Going Solar

Solar Water Heating

A Smart First Step

Solar water heating (SWH) is an easy way to "Go Solar" and can reduce your electric bill by up to 35% depending on the amount of hot water that you currently use.  Solar Thermal (the sun's heat) can heat water for homes and businesses. Solar water heating (SWH) costs more to purchase than conventional electric water heaters, but the operating cost is lower because the sun provides most of the energy to heat the water and this energy can be easily stored as hot water in the tank. Some electricity may still be used to circulate the water and provide backup heat during long periods of cloudy weather.

A rebate from Hawaii Energy in addition to state and federal tax credits can help offset much of the installation cost. Visit Hawaii Energy's website, https://hawaiienergy.com for a list of the contractors who participate in the solar water heater rebate program. If you find that your home is not well suited for a solar water heater, consider upgrading to a more efficient conventional electric water heater or an electric heat pump. Rebates for these may also be available from Hawaii Energy.


How Does it Work?

There are two basic types SWH systems, passive and active. In both, water is heated in a solar collector which is usually a flat, insulated box with a glass plate on top and a metal plate at the bottom with copper pipe inside. The efficiency of both systems is about the same and each has its advantages and disadvantages.

Passive
In a passive SWH system, a horizontal tank is installed above the collector and hot water flows by convection up from the solar collector to the tank. Cooler water at the bottom of the tank flows back to the bottom of the collector. No pump is required in a passive system. The tank also contains an electric backup heating element to heat water either on a timed schedule (early in the morning, for example) or when a thermostat indicates the water has cooled to a certain temperature to make up for heat loss of the stored water during the evening or overnight. Keeping the heating element at the lowest setting possible for most applications is recommended.

Because passive solar systems do not have a circulation pump or the associated temperature activated pump switches, they are less likely to have operational problems over time, and often come with longer warranties. Another benefit is that if a power outage occurs the system will continue to heat water. Most passive systems incorporate a single large water tank on the roof, which will increase the amount of weight your roof must support. If the strength of your roof is in question, it is always recommended and often required by code to seek the assistance of a structural engineer.

solar water heating

Active
In an active SWH system, the collectors are mounted on the roof and the tank is usually located in a water heater closet inside the home or garage. Similar to a passive system, the tank also contains an electric backup heating element to heat water when solar heated water is not available. Keeping the heating element at the lowest setting possible for most applications is recommended. A temperature controlled pump circulates cold water from the bottom of the tank to the collector where it is heated and returned to the tank. During the day the pump will turn on and off to circulate the hot water from the collectors and to move cooler water up to the collector for heating.

solar water heater 2

For both passive and active SWH systems, a solar hot water contractor will size your system based on the number of people in your household and the amount of sunshine that your location receives. The amount of hot water needed will determine the size of the tank. As a rule of thumb, a four-person household will require an 80-gallon tank, 20 gallons per person. The size and number of collectors to heat the water will not only be determined by the amount of water to be heated, but also by the amount of sunshine received along with the roof tilt and orientation of the collectors. A site that is generally cloudy will require more collectors or collectors of a greater size than a sunnier site.

Hawaii Energy has a simple guide to estimate a customer's SWH net system cost after rebates and applicable tax credits. Seek the advice of a qualified tax consultant for further information regarding tax credit qualifications.


History of Solar Water Heating with the Hawaiian Electric Companies

From 1996-2009, Hawaiian Electric, Maui Electric and Hawaii Electric Light Company administered energy efficiency programs, including a very successful solar water heating rebate program. Cash rebates were offered to homeowners to help defray the installation costs of a solar water heating system. As part of the program, Hawaiian Electric maintained a list of approved solar water heating installers and conducted a 100-point inspection after the solar water heaters were installed. Customers who also qualified for State and Federal tax credits were able to dramatically lower solar water heating installation costs.

The program was one of the largest and most successful solar water heating programs in the nation. By 2009, the Hawaiian Electric companies achieved a milestone 50,000 solar water heating installations. Combined with previously installed solar systems, the total statewide in 2009 was more than 80,000, making Hawaii a national leader with an estimated one out of three single-family homes equipped with solar water heating.

The utilities' solar water heating program reduced the demand for electricity by 111,328,000 kilowatt-hours annually (the equivalent of the estimated annual electric usage of more than 14,000 households), avoided the use of 210,000 barrels of oil annually and reduced carbon dioxide emissions - a major contributor to global warming - by 116,000 tons annually.

In 2009 the solar water heating program was transitioned to a Public Benefits Fund administrator selected by the Hawaii Public Utilities Commission. Information about approved installers and rebates are now available at https://hawaiienergy.com.

Starting in 2010, a new law required all new homes to be equipped with solar water heating as original equipment, with a few limited exceptions.

Solar Energy Technologies Program - U.S. Department of Energy.