Solar panels or solar modules (which are normally installed on the roof but can also be mounted on the ground or an overhead canopy like a parking structure) are made up of photovoltaic (PV) cells. When sunlight hits the cells, a current is made converting sunlight into power. The direct current (DC) power from those solar panels is sent to an inverter. The inverter does exactly what the name sounds like – it inverts the current from DC to AC (alternating current) power, which is what your home and everything in it uses. The inverter is also what monitors the performance of your system and regulates the voltage coming into your home. It’s the “brain” of your solar array.
The AC power then travels from the inverter to the electrical panel, which is often called a breaker box or main panel and into your home. Anything that uses power like your dishwasher, your kid’s video game system or your TV will draw it from the solar panels first. If solar is not available (because it’s dark or cloudy outside), you’ll get your power from the main power grid just like you do now. By the way, everything in your home that runs on power will run exactly like it does now. Your air conditioner can’t tell the difference between running on dirty power and running on clean kilowatt hours, but your power bill and your lungs sure can! Unlike fossil fuels, solar power doesn’t emit any harmful smog, waste chemicals or noise to mess up the air or water. Your home will also gain value as now you have a power plant on the roof that runs on unlimted free fuel. No more rising utility rates for you!
When it’s really sunny outside, you could even be sharing that clean power with your neighbors through Net Metering. Net Metering is how the utility pays you to send your clean power back to their grid. Think of it like roll over minutes on those old cell phone plans before we all got unlimited talk time.
Net Metering in a Bar Graph: See the blue bars on the graph? That’s solar production. See the gray mountain? That’s consumption. With Net Metering, extra power you make in the spring, fall and winter pays for the bigger power demand in the summer.
Here’s how it works. In the spring, winter and fall, we have a lot of clear, sunny days and most of us are not using our air conditioners, so our power use is way down. During those months, your solar system will usually make MORE power than your house needs. Whenever that happens, that extra power goes back out through your meter to your nearest neighbor that needs power right that second. When you go solar, you are truly making your whole neighborhood greener and cleaner! The meter will spin backward and you’ll be banking up credits against your future power bills – just like those old plans would save your unused minutes until you needed them. When summer rolls around and the temperature soars, you’ll need more power than your solar is making. The power company will then use your banked up credits to pay for it before they bill you. Thanks to Net Metering, if we can replace all of your power need with solar, you could have an average power bill of about $15 a month year round. Neat, huh?
Central or String Inverters are old-fashioned technology. With this type of set up, all the solar panels are wired in series (or strings). All the power the system makes feeds from the panels, over your roof as high voltage DC current, and down through one inverter to be flipped to AC current and then into your main electrical panel. The inverter is usually a pretty big box (about 1′ x 2′) and is mounted on your garage wall – hopefully in the shade as they tend to fail in high heat conditions. If you have a lot of panels, you’ll likely get stuck with two or more of these clunky boxes on your wall because each inverter can only handle so much wattage. Once it hits the string size limit, you have to add another inverter.
Central inverters are the weakest link in your system. Because all the panels are connected together in strings, that means if the inverter has a problem, the whole system stops working. (Just like those old-fashioned Christmas lights when one bulb burnt out.) It also means that if one panel is in the shade from a roof vent, a tree or a bird mess, the entire string’s power output is reduced to whatever that lowest performing panel is doing. Some companies will install a power optimizer on each module to overcome this performance loss and help the system compensate for shade, but that can’t overcome the single point of failure in the system.[ See how power optimizers compare to microinverters. ] These kind of systems are much more complex to design, are subject to a lot of limitations with string sizing and far more prone to failure. They also produce less power over the life of the system due to the line loss from the longer trip over the roof as DC current.
With a mirco inverter, each panel has it’s own inverter right on the back. The DC current is converted to AC current right at the panel. This means the system is more efficient and produces more power over it’s lifetime because electricity loses strength the farther it travels before it gets flipped from DC to AC. (Lower voltage AC current moving across your roof is safer than high voltage DC current, too.) It also means that there is no single point of failure because the system is wired in parallel. If one panel or inverter were to fail, the rest of the system would just keep making power. Robco recommends Enphase micro inverters. They offer free lifetime monitoring with the Enlighten app as well as built in consumption monitoring at no additional charge.
More power will be produced over the life of your system.
Having an inverter on every panel means each one is truly independent. If one or two panels are shaded or dirty, the rest of the system couldn’t care less and keeps right on making power at maximum capacity. On a central inverter system, there has to be enough light hitting the panels to create the minimum voltage on the string in order for the inverter to wake up from sleep. Because mirco inverters are independent, each panel will wake up from sleep about 30 minutes earlier in the day and go to sleep about 30 minutes later in the evening. (Burst mode is the technical term for this.) That means you get roughly 1 hour more of production each day from a mirco inverter system. Over 25 years, that extra hour can really add up.
More reliable with a longer life span.
