Finding Out My Needs
Hello again! Looks like you're almost through the
tutorials. We at Teragy solar are super stoked
for you to join us at our final tutorial.If you haven't taken the Math of Solar
Tutorial, we highly recommend it. But if you have, we
think you'll have no problem putting all the pieces you've learned into
understanding how to
find the right gear.
Good luck.
Watt Hours vs Peak Watts What’s the difference?
As you learned previously, watt hours are the total amount of electrical demand in one day. It is the summation of every drop of energy you plan on using for the day - much like planning how much gas to pump, when going on a small road trip. Peak watts, is a different concept.
Peak watts is the highest energy demand you'll be using while off grid. In terms of our gas comparison, ask yourself: “During my road trip, what is the longest stretch of road between gas stations?”. The amount of gas you would need to drive between the two gas stations would be considered your “peak watts”. In simple terms, “What is my single largest energy demand in a given amount of time?”.
Peak Watts is the highest energy demand you'll be using while off grid
Here is a real-life example. If you power a laptop, and a cell phone at the same time, each at 75 and 10 watts respectively, your peak watt usage would be 85 watts. Simple right?
Let's add more circumstance to our example. Let's say between the hours of 8 am to 12 noon, you power your phone and laptop. Then around 2 pm, the hottest part of the day, you power your air conditioner, using 1100 watts of power. Would your peak watts be 1185? Good try, but not exactly.
Since peak watts is about calculating maximum demand, and the air conditioner was powered outside the hours of the laptop and phone, your peak watts would be 1100 watts.
Exercise
Do you understand the concept of peak watts? Answer the word problems below, and see if you have the right idea! Feel free to refresh the questions, to strengthen yourunderstanding.
So Why Do I Need to Know This?
Peak watt calculations help you figure out how to size your solar array as well as your inverter.Your inverter has a maximum watt output, just like your solar array does as well. Knowing your peak watt number helps you estimate how much power to manage, and collect. Pair this with your total needed watt hours, and we can size a solar / battery array that is perfect for you!
Putting It All Together
So how does peak watt and watt hour usage affect your solar choices? In an idealistic scenario, we would like to build a system that is 100% sustainable to your energy usage/life.This means we need to size your battery bank to run the appliances you need at night.It also means we need to build a solar array that will collect enough energy to consume your day time energy usage, and replace your night time battery usage.Let's break it down by component.
Picking Solar Panels
Is your appliance used during the day time or night time? Because the sun is nonexistent during the night, watt hours used during the night come from your battery bank. Watt hours used during the day, can be offset by your solar panels.
Watt Hours used during the day, can be offset by your solar panels
Consider the exercise
You building an off grid solar system for your cabin retreat. During the day, you work remotely on a laptop, and make coffee for the morning. At night you relax, watching your TV, read something, and fall asleep listening to a radio on sleep. You've figured out your watt hours, and bought a 12 volt 200 amp hour battery. But how do you size your solar panels?
12 Volts x 200 Amp Hours = 2400 Watt Hours
|
Laptop |
|||||
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Volts 120 |
Amps 0.5 |
Watt 60 |
Hours Used 10 |
Watt Hours 600 |
|
|
Coffee Maker |
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|
Volts 120 |
Amps 12.5 |
Watt 1500 |
Hours Used 0.25 |
Watt Hours 375 |
|
|
|
|||||
|
Total Watt Hours 975 |
|||||
According to the chart above, our panels need to generate about 975 watt hours of energy to make up for the appliances we used during the day. But what about the night time?
|
TV |
|||||
|
Volts 120 |
Amps 0.75 |
Watt 900 |
Hours Used 3 |
Watt Hours 270 |
|
|
Radio |
|||||
|
Volts 120 |
Amps 0.20 |
Watt 24 |
Hours Used 1 |
Watt Hours 24 |
|
|
|
|||||
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Total Watt Hours 294 |
|||||
975 Watt Hours (Day Time) + 294 Watt Hours (Night Time) = 1269 Total Watt Hours
Now we are getting somewhere! At very least, we need our solar panels to collect 1269 watt hours for the entire day. But the sun doesn't last an entire day. A typical day has 13 hours of daylight.
1269 Total Watt Hours / 13 Day Light Hours = 97.61 Watt Panels
Looks like we can collect enough power to offset our daily usage with a single 100 watt panel (97.61 rounded to the nearest hundred). Now keep in mind, this is the bare minimum your solar panels need to charge to stay a float. We always recommend installing enough solar panels to fill half your battery bank in a days sunlight.
1200 Watt Hours (Half of a Battery) / 13 Hours in the Day = 92.30 Watt Panels
According to the calculations above, we'll need two 100 watt panels for the perfect solar system.
Exercise
Do you understand the concept of peak watts? Answer the word problems below, and see if you have the right idea! Feel free to refresh the questions, to strengthen yourunderstanding.
Calculate your day time usage here:
Appliance #1
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #2
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #3
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #4
Volts
Amps
Watts
Hours Used
Watt Hours
Total Watt Hours
Calculate your night time usage here:
Appliance #1
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #2
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #3
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #4
Volts
Amps
Watts
Hours Used
Watt Hours
Total Watt Hours
Picking a Battery
Remember in the Math of Solar tutorial when we added your daily watt hour usage? Picking a battery is simple. How sustainable would you like your system to be? Would you like to power your life style for 1 day without sun? Or 3 days? The calculations are simple.
