The Math of Solar
Welcome back to our tutorials! We hope our last lesson helped you understand what each solar component does, and how each piece functions in your soon-to-be off grid solar system. If you haven't taken this tutorial, we'd highly recommend it.
In this lesson, we want to lay the foundations of off
grid solar system planning. To do this, we
need to teach you the science of how electricity is calculated, and measured. Then,
we've set up
a few fun exercises to help you solidify your new understanding. Good luck, get
excited, and
don't forget to have fun!
Let's get started.
How's Solar Calculated?
If you were to go on a long-distance hike, how likely would you guess correctly the amount of food and water to bring with you? Not making an educated decision on your supplies could drastically change and endanger the outcome of your hike. You'd want to read about how long the hike would be, read research about the supplies previous hikers brought, and perhaps locate natural water sources along the hike. Just like doing research for a hike, we need to quantify how much electricity you will be using, to build a solar system that fits your needs. To do this, we need to learn a few simple terms.
Basic Terms You Need to Know
Current or Amps
Current, or amps is the measure of electricity flow per second. Electrons are quantified by current and amps, the same as water is quantified by gallons or pints. It is a unit of electricity.
Amps is a unit of electricity
Voltage
Voltage is the measure of the force moving electrons. It is the pressure which causes electrical current or amps to flow, pushing electrons through wires, and into your appliances. Going back to our water example, we can easily compare the pressure provided by volts with the same pressure provided by water pressure in a pipe.
Voltage is the measure of the force moving electrons
How long would it take to fill up a glass of water at a faucet, versus a fire hose? In one example, you'd have to wait a few seconds for the glass to fill up. In the second example, there would be so much water coming out, it would be hard to control. This is because of pressure (volts). In both examples, we are looking for the same glass of water (quantity of amps), yet both water sources deliver the same glass of water at dramatically different speeds. Due to this pressure (volts), the total amount of water is delivered within the same amount of time is also different. This total amount of water, brings us to the definition of watts.
Watts (Power or Rate of energy production/use)
Watt is the term we used for the total amount of power transferred, or needed within a wire or electrical device. Calculating watts is as simple as multiplying Volts by Amps. Think, total amount of electricity. In comparison, if we moved 2 gallons of water (amp) at 100 gallons per hour (volt), we would have moved 200 gallons (watts) within that hour.
Volts x Amps = Watts
OHMS
Think of Ohms as a bend in the pipe you are moving the water in. Each bend would add resistance in the pipe, and ultimately slow down the flow of the water. In electrical terms, we call this effect Ohms. Different types of metal wires, along with the distance at which you are moving electricity, will have a level of resistance or Ohms attached to it. The more Ohms (resistance), the more heat, and loss of electricity via the heat. Ohms are used to calculate voltage drop, and wire gauge within system planning, but we'll get to that later.
Ohm is the unit of electrical resistance
How Does This Relate to You?
Now that we have a good picture of how Amps, Volts, and Ohms work together, knowing these terms will allow you to correctly read the UL tags on your electronic devices. UL tags inform you about how much electricity a device uses per hour.
A UL tag has the number of volts and amps you can use to calculate how many watts per hour your appliance needs. You can also find the wattage, by doing a simple web search for your product. Multiply this number by the hours used per day for a daily estimate. Rinse and repeat, and add up all your daily estimates to a build your daily consumption number. An example below.
Exercise
Take a look at your refrigerator and fan. Look for the UL tag, or do a quick web search to note the amps and volts your appliance uses. Multiply the figures to get the total number of watts, then multiply it by your hours of usage. What number did you get to? You can also try this with your air conditioner or any other appliance. Use the calculation chart below to double check your figures!
Fridge
Volts
Amps
Watts
Hours Used
Watt Hours
Fan
Volts
Amps
Watts
Hours Used
Watt Hours
Total
What Exactly is Amp Hour / Watt Hour?
You just learned it! A watt hour is the amount of watts consumed within an hour. Think of your fridge as an hourly employee, needing 300 watts per hour to do their job. If you've read that your fridge is 300 watts, that means it takes 300 watts per hour to power it.
On the flip side, amp hours describe the appliance in the volume of energy needed versus the total amount of energy. So if your fridge is a 2.3 amp fridge, we would describe it as a 2.3 amp hour fridge - since it needs 2.3 amp hours at 120 volts to run itself.
Batteries also describe themselves in amp hours. You'll see labels like 200 AH, or 50 AH, signifying 200 or 50 amp hours. In these descriptions, its important to look at the voltage since it directly affects the amount of stored energy within the battery. 50 AH is much different in a 12 volt setting vs a 48 volt setting. Its 4 times the amount of wattage!
Watt Hour is the amount of watts consumed within an hour
On the flip side, amp hours describe the appliance in the volume of energy needed versus the total amount of energy. So if your fridge is a 2.3 amp fridge, we would describe it as a 2.3 amp hour fridge - since it needs 2.3 amp hours at 120 volts to run itself.
Batteries also describe themselves in amp hours. You'll see labels like 200 AH, or 50 AH, signifying 200 or 50 amp hours. In these descriptions, its important to look at the voltage since it directly affects the amount of stored energy within the battery. 50 AH is much different in a 12 volt setting vs a 48 volt setting. Its 4 times the amount of wattage!
Exercise
The calculation chart below uses the numbers calculated in the chart above. Using the watt hours and rounding up or down, answer the questions below. Feel free to change the table above, to generate different exercises.