Mirco inverters have an expected lifespan of 25 years and are warrantied (parts and labor) for all 25 years. Central inverters – even the ones with optimizers – typically last 12 to 15 years and often fail even before their expected end of life – especially if they are not installed in a shaded area. While it is true that it’s pretty simple to replace a central inverter and that it does not require anyone to get up on the roof, they cost $3,000 or more every time they fail! If you had to replace it twice in 20 years, you just added $6,000 to the sticker price of your system and added 2-3 years to the payback time. How do we know that micro inverters are more reliable? Simple. In the last 7 years, Robco has installed over 22,000 Enphase micro inverters and replaced only 59 of them (.27%).
Expanding your system
As we’ve learned, central inverters are set up with panels wired in strings. If you want to add on to a central inverter system, you’ll need to add a full string of panels (at least 8). On the other hand, micro inverter systems will allow a single panel at a time add on . You can perfectly size your system as your needs grow. No need to spend extra money on equipment you don’t need or pay the power company more than you have to. This design flexibility also means that your install team can easily alter your panel layout in the field if needed.
A solar panel is two sheets of silicon solar cells connected by bus bars encased in glass often surrounded by an aluminium frame, but that doesn’t mean all solar panels are created equal. The grade of the silicon and the wattage of the panels as well as how those cells are assembled all matter when it comes to lifetime power output and reliability.
Mono or polycrystalline
All solar panels are made from silicon crystals. In a polycrystalline panel, each of the wafers used to make the solar cells is composed of multiple silicon crystals. The process used to make the silicon ingots is a bit like the one used to make particle board in construction. A single crystal seed is placed in a vat with lots of chunks of silicon and it’s all melted together. Many crystals grow as it cools. It’s cheaper to make and creates less waste, but the borders where the crystals meet make for a slightly rougher surface which causes electricity to move around less efficiently. This type of panel has a 13-16% efficiency rating. It also has that typical blueish hue due to the composite nature.
For a monocrystalline panel, the silicon wafers are sliced from a single crystal, giving them a much more uniform surface which converts sunlight into power at a higher efficiency (15% to 22%+). Due to the purer nature of the silicon, monocrystalline panels have a dark black appearance. They are also perform better at higher temperatures – a big plus in our desert heat. Robco recommends high efficiency LG NeON 2 Mono Crystalline solar panels.
These days, most solar panels are at least 280 watts. That is the rated output of the panel under perfect test conditions. On your roof top though, the output will depend on which way the panels face, how much sun is hitting them, what angle they are tilted at, how much shade is present and how hot it is outside. When your solar installer is providing an annual power estimate, they are taking all these variables into account. Obviously higher wattage panels will produce more power over their lifetime then lower wattage panels even with these variables.
Silicon crystals break down over time. This is called degradation and it is a natural quality of the material so every single panel will lose a little bit of performance as it ages. It can be increased by environmental factors and stress to the module structure – like someone walking on your solar panels. Your power output in year 20 will be lower than it was in year 1. How much will it drop off and will you notice it? That really depends. Manufacturers guarantee that the typical solar panel will produce at least 80% of the day 1 power at Year 20, but manufacturers can reduce degradation in a number of ways from using more and thinner bus bars to the surface of the cells. Those technology advances mean that long term power production from panels is getting better and better and that you should not be concerned over degradation.
A bifacial solar panel is transparent and has no aluminium frame. It creates electricity using the light that hits the front of the panel and reflected light hitting the back of the panel. When they are installed at an angle over a light colored surface, like a on a flat roof, in a parking canopy or on a raised rack on the ground, these type of panels can increase power production.
However, when they are installed flush to the roof over dark shingles or roof tiles, there is very little reflected light for the back side to gather, so you would be paying for technology that offers no benefit.
As you’ve learned, most solar systems don’t have batteries attached to store power. When the power goes out, your solar system will shut off, too. It’s for the safety of the utility workers while they restore power. Unexpected power coming from your house would be a hazard to them. As soon as the grid is back up, your system will wake up on it’s own and start making juice again. Before you freak out.. think about it. Here in Las Vegas, our grid is really reliable. When was the last time the power was out for more than an hour or so?
What if you live in a place where the power goes out a lot or you just want to “cut the cord” with the power company and run on truly clean, local power all the time? That’s where batteries or energy storage come in. Instead of your extra power going back to the grid, it will charge the batteries. If it’s cloudy or dark, your power will come from the batteries instead of the grid. If the power grid goes down, your solar system will keep making power during the day and you’ll be able to draw power from your batteries overnight. With storage, you can save the energy you produce for when you need it most and be the only house on the block with power after the storm.
Here in Las Vegas, where the grid is really reliable, the cost of having backup power or going off grid entirely is usually a bit too high to make sense. Right now, batteries cost at least $6,000 each and you’ll probably need 3 or 4 of them to run that energy hungry air conditioner overnight in August. If you live in an outlying area like Lee Canyon, Caliente or Cold Creek, the extra cost can make sense. We do quite a few off-grid systems in those places, but until the tech matures and costs come down, we don’t recommend batteries for customers in Las Vegas, Henderson, Mesquite, Pahrump or Boulder City.