Solar electric systems, also known as photovoltaic (PV) systems, convert sunlight into electricity. Because they are made up of individual modules, PV systems can be designed to meet most electrical requirements. The size of a residential PV system is expressed in terms of kilowatts (kW) of power, and the electricity produced by a PV system is expressed in kilowatt hours (kWh) of energy. Systems are said to be “grid connected” when they remain plugged into the local utility. Grid-connected PV systems may have a battery back-up system to provide energy in case power from the utility is not available, but most do not. Grid-connected systems generally rely on their utility to provide power at night. “Off-grid” PV systems are stand-alone, use batteries for storing electricity, and are not connected to the utility.
If you are considering solar energy, include these important steps in your plans:
Install a solar water heater first
Heating water is one of the biggest uses of electricity for many households in Hawaii. Installing a solar water heating system is a smart first step that could reduce your monthly electric usage by up to one-third. 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, www.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.
Conserve and use energy more efficiently
A PV system is a substantial investment so it is important to reduce your household’s electrical consumption as much as possible with energy-efficient appliances and other energy-saving improvements, especially if you are considering a PV system sized to produce all of the electricity that you need. Becoming more energy efficient will reduce the size and cost of the PV system and allow your PV system to meet a greater percentage of your electricity needs.
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Do you still have one or more old refrigerators that could be removed or replaced with an Energy Star rated model?
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Are you using energy-efficient light fixtures and CFL or LED bulbs where possible?
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Instead of using an air conditioner, do you cool your home with shade from trees, a solar attic fan, window awnings or solar curtains?
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If you have a swimming pool, check the condition and age of your pool pump and consider installing a new, energy efficient model.
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Where are phantom loads and can they be eliminated?
Looking for more ways to reduce the amount of energy you use and save money? In Hawaiian Electric’s Energy Saving Toolkit, you'll find links to our publications, calculators, and online home energy audit, or call 543-7511 to request copies of the publications.
For a handy guide, see our Going Solar PV flyer.
Solar Basics:
Solar energy can be captured in various ways:
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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.
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Solar Electric is the conversion of sunlight into electric energy. Photovoltaic (PV) cells, usually made of silicon, are used in a wide range of applications. Single cells can be used to charge calculator and watch batteries. To provide more power, such as for a home or business, cells are grouped together to form a PV module. Several modules then can be grouped together to form panels, and then a number of panels can be put together to form a solar array. Arrays can power single homes or businesses. The number of modules in an array and the size of the array depends on the amount of energy the PV system is designed to produce. Large solar power plants can cover many acres. For a simple explanation, see our PV Basics brochure.
The main benefits of solar energy are:
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Solar energy systems do not produce air pollutants or carbon-dioxide.
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When located on buildings, they have minimal impact on the environment.
Two limitations of solar energy are:
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The amount of sunlight that arrives at the Earth's surface is not constant. It varies depending on location, time of day, time of year, and weather conditions.
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Because the sun doesn't deliver that much energy to any one place at any one time, a large surface area is required to collect the energy.
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.
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.
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.
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.
For more information on how solar energy works, visit:
