Courtesy of DOE/NREL

Part 6 – Cost Comparison – Fossil Fuel Generated Electricity VS Solar (PV) Generated Electricity

In the last segment, I calculated an approximate cost for a solar electric system sized to provide the same amount of electricity I now use in my home. In this segment (Part 6), and the last of this series, I spread the cost over the estimated life of the system (30 years), and compare it to the amount I’m currently paying for my electricity (fossil fuel generated). Hopefully this will provide a real-world cost comparison between fossil fuel generated electricity and PV solar generated electricity.

In segment one of this series I determined my total electricity use for the previous year and the cost of that electricity. The electric power use for the year was 8455 kWh at the cost of $1701.12. This is an average monthly use of 704.59 kWh of electricity at a cost of $141.76 per month. In the last segment I determined the cost of the solar electric system and after subtracting the Federal tax credit and State rebate, my cost was determined to be approximately $23,171 ($41,822 – $18,651 = $23,171).

At $23,171 for the system, divided by $1701.12 (cost of my home electricity for previous year), the payback works out to be just under 14 years (excluding the interest you may have to pay on the initial capital cost). This means that the remaining 16 years in the 30-year life of the system would pay out an additional $27,000 for the homeowner.

Although I didn’t take into account the amount of interest on the initial capital cost, that amount would probably be more than offset by the rise in energy cost over the next thirty years. With the proper care and maintenance you could have your own electrical generator in the home for at least the next 30 years. As for required maintenance, you should try to clean the panels at least two times a year, at the end of the rainy season and towards the end of summer because a 10-15% decrease in solar output can be noted when panels are dirty.

The energy cost and consumption for this analysis comes from one years worth of electric bills provided by an average homeowner. The conclusion of this analysis is that the PV solar system would pay for itself in just under 14 years and then provide, basically free power for the remaining life of the system. The payback of 14 years is a long time, but when considered over the life of the system, the remaining 16 years provide an excellent return on the investment.

Part 4 – Balance-of-System

This is Part 4 in the “PV Solar – Homeowner Analysis” series, comparing the cost of fossil fuel generated electricity used in my home with the cost of electricity generated by a theoretical photovoltaic system. In part one we went through the process of sizing a system, in part two we continued by examining how this type of system would work with the existing utility grid and in part three we covered solar panels (modules), which make up approximately 50-60% of the installed cost of the system. In this fourth segment I’ll cover the remaining elements making up a grid tied residential solar electric system.

The remaining elements of a solar electric system are called Balance-of-System (BOS) components. The major BOS elements include such items as the grid Tie inverter, the combiner box, the AC & DC disconnects, the module mounting system and the cable and wiring. There are other optional parts, but these elements make up the major BOS components of a residential grid tie system.

• Grid Tie Inverter
  The solar inverter converts the variable DC output of the photovoltaic cells into AC current. The grid tie inverter is a pure sine wave inverter that synchronizes with and feeds power back to the grid. A grid tie inverter connects directly to the utility grid without the use of batteries. One important thing to consider when purchasing the inverter is the efficiency. Even though you may have purchased solar panels with a very high PTC wattage rating, a low efficiency rated inverter can negatively effect the system’s performance.




• Combiner Box
  The Combiner box is where all the wires from the Solar Panels are routed and allows multiple solar panels to be combined in parallel. A combiner box is basically an electrical box that connects the input panels in parallel to produce one circuit. The breakers and fuses for the solar panels will also be placed here as well. The box is typically made to go outside near the solar panels.




• AC & DC Disconnects
  The DC disconnect is used to isolate the PV array from the inverter and an AC disconnect is used to isolate the inverter from the AC panel or load center. The AC disconnect (switch), installed on the inverter output, allows the system to be disconnected from the grid by utility personnel.




• Module Mounting System
  After you’ve establish the number of panels and the array arrangement, the mounting location is determined. Although module mounting systems are available for ground and roof installation, roof mount installation is the most common and cost effective method. The roof mounted panels are attached to a mounting system typically consisting of an aluminum or steel support structure which attaches the panels to the roof. If the roof space is limited, shaded or unsatisfactory for any reason, the array can be mounted on the ground. The most common type of ground mount is a wedge shaped steel structure anchored in concrete. Another type of ground mounted system is the pole mount, in which the panel array is mounted on top of a single steel pole. There are numerous variations of these installation types, including rail mounts, tilt adjustment, automatic tracking, etc.




• Cables and Wiring
  This includes the Wiring that connects the components of the PV system together. Most solar panels are now manufactured with multi-contact connectors (MC) which makes series and parallel connections a snap. The panels are wired together as needed and then routed into the combiner box paying special attention to safety and grounding requirements.

That’s it for the Balance-of-System components. In the next post I’ll go over the component and installation costs along with the Federal and State tax incentives and rebates to come up with a fixed cost for the estimated life of the system. As I stated before, this is a learning experience, so if you see any errors or omissions, please feel free to share and leave a comment.

