The simplistic view of solar energy might lead one to think that a person can mount a panel on the roof and run a cord to the back of the TV and—viola!—start watching a favorite show. But solar panels don't capture the kind of electricity our households run on. And that's by design.
For efficiency, solar panels provide energy as direct current, a current flowing one way. When cells in the panels absorb sunlight, they kick electrons loose and the electrons flow out of the cells in one direction. Since this energy is generated as a direct flow, it is most efficient to keep it flowing in one direction until needed: direct current.
In contrast, most household appliances and consumer electronics require alternating current, which is an electrical current that changes direction at regular intervals. Household electricity changes direction 50 times every second. If you pumped DC into devices expecting AC, they'd overload.
Get Home Solar EstimatesBefore the energy generated by solar panels can power up a TV or washing machine, it must be inverted, the proper term for converting an electrical current from DC to AC or vice versa.
If you have a laptop, then you've been carrying around an inverter, probably without knowing it. The inverter is the "brick" part of the power cord. Laptops run on DC. So you're already familiar with the concept even if you hadn't thought about inverters until today.
Let's look at the three main types of solar energy inverters you have to choose from when you upgrade your home with solar panels. Each type has pros and cons.
Standard String Inverters
The simplest and cheapest inverter setup is a standard string inverter. Each panel is connected to its neighbor until all are included, like a string of Christmas lights. DC current flows from one to the other and arrives at a wiring harness, which feeds the current into the inverter. See illustration below.
Smaller solar arrays often use a standard string inverter. It's common to find them mounted on the side of a house near the power meter or in the garage.
Benefits of standard string inverters.
- Least expensive usually
- Simpler installation
- Easier to diagnose trouble
- Easier maintenance because of where inverters are usually mounted, e.g., the garage
Drawbacks of standard string inverters.
- Total energy production can drop due to one struggling panel
- A bad panel could shut down the array, like when a string of Christmas lights goes dark because of one faulty bulb
- Not good for complicated rooflines or near trees, where shade could fall on the array, because shade over one panel will lower the energy production of the whole array
Optimized String Inverters
With an optimized string inverter you can manage the output of the panels individually so that a poorly performing panel does not drag down the rest of the array. A power optimizer inside each panel measures the output until it reaches an optimal threshold, then it sends the current straight to the inverter. See illustration below.
With an optimized string inverter setup, the inverter always receives the optimal DC output of any given solar panel regardless of how neighboring panels are performing.
Benefits of optimized string inverters.
- Consistent energy flow to the inverter
- Can offset production impediments such as shade falling on one panel
- Each optimizer supplies data, making for more detailed production reports
- Power optimizers usually come with 25-year warranties
Drawbacks of optimized string inverters.
- They generally cost more than standard string inverters
- Some power optimizers are factory-installed and unrepairable, so if it fails, the panel needs replacing
- When optimizers are repairable, labor is sometimes not under warranty
Microinverters
Space between solar panels and the roof is tight, but there's just enough room to hold an inverter if it's small. Microinverters, one per panel, change the DC to AC before sending the current to the house. See illustration below.
Benefits of microinverters.
- Each panel performs independently, unlike with standard string inverters
- A faulty panel won't affect the rest of the array, and neither will one in shade
- Microinverters supply plenty of data, which can be monitored on a smartphone or computer
- Usually backed by 25-year warranties
Drawbacks of microinverters.
- Generally the costliest solar inverter setup
- More parts mean longer installation
- Location under the panels makes them inconvenient to inspect, repair
- Adding storage later is somewhat less compelling since the most efficient way to add a battery doesn't apply
Our Solar Success Story "A Home in the Woods" illustrated the installation of a system that used microinverters.
Adding Energy Storage After the Fact
Your solar inverter setup affects how you might add battery backup later.
Down the road, if you decide to add battery backup to your home solar energy system, you will face two options: AC coupling or DC coupling. One is less expensive, but the other is much more efficient.
With AC coupling, the battery is attached to the house like another appliance, maintaining its charge by sipping the AC current. But since batteries store electricity in DC, the AC must be inverted when it enters the battery and again when it comes out. This is on top of the inversion that took place when the solar panels supplied that energy to the home. Three inversions total. Each time energy undergoes transformation, some is lost.
DC coupling slashes the number of inversions to one. The greater efficiency of this method means that more captured energy becomes usable energy. With DC coupling, the old inverter is replaced by an inverter connected to a battery. The DC energy generated by the solar panels flows through the battery first, then to the inverter when the house needs AC.
Help choosing between AC coupling and DC coupling.
When cost is no object, DC coupling is the best way to add a battery to a system that has a standard or optimized string inverter. You let the new battery-and-inverter combo do the work of the old inverter, with storage as a bonus.
On the other hand, if you already have microinverters making your home's AC, you should opt for AC coupling. Since you are going to suffer an efficiency hit either way, you might as well go with the less expensive and simpler way to add a battery after the fact.
Efficiency Ratings for Solar Inverters
As mentioned, every time energy is transformed, some is lost. Did you know the classic incandescent lightbulb is only 10% efficient turning electricity into light? The other 90% of energy is lost to heat.
Naturally, solar inverters can't be 100% efficient or it would defy the laws of physics. When you're comparing solar inverters, you'll encounter two ratings: Peak and Weighted. Here is the difference.
Peak efficiency reflects how well the inverter works under ideal circumstances, such as its optimum operating temperature (no equipment likes to be too cold or too hot). This rating is akin to your car’s fuel economy label. You would use this rating to compare various inverter brands and capacities, but you wouldn’t expect the inverter to reach that efficiency every day.
Weighted efficiency tells you how the inverter's doing under real-world circumstances. You might be able to monitor data on your computer if your inverter supports data monitoring. Environmental factors like sunshine and ambient temperature can affect efficiency, as can the amount of sunlight the panels are converting to DC at any given time of day.
Conclusion
Fortunately, you don't have to become an expert in inverters to reap the money-saving benefits of solar panels. Rely on an experienced installer to outline your options and guide you to the right decision. Once your system is activated, you can put it out of your mind. No moving parts—just clean, quiet, renewable energy!
Key Solar Inverter Takeaways
- Solar panels generate DC power, while households run on AC.
- Every home solar energy system requires at least one inverter.
- A string inverter is often mounted in the garage or on the side of the house, while microinverters are small enough to sit under each panel.
- The inverter setup of your solar energy system affects how you might add a battery later.
