The First in the "Everything You Need to Know" Series
Last updated on 9/5/19 | Written by Dan Martin
Welcome to the "Everything" Series!
We're glad you made it here! Buying solar panels is a big investment, so we want to make sure you know everything you need to know before making the jump.
The purpose of this article is to help you understand all the factors you should consider before you buy solar panels. This is the first of a several part series that will explore everything from solar panels to inverters to batteries.
We hope this is a useful guide and provides you with some answers to questions you may have in your solar journey. If you do have any other questions, feel free to ask them by emailing the author at firstname.lastname@example.org or give us a call at (480) 964-0260.
What is a Solar Panel?
Solar panels are made up of silicon semiconductors that absorb light from the sun. The silicon used in solar panels is classified into two types, monocrystalline and polycrystalline. Monocrystalline solar panels, sometimes just called mono, are more efficient, but a little more expensive. Polycrystalline, or poly, are typically cheaper, but less efficient.
When light hits these silicon cells, the cells absorb the light and knock loose electrons. When these electrons are knocked loose, an electric field guides these electrons in a current which is directed to a designated point, metal contacts placed on the outsides of the cells. Those electrons are then transferred out of the solar panel and into other appliances through a junction box on the back of the solar panel.
So, in short, light hits the silicon cells in the solar panel, which frees electrons. The electrons are guided to a metal point at the end of the cells and extracted through a junction box. The junction box has two cables that come out of it, and those are what carry the electrical current to other devices.
How Stuff Works has a great article on these. You can check it out here!
Terms to Know Before Buying
Cost Per Watt
Cost per watt is a metric to determine how much you pay per watt gained from the solar panel. To determine the cost per watt, you divide the price of the whole solar panel by the listed wattage, for example here at SanTan Solar we have a SunPower P17 340W solar panel listed for $110. If we divide 110 by the wattage, 340, we get the price per watt, $0.32 per watt.
Ok now we have that number, but what does that mean? You can add context to that price by doing the same calculation for other solar panels and see if you’re getting a good deal. SanTan Solar has another solar panel, a 250W panel listed for $55.
This panel comes out at $0.22 per watt. So you’re paying less per watt, that could be a good deal. Some of our competitors offer a 335W solar panel for $215, which puts it at $0.64 per watt. This is one way you can determine if you’re paying too much.
Solar panel efficiency can be tricky if misunderstood. Another term that is synonymous with efficiency is conversion rate. What these two terms refer to are the solar panel’s ability to convert incoming solar energy into electrical power. Most solar panels are able to convert between 11-19% of solar energy into electricity.
One other number that gets thrown around is a percentage of the rated efficiency. This is usually in reference to how long a solar panel will be able to be efficient. For example, if a solar panel is five years old, it will still be between 11-19% efficient BUT might only be able to function at 95% of the rated efficiency. Solar panels do degrade slowly over time, and this means that they’ll be able to convert less and less energy, but don’t fret!
Solar panels are made to last between 10 and 25 years depending on the solar panel. All this means is that as your solar panel gets old you might see a slight dip in how much solar energy it’s able to convert into electricity.
Not all solar panels are created equal, in fact most are created quite different! Each manufacturer has a different theory in crafting their solar panels and maybe even a different goal for application. For that reason you might see variations in the number of cells each solar panel has.
Some are 60 cells, some 72 and as many as up to 144! There are several reasons behind these discrepancies. Monocrystalline, or mono solar panels usually only have 60 or so cells, but that’s because they don’t need more!
A 60 cell mono solar panel usually produces 300+ watts, while 72 cell solar panels are typically polycrystalline and usually stick in the 220 watt range.
Some companies, like QCell, will even cut their 72 cells in half making them 144 cell polycrystalline panels. The logic behind this actually has to do with shading. On a typical solar panel, if shade covers a corner of the solar cell, the whole cell is inhibited from producing electricity.
Cutting the 72 cells and making 144 means that if shade covers the corner of a solar cell, there are still many more cells that can produce, and are not inhibited from producing.
Temperature Coefficient (Real World Conditions)
Solar panels are built and tested in laboratories where scientists are able to create ideal conditions for their tests and production. These tests are used to determine the wattage output and in what conditions can the solar panel achieve the highest output possible. Most solar panels have a sweet spot, and the temperature of the solar panel can be a big factor in reaching optimum output.
The ideal temperature for most solar panels is between 43°C and 48°C (109.4°F to 118.4°F) which is pretty warm. If the temperature of the solar panel is outside of that range, it won’t be able to produce as much as it has the capability. These numbers are sometimes called optimal conditions vs real world conditions, and this is something to take into account when you buy your solar panels. You are going to produce less than the listed wattage in real world conditions most of the time.
How Many Solar Panels Do I Need?
When it comes to determining how many solar panels you need, there are a few factors to consider: Your electricity usage, sunlight hours your house receives and your budget.
