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Question:

Can you give me information on combination systems of solar and wind power? I would like to take advantage of the plentiful sun and wind where I live — to either power the whole house, or to use as back up during power outages.

Answer:

The following information should be of help in your decision making.

Back-Up Power

The purpose of back-up power for a home is to provide short-term electrical power during relatively brief interruptions of power from your utility. Such a system is sized to provide for your basic safety and comfort needs, and if money allows, a few luxuries as well. A careful self-assessment of needs, wants and luxuries is in order and they will not be identical for everyone. Someone whose medical needs depend on specific equipment needs to plan for it. Television or computer games could be seen as an unnecessary luxury to some, while others may consider them essential. No electrical heating device should be used (such as electric blankets, space heaters, or hair dryers) as they are extremely inefficient uses of self-generated power.

As you already have natural gas for heating, cooking and hot water, you are in a good position to operate on back-up power. If you have a furnace fan, you still need electricity for heat and may want to consider a back-up appliance such as a freestanding stove that requires none. Know what is required of the thermostats involved during power outages.

Refrigeration is usually the largest energy consumer after heat and hot water. You were wise to purchase an Energy Star refrigerator—it will save a lot of energy over its lifetime. Are you aware there are many other ENERGY STAR products such as televisions and computers? Refrigerators and freezers are often the main loads required of a back-up generator. Keep them closed during power outages, use a cooler with ice for frequently used items, and if the unit is in a heated space add blankets around it for insulation to slow heat gain. Do not cover the condenser or compressor while running. Freezers must stay below freezing, 0 degrees Fahrenheit is better; refrigerators should stay below 40 degrees Fahrenheit. A thermometer that can be read without opening the cabinet can be useful to monitor when you need to run the compressor. I have an indoor/outdoor model that also shows maximum and minimum temperatures.

Though you currently are not pumping your own water, if you decide to do it, determine what equipment will be used so that your back-up power system can handle it. Water pumping loads are large and may be the most critical during emergencies. In the meanwhile you may want to consider a water storage tank at your site, which is also an asset if you if you move to renewable power generation full time. With a large storage tank it may be possible to use a small DC pump and wind or solar energy to keep it full.

Lighting, communications, and battery re-charging are the most common remaining needs. Flashlights and battery-powered lanterns provide very safe lighting, and rechargeable batteries can reduce the amount of solid waste they generate. If you are using a generator for power, it is very inefficient to operate for only a couple of lights. If you are using your generator for lighting while running other loads, compact fluorescent lamps using 1/4 to 1/3 the energy of incandescent lamps provide high quality light and enable you to light more space than standard light bulbs. They are an excellent choice in any high use application. In the case of outdoor lighting be sure the model you choose can operate in the conditions of the location.

Other lighting options with higher fire hazard and potential indoor air quality concerns are candles and kerosene lamps which are adequate for general visibility, but not critical seeing such as reading. It is always good to have smoke detectors, carbon monoxide detectors and fire extinguishers in your home. Being vigilant about fire safety is always important, but even more so when you increase the sources of open flame. If your detectors are electrical, battery-operated ones will provide some protection during times when your risks may be greater.

Cordless telephones—and sometimes corded ones—are dependent on electricity, and cellular phones require electricity for recharging. A battery-operated radio should be available for news and weather information at least. Battery-powered tape and CD players, as well as televisions, use considerably more energy and will go through batteries in a hurry. If the generator is on for other loads, and there is enough power, this is a good time to spare the batteries—besides it may help drown out the sound of the generator. There are also solar and hand-crank powered radios available. Solar battery chargers are available, but the sun doesn't always cooperate when you need it most, so an electrical charger may be useful too. Another option is to use a 12-volt battery and a DC compact fluorescent lamp, radio, or television; but again once the battery is drained it must have a means to recharge. Many generators have this option.

Where Does The Power Come From?

The most common back-up power supply is from a generator. Most residential units are gasoline powered, though propane is gaining popularity. The Backwoods Solar website contains information on selecting a generator.

As generators represent a substantial investment, choose one for durability and ease of use. Electric starters are nice, and very important to people with limited physical capabilities (broken arms and pulled muscles never occur at a convenient time). They can be set up to come on automatically at time of power failure. Consider purchasing one a size larger than you calculate you'll need. It will allow you extra power if you underestimated your need, if your needed load increases, or to provide more flexibility in using equipment, especially if the equipment is motorized. Generators do not continuously produce the wattage on their label (which is their surge power), so it is not feasible to purchase a 4000-watt model and put a 4000-watt load on it regularly, and especially not if there are motors involved. Running it at about 80% capacity gives you some protection against overloading or incurring a low-voltage situation. If you are looking at the generator as part of an off-grid renewable energy system for long term use, you want to choose carefully for durability, maintenance, fuel availability and storage considerations. At this point diesel fuel may become more attractive. The Consumer Reports Generator buying guide includes an appliance wattage calculator.

The other source of back-up power is a set of batteries and an inverter to convert them to AC power. You can keep such a system trickle charged while on utility power and run small loads (a couple of compact fluorescents lamps and radio or television) quite awhile before recharging. The size of the battery bank and the loads you put on it determine how long before recharging is necessary. Recharging may be accomplished with a generator, solar photovoltaic system, wind, or micro-hydropower. Advantages of this system are that, while it is operating, it is silent except for appliances you are using, and that it only uses the energy required (rather than burning a fixed amount of fuel per hour as a generator will). This power source is run through a sub-panel of the main power panel for your house, so you use the circuits as you normally would, within the limitations of the system. It will come on automatically when power is interrupted. Pre-packaged units or components may be assembled for these systems.

