Question

We want a solar energy system that will allow us a week of power in the event of an outage, and save on electricity each month. Suggestions?

Asked by Beth
High Rolls Mountain Park, NM

We are building a new home and we want to install solar panels with a battery backup, in addition to electric. What are your suggestions for the best system for a 2000 sq. ft. home, which includes a power backup, taking into account the cost of the system and installation? We are also interested in a solar water heater and any other solar devices we can add during construction that make sense.

Answer

Beth,

I commend you for considering the implementation of renewable energy systems in your new home design.

If you have not done so already, choose an architect knowledgeable in solar design or find a renewable energy consultant to help with your project. It is with their help that you can properly design the home to be at a peak of performance with the minimum power consumption.

Begin by answering the two following questions:

  1. Do you wish for the whole house to be able to function on solar power during this period of time?
  2. Do you wish that only a portion of the home such as critical loads (furnace, refrigerator, specific lights…) operate during this period of time?

Which one of the questions you answer “yes” to will greatly impact the system's design, size of the photovoltaic array, battery bank and controls.

Figuring out what's essential

To help in answering the questions above, here is a small exercise you can perform.

  • Begin by creating a chart for each room in your house. On this chart create rows and columns listing the days of the week and a 12-hour period of time (example: from the time you wake up to the time you go to bed each day) with room for notes.
  • Track your behavior for this period of time, noting the function performed, appliances used, lights turned on and how long they were on for, for each room.
  • Review the charts and determine which rooms require lighting for each day, which appliances are crucial to maintain your routine, and what items you can eliminate during this time of emergency.

You will be surprised by your results and will find that you may only need to have certain critical loads tied to the solar power system. However, if you find that you wish to power the complete home, you may wish to look into a hybrid system utilizing a combination of solar, wind, and natural gas generator.

Other items to consider

You should also consider these issues in your new home's design:

  • daylighting
  • solar hot water
  • photovoltaic system

Natural daylighting

Daylighting is key to a good energy performance as well as developing a space that will allow you and your family to function comfortably without the use of electrical lighting.

There are two types of daylighting to consider:

  1. Toplighting: Through the incorporation of skylights, solar tubes, light shafts, etc., toplighting allows added natural daylight into areas that usually would not receive natural light, such as internal hallways, closets and bathrooms. Toplighting also allows even distribution of diffused light in rooms that are too large for sidelighting alone to be implemented.
  2. Sidelighting: Through the incorporation of windows, glazing, transoms, light shelves and sun shades, even levels of natural daylight can be transmitted into each room. This method, however, is more complex than toplighting. To properly design sidelighting, you must carefully take into consideration location, glazing type/transmittance, energy performance, depth of room, and room function.

It is important to remember the difference between sunlight and daylight.

  • Sunlight is associated with excessive heat gain and light levels which could be uncomfortable.
  • Daylighting is the control of sunlight by the incorporation of diffusers, glazing films, and light shelves.

Solar hot water

Regardless of whether you are intending to use solar for backup or everyday use, solar water heating is simple and cost-effective. The key to designing an efficient system is the amount of storage available.

Types of solar hot water collectors include flat plate and evacuated tube collectors.

Flat plate collectors: Most commonly used in the industry, these collectors can be either passive or active.

  • Passive flat plate collectors contain all of the heated water within its tubes on the roof. A typical collector can hold 50-100 gallons of water at a time and distributes the heated water when it is called for, while replacing the heated water with cold water to be heated in the collector. In periods without sun, this collector will require the use of a backup whole-house water heater.
  • An active flat plate collector is tied to a hot water storage tank in the home. With the use of thermostat controls and circulation pump, water from the storage tank is pumped through the collector during periods of sunlight, heating and storing it for use. This type of system can sustain longer periods without sunlight and may require the incorporation of a small point of source flow through the water heater.

