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What Can A 100 Watt Solar Panel Run?
What can a 100 watt solar panel run? This is a typical question from newcomers in the world of solar power. Generally, when we design a solar power system, we start with a load list. We determine what you are trying to power. From there, we figure out what size solar panels you need. But for this scenario, we will be doing things in reverse. What can you power with a 100W solar panel?
A solar panel is rated by the amount of power it creates at Standard Test Conditions (STC). These conditions include:
the intensity of the sun, 1000 watt per square meter
the angle of the light hitting the panel directly
the temperature, 25℃ or 77℉
So the actual output from solar panels may vary, based on these factors in the real world. We generally reduce the calculations based on the difference between the lab setting and your actual installation. When a 12V solar panel is rated at 100W, that is an instantaneous rating. If we meet the test conditions, when you measure the output, the voltage will be about 18V and the current will be 5.55A. Since watts equals volts times amps, 18 volts x 5.55 amps = 100 watts. To figure out how much power is generated over a period of time, you can multiply the watts times the number of hours it is running. So in one hour, 100W x 1 hour = 100 watt hours (WH).
Now, we need to figure out how many hours to plug into the equation to determine how much power the solar panel will generate in a day. How many hours of sunlight that is equal to the intensity of standard test conditions? This is basically the sun at noon. How many hours will the solar panel be exposed to during the day? The number of hours of sunlight equal to noon is called effective sun hours (ESH). As you well know, even though the sun is up at 8 in the morning, it is not as bright as it is at noon. You can’t say that the sun is shining for 10 to 12 hours and multiply 100W x 10 (or 12). This is not the way it works. The hour between 8AM and 9AM is probably only half as strong as the sun from 12 noon to 1PM. The morning hour would probably only be equal to ½ sun hour. The number of sun hours would be dramatically different throughout the year.
Your location also determines the amount of effective sun hours you will get in a day. I live in the Philippines, where we get an average of 4.28 ESH/day according to NREL’s PVWatts Calculator. Looking at two major cities in the Philippines, the amount of sunlight I’d get in Baguio City (5.52 ESH, average) would be different than the amount of sun hours I’d get in Cebu City (4.8 ESH, average). The solar data can be obtained from the Solar Electricity Handbook Solar Irradiance tables. From the pictures, the ESH is broken down by month and even the tilt angle that the panels are mounted. Looking at the charts, if I have a 100W solar panel, in Baguio City, installed facing directly south, on annual average, I’d get 5.52 sun hours a day. Likewise, if I took that same solar panel in Cebu City, installed it facing directly south, I would have an annual average of 4.8 sun hours. I want to make sure that you see, during the month of April, I am going to get more power out of that solar panel in Baguio City than I will in Cebu City. For your specific location, play around with the solar panel direction to get the best ESH for your planned installation.
Back to the question at hand, what can I power with a 100W solar panel? Will I use the solar power for a specific period or do I plan to use it the whole year round? If the plan is to use it the whole year round, I need to figure out my worst case scenario. This is the worst performing month that I will be using the panel. Let us assume that the panel will be used in Cebu City. I need to use the data for December which has the lowest projected ESH for the year. So how can I squeeze out as much power as I possibly can in December? Based on the solar panel direction choices from the chart, no matter how I tilt my panel, I will still get an ESH of 4.05 sun hours. We will use what is in the picture, panel facing directly south for our example. 100W x 4.05H = 405WH. Our panel can produce a minimum of 405WH/day. Let’s move on to the next step.
Nothing in real life is perfect, so I have to figure in losses that I’ll likely incur, such as voltage drop across the wire, dirt accumulating on the solar panel, losses through the charge controller, etc. We are going to multiply the 405WH by 0.7 (30% loss factor). It is like losing ⅓ of your power, that is the reality. We have to compensate for losses all the time. I now end up with 283.5WH of power that I have made with my 100W solar panel on a December day. What can I do with that power?
First, I need to store it in a battery so that I can use it later when I need it. Before doing so, I need a charge controller (SCC) to manage putting the power into a deep cycle battery that can be charged and discharged on a regular basis. WIth our 100W panel and system voltage of 12V, 100W / 12V = 8.33A. We need an SCC of at least 10A for this purpose. The TOOGOO 10A 12V/24V Solar Charge Controller is an example. The Renogy Adventurer – 30A 12V/24V PWM Flush Mount Charge Controller with LCD Display is 30A PWM charge controller, a good option if you plan to expand your system up to a maximum of 400W. If you have a plan to expand in the future, you only need to buy more panels instead of buying a new charge controller.
What size battery do I need? I have my 283.5WH that I am producing. I am also putting it in a 12 volt battery. Because watts divided by volts equals amps, 283.5WH divided by 12V equals 23.63AH. Even though I will be using a deep cycle battery, most batteries still don’t like being drained down more than half way, so I’m going to make sure I get a battery that can hold at least twice as much power I will be using, so I’ll only use half of the power in it. 23.625AH x 2 = 47.25AH. The amount of power a battery can store changes depending on the temperature of the room it is in. If my battery is going to be as cold as 60 degrees Fahrenheit, I need to increase the size of my battery by 11% to accommodate the cooler temps. This applies to installations in very cold areas. 47.25AH x 1.11 = 52.45AH. DO NOT FORGET THIS IF YOUR INSTALLATION SITE WILL HAVE CLOSE TO FREEZING TEMPERATURES. But in our example, since this is a tropical location, we don’t need to compensate for this and we will continue our calculations using 47.25AH. We are also going to use an inverter to convert the DC power from my battery to AC. I am going to lose about 5% of my power through that conversion, so 47.25AH / 0.95 = 49.74AH. Bestek inverters are a good choice. To save on money, some will opt to use 12VDC appliances. You may skip this step if that is your plan. The sun doesn’t shine every day as we have to compensate for cloudy days and rain (or snow). So I need to figure out how many days without sun that I need to store the power for me to get through those sunless days. This is called days of autonomy (DOA). Let us say I need it to last at least two days without sun. 49.74AH x 2 days = 99.47AH. So finally, I am going to get myself a deep cycle battery that is 100AH at 12V. The Renogy Deep Cycle Pure Gel Battery 12V 100AH is a good choice. The Trojan T31-AGM 12V 100Ah Group 31 Deep Cycle AGM Battery is also another option.
I can finally figure out what I can do with that power. I can run my Macbook Pro that uses 61W for 4 hours and 40 minutes. Because 283.5WH Watt hours/61W=4.65 hours. Or I can power 3 of my 10W LED lights for 9 hours, and still have a little power left over. Or I could watch TV, listen to the radio while reading a book with a 10W light on for 3 hours, and use my laptop for 2 hours. This should give you enough information so that you can figure out how to fit this into your situation. You can change the numbers to fit your area, and your power needs. You can make a load list to figure out what you can use per day, just do not exceed the calculated 283.5WH. This is so that you still have left-over power for the next day in case the battery does not fully charge because of cloudy conditions or rain. It is all up to you.