Before I end this segment, I want to refer back to part 1 and revise the calculated system size. The last step used in determining the size was to multiply by a real-world factor of 1.2. Further study suggests that a factor of 1.3 is actually closer to a real-world situation. Due to module and inverter inefficiencies along with other power losses, the calculated system size of the 4.271 kW system is multiplied by a factor of 1.3 to give a more real-world estimate (4.27 kW x 1.3 = 5.55 kW) of 5.55 kW.


Recommended Site
Pennsylvania Solar Course
This is an online solar training course from the state of Pennsylvania designed for residential applications and specifically includes the siting, sizing and installation of solar electric (PV) and solar water heating.

Part 3 – PV Panels

As I noted in part one, the object of this analysis is to provide a simple comparison of the cost of the fossil fuel generated electricity used in my home with the cost of electricity generated by a theoretical photovoltaic electric system. In part one we went through the process of sizing a system and in part two we contiued by examing how this type of system would work with the existing utility grid. In this third segment I’ll give a brief overview of photovoltaic modules (solar panels) which make-up a major portion of the system and cost.

A solar panel or solar module (terms are interchangable) is a collection of solar cells wired together in a series/parallel configuration so as to produce a desired voltage and current. The panels are made of aluminum framed glass with the individual cells mounted to the inner surface of the top glass (tempered, low reflective, etc.) with the back of the panel protected by another sheet of glass or glass-like material.

The following ratings, warranties and certifications are provided by the manufacturer and independent testing agencies to provide information about the panels and help the consumer compare apples with apples when shopping for solar panels. Be sure to check the panels specification sheet for this information or have the dealer provide it.

Efficiency Ratings
Most panels today are manufactured using two of the most common types of cell technologies, monocrystalline silicon and polycrystalline silicon. Solar cells manufactured with the lower efficiency material (polycrystalline silicon) result in larger cells and panels than those manufactured with higher efficiency material. The higher the efficiency rating of the module, the more power you’ll get per square inch of panel surface. In other words, the higher the rating the less roof area will be required for installation.

Panel Output (Wattage) Ratings (STC Ratings vs PTC Ratings)
The individual modules are manufactured to provide a specific amount of power output. Manufacturers rate the nominal power output (wattage) using a set of standard testing conditions (STC). The STC rating is the wattage specified on the panel’s nameplate. The independent rating anency, PVUSA, provides the PTC (PVUSA Test Conditions) rating which provides a more realistic or real-world measure of a panel’s output. Because the PTC rating uses more real-world conditions than the STC rating, the PTC rating is lower. A module with a STC rating of 200-watts, for example, may have a PTC rating of only 180-watts. When determining a system’s cost, it’s important to know that the PTC rating is used by California and various other states as the basis for determining system rebates. Ratings are listed as DC (direct current) watts.

Minimum Power Ratings
This is the manufactuer’s guarantee that the panels’ actual power output, out of the box, will not fall below a specified amount. This is sometimes called minimum warranted power and negative tolerance rating. A 200-watt solar panel (STC rated) with a negative tolerance rating of 5% will only be warranted for 190-watts (Minumum warranted power) out of the box.

Certifications
IEC 61215 – This is the international design standard for crystalline silicon modules. Conforming to IEC 61215 only guarantees that a test batch of modules has passed the required tests. It is not a guarantee of a manufacturer’s quality control in production.

UL listing – In terms of solar panels, Underwriters Laboratories tests for various safety considerations. Modules that pass testing are given UL’s “UL 1703″ listing for Flat-Plate Photovoltaic Modules and Panels.

Warranties
Provide some type of guarantee against defective workmanship or materials, which covers failures or problems during a specified period of operation. The remedy may be replacement or repair of the defective product.

Provide a guarantee that the peak watts of a module will not reduce by more than a stated percentage over a certain number of years (limited power guarantee). For example, a manufacturer would warrant its modules to produce no less than 90% of their initial minimum stated power under Standard Test Conditions for a period of 10 years from the date of original purchase, and also to produce no less than 80% of their initial minimum stated power under Standard Test Conditions for a period of 25 years from the date of original purchase.

The life expectancy for newer crystalline panels is anticipated to exceed 40 years with manufacturer warranties varying anywhere from 20-35 years.

Module Pricing
Panel prices for polycrystalline and monocrystalline have continued to delcine to the $2.50/watt and $2.75/watt range respectively. Module price greatly effects the cost of the solar system, as they encompasses approximtely 50-60% of the total installed cost (pretax cost). For more details concerning panel pricing see the following article from solarbuzz.com “Solar Module Price Highlights: November 2009″.

Although the present economic conditions are not that great, this is good news for consumers who have the money to buy modules now.

Okay, we’ve covered what you should know when comparing solar modules and found out that they make up approximately 50-60% of the installed cost of the solar system. In the next post I’ll cover the remaining elements making up a grid tied solar electric system and and begin pricing the system sized in part one. As I stated before, this is a learning experience and I know I may have moved along too fast at times, so if you see any errors or omissions, please feel free to share and leave a comment.

Recommended Sites and Articles

• solarbuzz.com
• greeneconomypost.com “A Cloud Over Solar In 2009; What are the Near Term Prospects for the Solar PV Sector?”
• solar.calfinder.com “Solar Panel Ratings Breakdown”