Evacuated tube collectors: These are used primarily in colder climates where lower sunlight levels are achieved. Evacuated tube collectors do not contain any water; instead they use a sealed system within a double-walled, insulated glass tube. A glycol solution is pumped through a manifold located at the top of the tubes and heated and returned to a storage tank heat exchanger where that heat from the glycol is transferred to the hot water intended for use. A properly designed system can be used for domestic hot water and also the home's heating season needs, and is +/-95% efficient.

Photovoltaic system

Because of the complexity in design, listed below are the considerations which must be addressed, assessed, and taken into consideration during the design of the home to ensure an efficient operating system.

Solar orientation: This is the easiest to determine and key to the performance of any photovoltaic system.

  • Do you have a southern-facing roof slope? If not, a ground-mounted system will have to be installed. In many instances, city ordinances prohibit this type of installation. Also, it increases the overall cost substantially due to necessary installation of foundations and mounting racks.
  • Obstruction: What is the nearest tall object which could cast shadows on the solar system? Look at trees near the home as well as taller homes. To help determine whether or not they will cast a shadow, refer to solar path and angle charts for your specific latitude to help determine the length of shadow for each object every month.
  • Insulation: Determine average duration of sunlight. For New Mexico, you are between 6.0–6.8 hours of sunlight; this is really good. What this means is that, for example, a 4 KW photovoltaic system will generate approximately 24–27.2 KWh gross of electricity.

Building envelope: Is your new home's envelope properly insulated? With the aid of a professional, perform an energy audit of the home's design. This will determine both heat loss during the heating season and heat gain during the cooling season. This analysis will pinpoint areas of inefficiency. If your newly designed home is inefficient, installing photovoltaic panels is like placing a Band-aid on a sieve to prevent it from draining liquid.

Energy consumption: Review all energy bills for an entire year. This will give you a full understanding of how much energy is required to operate your current home. Also look at the existing systems to determine their efficiency. When installing equipment in your new home, look at purchasing Energy Star rated products to reduce the overall load requirements.

State, federal and power supplier incentives: Visit www.dsire.org. This is a U.S. Dept. of Energy database for state and federal government incentives. The list is extensive but worth the research.

  • For example, on a current project of mine here in Michigan, my client will receive 50% of the installation and material cost from the power supplier plus the ability to sell back any power produced and not consumed.
  • Also, he will receive a 30% federal tax grant plus additional state tax incentives and exemptions.
  • These incentives will reduce his return on investment for a 4 kW system to just about 7 years with an approximate gross savings/earnings of $40,000 over the course of 25 years.

Type of photovoltaic panels: There are two predominant types of photovoltaic panels available, multi-crystalline and amorphous (thin film).

  • Multi-crystalline: These panels work best under full-sun conditions and cooler temperatures. They are required to be installed on a rack system to allow for airflow below the panels to reduce their overall heat gain and maximize their performance. These panels do not lend themselves easily to Building Integrated design.
  • Amorphous: This is a thin film panel which is flexible and virtually indestructible. These panels are less efficient then the multi-crystalline panels, but produce a greater amount of power over a longer duration of time. They work well in low sunlight and high heat conditions, and produce electricity from ambient light as well as direct sunlight. They can be installed directly to standing seam metal roof panels and membrane roof systems. They come in many configurations, including shingles and strips.

Battery bank: There are many types of batteries available on the market which can be utilized for your system and must be sized properly for the amount of demand over a period of time.

  • To incorporate batteries will also require additional equipment such as charge controllers, disconnects and construction.
  • If it is not sized properly, you will find yourself without power quickly or absorbing a large upfront cost for an oversized battery bank.
  • On average, batteries cost $400-600 per battery and will last an average of 5 years if maintained per the manufacturer's specifications.

Due to the cost and maintenance associated with batteries, I would highly recommend utilizing the batteries for only critical loads.

 

For more information:

Read Harold Remlinger's Q&A "If I install solar panels, will they support an all-electric house (4 BR 3 BA)?"

Tagged In: solar panels, daylighting

Do you have a question about greening your home? GreenHomeGuide invites you to Ask A Pro. Let our network of experienced green building professionals – architects, designers, contractors, electricians, energy experts, landscapers, tile & stone specialists, and more – help you find the right solution.