Grid Tied Systems

 Grid Tied Systems

A grid-tied system is tied to the power grid that your local electric company uses to provide electricity to homes and businesses in your area. The solar panels direct solar energy into your building’s electric service, providing another source of electricity needed for your home’s lights, heating, cooling, and appliances, etc. This electricity production reduces your electric bills since you are using less electricity from the utility. You don’t change the way you use electricity – you just have two sources of electricity.

If your solar array produces more energy than your house needs this extra energy will flow backwards through your electric meter, registering a credit which can be used in the future. This is called Not-Metering, which allows you to “store” excess electricity in the utility grid and use it at any time during the calendar year. 

However, this type of system is required to shut down when the power from your utility company goes out, so it doesn’t provide a backup for your utility company’s electricity the way a battery backup system does.

This style of PV system is the most popular worldwide for residential and commercial applications. It offers a very high energy conversion efficiency and is incredibly low maintenance.  There are only three parts involved in these systems:

  • PV modules, also known as solar panels, which harvest the sun’s energy
  • One or more inverters to process the solar energy and change it from direct current to alternating current — the kind of electricity your household appliances use
  • Interconnection equipment to put it all together

 

Grid connected PV system financial examples:

Residential example:
11,020 watt (11.02kW) grid connected PV system with 38, 290 watt US made PV modules and German engineered US made string inverter.
Turnkey system cost including tax: $33,259
Less federal tax credit: $9,977.70
Final system cost : $23,281.30

This system will produce 15,428 kWh of energy per year worth $1,511 per year. Your $23,281.30 investment will return a tax free dividend of $1,511 per year, a 6.4% ROI at current electrical rates. This ROI will improve as electrical rates increase. This system will produce 385,700 kWh of energy over it’s 25-year guaranteed lifetime at a cost of $23,281.30. The levelized cost of energy for this system will be $.06 per kWh of energy produced over the system’s warranty period, roughly $.03 less than current electrical rates in NW Arkansas.

Commercial example:
100,050 watt (100.05kW) grid connected PV system with 345, 290 watt US made PV modules and
German engineered US made string inverter.
Turnkey system cost including tax : $225,099
Less federal tax credit : $67,529
Less bonus depreciation : $33,764
Final system cost : $123,806

This system will produce 140,070 kWh of energy per year worth $13,726 per year. Your $123,806 investment will return a tax free dividend of $13,290 per year, an 11% ROI at current electrical rates. This ROI will improve as electrical rates increase. This system will produce 3,501,750 kWh of energy over it’s 25-year guaranteed lifetime at a cost of $123,806. The levelized cost of energy for this system will be $.03 per kWh of energy produced over the system’s warranty period, roughly $.06 less than the current electrical rates in NW Arkansas.

 

String inverters or Micro inverters?

String Inverters

Way back in the early 2000’s when grid tied solar power installations were just getting started, there were very few choices for inverters that legally allowed grid connection and back feeding. Of the inverters available most were string inverters that electrically “string” PV modules together into long circuits of around 8 to 14 modules in series. These electrical series circuits build the DC voltage within the circuit and lower amperage within the circuit, allowing installers to use smaller wires and/or deliver PV energy at greater distances with acceptable losses. This was a great technological improvement over the dark ages of battery based systems whose lower voltage circuits were very cumbersome because of the large wires and the short wire runs that were required to keep them cost-effective.

With string inverters, we were able to increase system efficiency to well over a 95% conversion from DC energy in, to AC energy out of the inverter and into your electrical system.

With all of these improvements there is one design drawback to stringing many PV modules together in series.  If any one module within the electrical string gets shaded, then that shade affects the whole string of modules and you will see a loss in power output because of that one shaded module.

There are many ways around this problem.  Good system design and wiring layout within the array can decrease the effect this will have on a partially shaded array. Adding another string inverter or specifying an inverter that allows two or three input circuits can help, too. My favorite technique? Don’t install solar panels in an area that will receive shade between 9AM and 3PM all year around.

The amount of shade in a particular setting may not be under our control, so there are times when string inverters are not the best solution. There are newer technologies that we can turn to when shade is an issue: Micro Inverters and Optimizers.

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Micro Inverters

Micro inverters provide an inverter for each solar panel. The major advantage of micro inverters is that they are more forgiving than string inverters. They don’t require as much concern for system design or the same level of expertise. If for some reason you’re planning to have your solar installation done by people who aren’t experts in solar energy, you might be better off with a micro inverter.

There are drawbacks, though.

Most micro inverter installations have the inverter mounted to the racking under the PV module or mounted directly to the module from the factory.  This places the sensitive inverter electronics in a very harsh and hot  environment. After installing both types of inverter technologies for years and on some sites installing both technologies side by side, we have found that string inverters do still outperform micro inverters.

We feel that micro inverters have their place in some sites with shading issues. Micro inverters do allow each PV module to operate individually without being influenced by its neighboring modules’ electrical characteristics.  If shade sweeps across an array during mid day then only the shaded modules are affected by the shade – not their unshaded neighbors. We feel this difference is the biggest positive attribute micro inverters have.

They are not a cure-all and cannot take the place of thoughtful site-specific system design. 

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Optimizers

In our ever forward march of technology, the PV industry has come up with another new way to get around the effects of shade on a PV array. Optimizers, like Micro Inverters, are placed under each individual PV module in an array. From this position they allow each individual module to produce as much power as it possibly can no matter what is happening to other modules in the same circuit. Another benefit of this technology is that it allows the installer and system owner to monitor each individual PV module online. Micro Inverters offer these same benefits but do all of the DC to AC conversion on the roof in an incredibly hot and humid environment.

In our opinion the fewest electronics you can place in this unforgiving environment the better. This is where Optimizers have benefits over Micro Inverters and where String Inverters have benefits over both. In our years of experience, the least complicated and fewest numbers of electronic components you can place on a PV system the better, especially keeping as few components on the roof as possible. At RSE, we use good system designs to mitigate issues caused by shade and if necessary will purposely complicate the system by using Optimizers to lessen the effects of shade. There is no magic bullet that allows one to install solar in the shade and maintain on acceptable ROI, although good design and electronics do help lessen the effects of intermitting shade on an array.