How Solar Works

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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. Your inverter will also provide the monitoring of your system. Our Enphase systems come with lifetime free production and consumption monitoring through the Enphase MyEnlighten application. 

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. 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 unlimited free fuel. No more rising utility rates for you!

How does solar work with the power grid?

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 to the right? That’s solar production. See the gray mountain? That’s power consumption. With Net Metering, the extra power you make in the spring, fall and winter pays for the bigger power demand in the summer.

In the spring, winter and fall, we have a lot of clear, sunny days. 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 and NV Energy puts a credit on your power bill. That power is then delivered to your nearest neighbor that needs power right that second. Your 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 needs with solar, you could have an average power bill of about $14 a month year round. 

How does this look on your bill?

NV Energy offers net metering on a Tiered system. Right now, the state is in Tier 4 at 75% of retail value buyback. At the end of the month, NV Energy will compare what you used (KWHD) with what you sent back (KWHR). What you send back wipes out what you used at 100%.

If you used more than you sent back, they bill you for the extra (KWHN).

If you sent back more than you used, they credit you for the excess energy sent back to the grid

Any extra (KWHA) is purchased back from you at 75% of whatever the current retail price per kilowatt hour is. It’s a credit on your bill (Excess Energy Credit). Just like rollover minutes on the old cell plans, the credits just sit there until you need them. The power company applies them as needed. They never, ever cash these credits out, so it’s important to size your solar to create the the amount of power you need and not oversize it. Click here to learn more about sizing your solar system correctly. 

Central Inverters

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 fed 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 might 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 or more panels are in the shade from a roof vent, a tree or a cloud, 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. Power optimizers provide panel level monitoring, but they are NOT inverters. If the inverter fails, the whole system stops working. In summary, these kind of systems are much more complex to design. They are subject to a lot of limitations with string sizing. They are far more prone to failure due to the large mechanical cooling fan inside the inverters. Anything that moves can break over time. This is why central inverters typically come with a 12-15 year warranty instead of 25 years. When that fan goes, the unit must be replaced. You will pay to replace a central inverter at least once during the 25 year life span of your system. Central inverter systems, even with optimizers,  produce less power over the life of the system due to the line loss from the longer trip over the roof as DC current. Centrals with optimizers are a kludge, not an equivalent to micro inverters.

Micro Inverters

With a mirco inverter system, each panel has its own inverter. 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. Micros also do not have ANY moving parts inside them, so there is no fan to fail. This is why they are warrantied for a full 25 years. Robco recommends Enphase micro inverters. They offer free lifetime monitoring with the Enlighten website and 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. These failures are especially common 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 and will take 4-6 hours to replace. If you had to replace it twice in 20 years, you just added $6,000 to the sticker price of your system and added years to the payback time.

How do we know that micro inverters are more reliable?

Simple. In the last 8 years, Robco has installed over 40,000 Enphase micro inverters. They have been through many punishing summers and the failure rate remains less than 1%. You can verify this by checking our reviews, there are no mentions of inverter failures.

Expanding your system

At some point, you may find you need more power than your system was sized to deliver. Maybe you put in a swimming pool or buy an electric car. Once your system is up and running, you can add on as needed. Find out more about adding on here.

If you want to add on to a central inverter system, you can only go up to the capacity limit of the inverter (3kw, 6kw etc.). So if you have a 6kw system with a 6kw inverter and you need to add on 8 panels because you converted your garage into an office, you’re going to need another inverter. Another inverter can cost upwards of $1,500 in addition to the added cost of the panels.

On the other hand, micro inverter systems add a panel and an inverter together. With micros, 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.

Types of panels

At it’s most basic, a solar panel is two sheets of silicon solar cells connected by bus bars encased in glass surrounded by an aluminum frame. Just as there are many kinds of cars with many different price points and specifications, there are a lot of brands of solar panels with different specifications and different prices. 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.

Monocrystalline or polycrystalline solar panels

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 where light is bouncing off the rougher surfaces and being reflected back to your eye.

In a monocrystalline panel, the silicon wafers are sliced from a single pure silicon crystal, giving them a much more uniform surface which converts sunlight into power at higher efficiency (15% to 22%+). Due to the purer nature of the silicon, monocrystalline panels have a dark black appearance. They also perform better at higher temperatures – a big plus in our desert heat. Robco recommends high-efficiency LG NeON 2 Monocrystalline solar panels.

Panel Wattage

These days, most solar panels are at least 300 watts. That is the maximum rated generating capacity of the panel under perfect test conditions. On your rooftop though, the output will depend on which way the panels face, how much sun is hitting them, what angle they are tilted at, how high the sun is in the sky, how much shade is present, and how hot it is outside. In practical terms, that means each panel is going to put out different amounts of power at different times of day and in different seasons. 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 than lower wattage panels even with these variables.

Degradation

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 25 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, to the actual composition of the cells themselves. Those technological advances mean that long-term power production from panels is getting better and better.  You should look for a panel that offers at least 90% power output guarantee at year 25 just like our recommended LG NeON 2 panels. 

Bi-Facial

A bifacial solar panel has a transparent backsheet. The cells used can create power from both sides. 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 on a flat roof, in a parking canopy, or on a raised rack on the ground, these types of panels can increase power production. They are typically reserved for commercial installations, however we may recommend them for flat roof installations.