How to Size Solar Hot Water Systems

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When considering how to size solar hot water systems there are many factors that come into play. The two most important are climate and your family's water consumption.
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“The Homeowner’s Guide to Renewable Energy,” by Dan Chiras, helps readers understand the sometimes confusing and broad array of renewable energy options available and presents the ways energy bills can be slashed while maintaining, or improving, the comfort of homes.

Homeowners scramble to cut utility bills and find alternatives to fossil fuels as the prices of oil and natural gas continue to rise. The Homeowner’s Guide to Renewable Energy (New Society Publishers, 2011), by Dan Chiras, offers excellent ways to improve energy efficiency by making the switch from fossil fuels to clean, affordable, renewable energy. The following excerpt examines one practical option for utilizing solar energy and details how to size solar hot water systems for any climate and family size.

You can purchase this book from the MOTHER EARTH NEWS store:The Homeowner’s Guide to Renewable Energy.

Things to Consider: Sizing Solar Hot Water Systems

Sizing a DSHW system is pretty straightforward process. Before you get started, however, it is important to be sure you have a good solar site. That is, you need to be sure you can position solar collectors so that they’re exposed to bright sunlight from 9 a.m. to 3 p.m. every day of the year. Fortunately, most homes, and even many apartments in cities, have good solar access somewhere on the site. Roofs are often free from obstructions that can shade solar panels.

If you have a good site, your next task is to scour your home for ways to make it more efficient with respect to hot water use. Remember: efficiency is the first rule of renewable energy system design! Make your home as efficient as possible, then size the system.

In his excellent article, “Solar Hot Water: A Primer,” published in Home Power magazine Issue 84, Ken Olson recommends the following steps to make your home more efficient:

1. Turn down the thermostat on your water heater to 120° to 125°F (48° to 51°C). “Many water heaters,” says Olson, “are set between 140° and 180° F (60° and 82°C),” but much lower temperatures are just fine.

2. Wrap the water heater with an insulated water heater blanket.

3. Fix drips in faucets in the kitchen and bathrooms to prevent the waste of hot water.

4. Install water-efficient showerheads and faucet aerators to reduce hot water use.

5. Insulate hot water pipes in unconditioned space.

You can also enact various water conservation measures, like taking shorter showers, washing dishes by hand and not leaving the hot water running, and washing clothes in cold water.

How to Size Solar Hot Water Systems

Once water and energy efficiency and conservation measures are in place, it is time to size your solar hot water system. The size of a system depends on many factors, the most important of which are (1) your climate — how hot, cold or sunny it is; and (2) your family’s water consumption. It also depends, in part, on your solar exposure. Will your system have unobstructed access to the sun from at least 9 a.m. to 3 p.m. each day? If not, you’ll need a larger system.

In the United States, most families consume 15 to 30 gallons of hot water per person per day for showers, baths, washing clothes, and dishwashing. By conserving water, you can easily slash daily water consumption to the low end of the range, about 15 to 20 gallons per day. Knowing a family’s daily water consumption, designers next turn their attention to daily storage capacity. If, for example, a family of three consumes 20 gallons per day per person, they’ll need 60 gallons of hot water storage to meet their needs. For most households of four, Olson recommends an 80-gallon hot water tank, based on daily water use of 20 gallons per person. Once you’ve determined storage capacity, you turn your attention to solar collectors. You will need to determine the number of square feet of solar collectors for your application.

Once again, this process is pretty straightforward and highly dependent on solar availability and local climate. Generally, the sunnier and warmer the climate, the fewer square feet of collector you’ll need. The cloudier and cooler your region, the more square feet of collector you’ll need.

As a general rule, say the folks at AAA Solar in Albuquerque, New Mexico, in the sunniest locations, like the desert Southwest and Florida, you’ll need about one square foot of collector for two gallons of tank capacity. Thus, for an 80-gallon tank, you’ll need 40 square feet of collector. (A single 4- x 8-foot collector provides 32 square feet of collector surface.) 

In the Southeast and the mountain states, which are a little less sunny and, in the case of the mountain states, also a bit cooler, you will need one square foot of collector for 1.5 gallons of tank capacity. For an 80-gallon tank, then, you will need 53 square feet of collector. In the Midwest and Atlantic states, you will need one square foot of collector per gallon of tank capacity. An 80-gallon tank will therefore require 80 square feet of collector. In New England and the Northeast, which are the least sunny areas and pretty cold in the winter, you need one square foot per 0.75 gallons of tank capacity. An 80-gallon tank will then require 107 square feet of collector.

Solar systems designed to these general guidelines will provide about 100 percent of your domestic hot water needs in the summer, and about 40 percent in the winter, says Olson. If you want to obtain more hot water in the winter, you will need a larger system. But bear in mind that you will have a huge surplus in the summer.

Designers use these general guidelines to determine how many flat plate collectors are needed for a solar hot water system. Because all solar collectors differ with respect to their efficiency, be sure to consult with a local vendor or your supplier before you order collectors — if you are going to install the system yourself (this is not a project for individuals with little or no experience in plumbing and building). Remember, if you undersize your system, you can always add another collector later, if you have room for one.

More from The Homeowner’s Guide to Renewable Energy

What Is Renewable Energy and Why Should We Use It?
What Is Passive Cooling and What Can You Do to Make It Work for You?
Using Attached Sunspaces in Passive Solar Design

This excerpt has been reprinted with permission from The Homeowner’s Guide to Renewable Energy: Achieving Energy Independence Through Solar, Wind, Biomass and Hydropower by Dan Chiras and published by New Society Publishers, 2011. Purchase this book from our store: The Homeowner’s Guide to Renewable Energy.

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