First, you need to know how much electricity you use each month. On your electric bill this should be a pretty clearly labeled section that lists how many kilowatt hours you used this last month. Typical American kWh usage is about 950 kWh a month, and this number will vary greatly depending on where you live. That’s the first piece of the puzzle.
The second piece is knowing how many hours of direct sunlight your house (or other preferred location) gets a month. There are several websites that can give you an average, here's one on wikipedia.
The last piece is all about how much you want to spend. Do you want to stop paying your electric bill entirely, or just in part? Do you want to start with just a few solar panels and build it up? Or jump in headfirst?
You’ll also need to know what solar panel wattage is best. Usually it’s better bang for your buck to buy as high a wattage solar panel as you can afford to meet your kWh needs.
Lastly take all these numbers and put them into a solar panel calculator like the one we have here. The calculation we’re doing in this calculator is pretty simple:
First we convert your kWh into watt hours, the easy way to do this is just add three 0’s to the end, so if you use 1000 kWh a month, you’re using 1,000,000 watt hours a month. Now we take that number and divide it by your average monthly sunlight hours, here in Arizona its something like 270. Now we take that number and divide it by the wattage of the panel you like, let’s say a 340W solar panel.
This gives you 10.89. This is the number of solar panels you need at minimum and in optimal conditions. Obviously, you can’t have 10.8 solar panels, so round up to 11. It’s also a good idea to do some math for non-ideal conditions, or simply to get one or two more panels than you think you might need to make sure you stay where you need to.
Once again, all this math is done for you in the solar calculator found at www.santansolar.com/solar-calculator. Here are a few other solar calculators that we like:
Which Solar Panels Are Best For Me?
The long winded answer above might have helped you get an idea on how to determine which solar panels are the best for you and your needs. To best determine which are best for you, you need to know how much electricity you want to create and how much space you have for the modules. Our general recommendation is to get as high a wattage of solar panel as available.
This saves on space because you’ll need less panels overall, it also saves on money because you’ll be buying less panels.
There are good reasons to buy lower wattage and lower priced panels though. Maybe space isn’t an issue for you, you just want to buy as many as you can within your budget. Maybe you just want to dip your toes in the solar panel pool.
Either way, there are lots of considerations to take into account before making a purchase: budget, spacing, desired panel wattage, kWh you use, other equipment you might have or be interested in, manufacturers or even country of origin.
Should I Consider Used Solar Panels?
One consideration you might not have thought of is opting for some used solar panels. We have a blog that addresses some of the benefits of buying used solar panels. In that post we compare buying used solar panels to buying a used car.
Should I Worry If The Panels Don’t Have A Label?
Sometimes here at SanTan Solar we get some panels second hand that have the manufacturers label removed and therefore don’t have a UL listing or a CE certification. These types of solar panels probably aren’t ideal for a typical residential on grid connection, although in the right situation they could be. These panels are more suited towards off grid installation such as a cabin in the woods, or an RV or boat.
In most areas inside the United States to get solar panels installed on your residential house and connected to the grid they need the UL or CE certification and they need to be inspected by an electrical inspector. In most cases he’ll double check to make sure the panels have that label. In some places in the US and Canada that isn’t a huge concern, but especially not in Mexico.
In Mexico you can install any type of panel however you want. So, if you do a lot of traveling and want some panels on your RV, boat, or a place in Mexico these panels would be perfect for you! If you have no interest in connecting your solar panels to the grid, then you could still use these on your residential house. If you fall into these categories then a solar panel without a label can be a great opportunity at a great price.
Best Panels On The Market
Now that you’re an expert on all things solar, it's time to take a look at the best solar panels on the market. We have a post that goes through the top manufacturers here
One of the top sellers here at SanTan Solar, and the author’s preferred panel, are the SunPower solar panels. SunPower is a well recognized and respected solar panel manufacturer. In fact, they’re one of the top sellers in the US!
They have a variety of solar panels. Some of their best products are the P, E and X series. They have both mono and poly variations at different prices and wattages. We here at SanTan Solar highly recommend these panels!
Through our years of experience and testing we can say that these panels are a real bargain.
Because SanTan Solar is a wholesaler and re-seller they often have several of the top makes and models come through at discount prices. It’s well worth your time to check in occasionally and see the inventory, as it changes regularly!
About The Author
Dan is an International Marketing and Sales Manager. A linguist turned marketer, he has a degree in Spanish and Linguistics with a minor in Portuguese and Mandarin Chinese. He is currently working with SanTan Solar developing markets in Canada and Mexico. He has written technical papers and academic reports on second language learners in various environments and explored the linguistics behind “internet words”. Currently he is a solar panel consultant simplifying the complex world of buying and installing solar panels by writing easy to read but in-depth articles on anything and everything that touches the solar panel industry.