn’t determine how much current flows, the equipment gets to do that.

Resistance

Resistance is effectively friction in the wires slowing down the movement of electrons and turning that energy into useful work or heat. For this simplification you can consider impedance and resistance to mean the same thing: something that slows down the electrons from flowing to their goal (the positive side of the power supply). Electrons can’t jump through the air from the – battery terminal to the + because the air has effectively infinite resistance. Resistance is measured in “Ohms,” represented with the symbol Ω. One Ohm is conveniently defined as the amount of resistance which will slow down the current to only one ampere if one volt of pressure is applied. The relationship is linear, if you keep resistance at one ohm, 2 amps will flow if the voltage is 2 volts, 10 amps will flow at 10 volts of pressure. This is the basis of the simple but  extremely useful equation for Ohm’s law.
Current = Voltage ÷ resistance or I = V / R
(I is current in Amperes, V is voltage in Volts, and R is resistance in Ohms)

Incandescent light bulbs make good simple examples. A 60W car headlight is designed with just the right amount of resistance in the hot filament to allow 5 amps of current to flow when 12V is applied; about 2.4 Ohms. If you were to hook it up to 120V or 230V in your house it would instantly burn up because the higher voltage would cause way too much current to flow vaporizing the filament. Just as the water pipes in your house will burst if you apply too much pressure, electric and electronic devices will burn up or break if too much voltage (pressure) is applied.

Wire resistance is usually ignored

Copper wire has extremely low resistance, with shorter or fatter wires having even less resistance than longer or skinny wires. In any electric circuit they are chosen to be fat enough that their resistance can be ignored. Because of this, the resistance of the device being powered (the “load”) is the only thing that determines how much current flows (amps). In the steam engine analogy, a valve can be used to allow more or less steam to pass and throttle the engine up or down. Likewise, many electric devices can vary their resistance and use more or less current, depending on what they are doing. Electric motors and most electronic devices like computers will vary the amount of current they use.

Amperage

I’ve talked about current flowing but haven’t defined it. In a wire the electrons actually flow through the metal. If you are measuring water flow, you would using a rate like “30 liters per minute” or “20 gallons per hour.” With electricity we do the same thing, a rate equal to 6.2 x 10¹⁸ electrons per second is called an Ampere, or Amp. Sounds like a lot, but compared to the number of atoms in even a short piece of wire, it’s tiny. That many copper atoms would only make a tiny fleck the size of a small grain of sand. That 6.2 x 10¹⁸ number is called a “Coulomb,” so even though an Ampere sounds like a quantity, it’s actually a rate of one Coulomb per second which also equals 3600 Coulombs per hour. How much work you can do with that rate of flow is directly proportional to the pressure behind it (the Voltage).

Power

Finally we get to Watts! You might think the amount of work you can do with you electricity depends solely on the amps, but that’s only half the story. Since we’re using the pressure difference between the input and output to do work, like a steam engine, the voltage is just as important.  Power is Volts x Amps. It’s an instantaneous rate, and is measured in Watts. How long you maintain that rate determines how much work you can get done, and that’s called Energy. (See below.) I like to think of watts as how fast I am draining my battery. A watt is defined as the rate of work supplied by one volt of pressure flowing at a rate of one ampere. So if your equipment is drawing 2A from a 12V battery, it’s using 24 watts.

Energy

Energy is a measure of how much work you can get done if you’re equipment was 100% efficient (no friction & no inefficiencies). People often mix up the terms power and energy. Just remember power is a rate, and energy is a total amount; it’s power x time and normally expressed as watt-hours. (watts x hours) Your electric bill is actually based on energy, not power and that’s what your “power meter” measures, in kilowatt-hours.

To get a feel for this, a good alkaline AA battery can deliver about 7200 Coulombs at 1.5V before it’s dead, but nobody marks their batteries in Coulombs. Instead they’ll say it’s 2 amp-hours (Ah) or 2000 milliamp-hours (mAh) which sounds better. That means the battery can (supposedly) deliver 2 amps continuously for 1 hour, or 1 amp for 2 hours etc. But amp-hours is not enough information. The amount of energy the battery contains is a mystery unless you also know the voltage; in this case approximately 1.5V on average. So 1.5V times 2 amp-hours gives 3 watt-hours in that little AA battery. For comparison, my entire house uses about 18kWh per day; that’s the equivalent of 6000 AA batteries every day.