|
Laptop |
|||||
|
Volts 120 |
Amps 0.5 |
Watt 60 |
Hours Used 10 |
Watt Hours 600 |
|
|
Coffee Maker |
|||||
|
Volts 120 |
Amps 12.5 |
Watt 1500 |
Hours Used 0.25 |
Watt Hours 375 |
|
|
TV |
|||||
|
Volts 120 |
Amps 0.75 |
Watt 900 |
Hours Used 3 |
Watt Hours 270 |
|
|
Radio |
|||||
|
Volts 120 |
Amps 0.20 |
Watt 24 |
Hours Used 1 |
Watt Hours 24 |
|
|
|
|||||
|
Total Watt Hours 1269 |
|||||
1269 Total Watt Hours x 2 Days = 2538 Watt Hours
2538 Watt Hours / 12 Volts = 211.5 Amp Hour Battery
Now if you're a stickler about absolutely being able to power your appliances for 2 days without sun, a 300 amp hour battery would go above and beyond what you need. But if your budget conscious, a 200 amp hour battery could be a close fit to your energy demands. Now keep in mind, if your energy demands go up, you'll most likely need multiple batteries that total your daily consumption.
Exercise
Go ahead and try the exercises below yourself! Pick a list of appliances you'd like to power throughout the day, then use the calculation chart below. See if you can pick the right battery for your needs, in the multiple choice below. Feel free to generate as many exercises as you need by hitting the refresh button, and refilling your appliances.
Input your appliances here:
Appliance #1
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #2
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #3
Volts
Amps
Watts
Hours Used
Watt Hours
Appliance #4
Volts
Amps
Watts
Hours Used
Watt Hours
Total Watt Hours
Picking an Inverter
We hope you did your homework to understand what peak watts are! At Teragy, our basic special formula for picking inverters is the average watt rating of all your appliances, added to your highest watt rating of all your appliances. Examine the calculation chart below to see this principle in action.
Laptop
Volts
Amps
Watts
Coffee Maker
Volts
Amps
Watts
TV
Volts
Amps
Watts
Radio
Volts
Amps
Watts
Highest Watts
Average Watts
According to the calculation chart above, 1919.00 watts would be what you're looking for in an inverter at minimum. We would recommend rounding to the nearest 1000, or 500, so in this case you would be looking for a 2000 watt inverter. Figuring out what you need, really is that simple!
Exercise
Do you understand the concept above? Write and calculate the volts, amps, and watts of all your own appliances above. Gather your highest watt rating, average watt rating, and add them together for your Inverter watt rating. Use the calculation chart to double check your work!
So what about inverter / chargers?
Inverter / chargers do the same thing as an inverter, with the added benefit of charging your battery on grid. Having an on board charger opens up your charging diversity, allowing you to plug your off grid system directly into a generator or 120 / 240 AC outlet.
Make sure your batteries can charge at the given amps your charger will put out. And don't for get to ask about charging profiles - charging profiles allow you to safely charge AGM, Wet Cell, Lithium or all battery types while prolonging life span.
Picking a Solar Charger
Solar chargers (controllers) translate the inconsistent energy of the sun, to a smooth consistent energy for your battery and DC system. Picking the right solar charger comes to calculating the total amps from your solar panel(s), and the volt rating of your battery, DC system, and solar panels. But before we dig into this, lets go over some vocabulary.
In / Out
- Similar to an old VCR, DVD player, or a stereo receiver, solar controllers direct energy coming “in” from the solar panels (sometimes referenced as the solar panel side), and translates them “out” to your battery or DC power system.
Step Up / Step Down
- Your solar panels can have a different voltage than your existing DC or battery system. Step down and step up controllers help bridge the voltage difference between your solar panels and your native DC system. When you put together a 48 volt solar panel system with a 24 volt DC system, you are effectively “stepping down” the voltage via the solar controller. Same applies for “stepping up”. Most MPPT chargers are built with a “step down” ability to match your higher output solar panels to your lower voltage batteries or DC eco system.
MPPT
This stands for Maximum Power Point Tracking. This is the latest technology in solar controllers, and for our purposes, we will only be covering MPPT charge controllers. MPPT charge controllers deliver more power, and adapt to a much wider range of solar applications than PWM controllers.
Smaller Off Grid Systems
Choosing a solar charger for a small system is much easier than a larger off grid system. Smaller off grid systems include camper vans, recreational vehicles (RVs), boats and yachts, and small off grid cabins. In a small system managing less than 1000 watts of solar, finding the right solar charger / controller is easy to learn.
The first step to choosing a controller is understanding how much amperage and voltage you need to manage. Behind your solar panel is a silver sticker that will list your panel's voltage, and amperage. If you have multiple panels, multiple your single panel amperage by how many solar panels you have. See the example below to illustrate this principle.
Wiring Strategy
Panel Quantity
Voltage (Voc)
Watts (Pmax)
Amps (Imp)
Controller Max Amps:
Controller Max Volts:
In the example above, we would be looking for a solar controller with the ability to manage 14.58 amps at 24.00 volts - with panels wired in Series. To make sure we don't damage our equipment, we will be rounding up our specs, making our approximate product spec to be 40 volts at 20 amps. Do you need to make sure your setup is scalable for future expansion? The more amps your controller can handle, the more you can expand your solar array!
Exercise
Do you understand the concept above? Write down the voltage, watts, and amps of the solar panel you are trying to buy. Add up your total amps. Did you get it right? Use the calculation chart above to check your work. Looking for extra credit? Play around with the Wiring Strategy feature and see how it affects your solar controller specs!
Learn More About Our Solar Controllers >Larger Off Grid Systems
There are multiple skill sets involved in choosing the proper set of solar controller(s) for larger off grid solar systems. In the next installment tutorial, we will cover picking solar controllers for large solar arrays. The tutorial will go over series and parallel wiring, voltages, wiring thicknesses, and all-in-one off grid solar controllers.
Do you need help picking out the right solar controller for your system? Call our solar specialists today, and let us find the right fit for you.
