Amps, Volts and Watts

So one of the most frequent misconceptions or at least misunderstandings we hear from our customers is how amps, volts and watts relate to each other in a solar system (or any system for that matter) and why more amps is not necessarily more power.

The quick mathematical formula is E = I x R or V = I x R (Ohms law) where E (Electromotive force AKA Volts) = Amps (whose symbol is "I") x Resistance. Which boils down to: Volts = Amps * Resistance. To find any value, cover that value with your thumb on the Ohms law triangle and perform the math that remains (either division or multiplication).

For example, if you want to know how much resistance there is in a wire (circuit) and you know the amperage and the voltage, then cover up the "R", which leaves V / I. To solve for volts, cover volts and you have I x R.

What the heck is a WATT, and how is it different than an AMP?

Electrical plumbing. The goal is to fill a swimming pool with water. The swimming pool is our battery bank. We need water (electrons) and a length of water hose for the electrons to flow through (wires). Next we need pressure from a pump (PSI) AKA electromotive force (volts) to push the water from our source (the solar panels) to the swimming pool (our battery bank). The more pressure we have the faster the water will flow through the hose. The rate of flow is current AKA amperage. The higher the rate of flow (AMPS), the hotter things get.

If we want the swimming pool to fill within a day we better have a pretty large hose (wire size) since a bigger hose has more capacity and a small hose will be very restrictive (wire resistance). If the hardware store only has garden hose, we could increase the pressure (voltage) which will increase total flow (watts per hour) or we could shorten the hose (to reduce the resistance).

If we increase the pressure (volts), we can reduce the hose size (wire size) and get the same result. If we increase the hose size (reduce the resistance), we allow a greater current flow rate and can reduce the pressure (volts) and get the exact same result. Amps and volts are inversely proportional.

What we are really after is accumulated flow, or in electrical terms POWER measured in watt hours. Volts and amps are the variables we can manipulate, but it's all about accumulated power.

If our battery has stored up 10 amp hours of power, it theoretically can power a load that consumes one amp at the battery voltage for one hour (or a 1/2 amp for two hours). We can just as easily express accumulated power as watt hours. If our battery has gained 100 watt hours, it can power a 100 watt light bulb for one hour (or a 50 watt bulb for 2 hours). Watt hours is a measurement of total power whereas amp hours is a measurement of amp (flow) capacity at a certain voltage.

• The bigger the wire, the easier it is to get more volume into the batteries.
• The higher the voltage of the system, the lower the rate of flow (current) can be in order to accomplish the same result.
• The smaller the wire, the higher the voltage should be or the amperage would have to be increased (to have the same power) which might cause the wire to heat up and losses to accumulate.

Bigger wire, higher voltages and lower current is the most efficient system, yet budgets often dictate design not pure math, so use this as the design goal and then build safely with what you can afford.

Craig
Coleman Air, Inc.

Source: getawaypower.com/2019/02/amps-volts-and-watts/