In the past little has been available to guide homeowners in making a safe installation. Though it is possible to just plug extension cords into a generator for short term, infrequent use, it presents some hazards such as tripping, drafts from doors or windows cracked open for passage of the cords, and fire hazards from undersized, overheated cords. Too often users have rigged up an extension cord with a plug at each end so they can energize to house wiring and use the normal outlets. The dangers of that idea are numerous as you can well imagine. When one end is plugged in, the exposed male prongs on the other can be hot and endanger the homeowner. Worse yet, if the homeowner fails to shut off the correct breaker to isolate the circuit, the generator can backfeed into the utility system and electrocute utility personnel who are repairing the outage. Many utilities sponsor clinics on safe generator use specifically to avoid these problems, and have generator safety information on their websites.

The key component of a proper installation is a transfer switch. This is just a three-position double throw switch with the circuit to be served connected to a terminal that toggles between the generator terminal and the utility terminal. The load circuits can be switched to either utility power, generator power, or "off" (neither). With this arrangement, it is virtually impossible to backfeed power onto the line or simultaneously try to feed the same point with both generator and utility power. A qualified electrician should do installation of the transfer equipment. Check local regulations regarding inspections.

Since only the circuits in this transfer switch will be useable from the back-up power, it is important to size it carefully if you expect to later run your whole house on the system. Be sure to include any hard-wired appliances or those which are difficult to reach such as refrigerators. Keep in mind that, just because you may have many circuits connected to the transfer panel or switch, it does not guarantee you have enough power to use them all at the same time, especially if motorized equipment is involved. Keep several copies of the audit list you made of all your equipment handy so you can figure out before you trip the circuit breaker how much load you are drawing from your system (see discussion of self-audit below under How Much Power Do I Need? It is important that all occupants understand the limitations of the system and do not overload it, and turn off whatever is not being used.

Renewable Energy

Renewable energy offers us the opportunity to use sustainable resources and reduce our dependence on fossil fuels. The investment in the system is too large to be considered as only a back-up power system and should instead be viewed as your main power system, or an integral, supplemental component of it, even if you are connected to the utility grid. The cost of installing solar, wind, or micro-hydropower systems is similar to paying 20 years worth of electric bills at one time. Since you are already connected to the grid you should check with your local utility to find out any policies they may have regarding net metering, a process that allows you to "sell" any surplus power you produce back to the utility, in effect making your meter run backwards. If they participate in this they will have regulations about the power quality and installation of your system being connected to theirs, information you need before choosing your inverter.

How Much Power Do I Need?

Energy efficiency is a natural companion to living more gently on the earth. It also allows you to install a smaller, less expensive power system than otherwise. Whether you want to reduce your current electric bill or put in your own off-grid power system, the first step is to look at and evaluate your current use and then reduce, reduce, reduce! Enclosed are several lists of the energy consumed by various appliances and tools.

Conduct a self-audit to determine your energy needs. Keep in mind the difference between needs (enough heat to keep the water running) versus wants and luxuries (like wearing shorts and t-shirts in the house all winter). An emergency system cares for needs; a full-time off-grid system covers wants, and luxuries should be carefully evaluated. To really know what your equipment uses, you need to measure the ampere (amps) draw and multiply by the voltage to obtain watts. Nameplate information is the average for the model of appliances, not any particular unit. The total you use at any one time determines how much power your system must produce, and the total energy you use in a day determines how many watts you must have available from your production and storage system. Both the peak draw and operating consumption of motorized appliances should be measured. While doing your audit of electric appliance use, carefully consider which appliances need to operate at the same time, how often and how long. Both watts and amps are important--remember, volts times amps equals watts. If voltage drops, amps increase causing your equipment to run hotter than designed for and damage may result.

Heating and refrigeration are the most energy intensive uses of electricity. Do not plan to heat your home, produce hot water, use a dryer, or cook with electricity you produce yourself. I am unaware of anyone in the renewables industry who considers that practical. At a cost of about $6.00 per watt for solar panels, a small 1500 watt electric heater or even hair dryer would require a $9000 investment! Modern homes are filled with "ghost loads", electrical use by appliances that don't appear to be on. Instant-on televisions are drawing some power even though you've turned them off. Phone answering machines use power whether or not they are actively answering the phone. The small transformers that adapt battery equipment to AC power are using energy and producing heat whenever they are plugged in. These loads add up and, if you are generating your own power, provide a constant and unnecessary draw on your storage. Load management is an important part of energy efficiency, and critical when producing your own power.

What Types Of Systems Are Available?

Often a hybrid system will provide you with the most reliable energy source. In the right location solar, wind, and back-up generators can be combined to provide a constant source of energy. When conditions aren't right for one system, they often are for another. Having a system you understand and can maintain is very important when you are isolated from assistance.

Photovoltaics (PV)

Photovoltaic systems use the sun to produce electricity. Unless you can store the energy in batteries, you will only have power while the sun shines. Solar systems with the proper inverter can be connected to the utility grid for back-up power as an alternative to batteries. When you are producing more power than you need, some utilities will buy the excess through a process called net metering. The drawback is if the utility is not generating power, you have no back-up power. Another option is to charge up a battery bank first, then sell the surplus power to the grid.

Backwoods Solar has a lot of information as well as products for sale. Home Power's website is excellent as well and their magazine should be added to your subscription list if you plan to go off the grid. The Solar Living Sourcebook is an excellent book for planning and using your system.

Once you have analyzed and reduced your power requirements, you can shop for an appropriately sized system. Inverters are expensive, but essential for AC power. Running off battery power through an inverter is more efficient than running off a generator, drawing only what you need, and quieter too. If you foresee future expansion, keep that in mind. Batteries should all be purchased and installed at once. If not, fresh batteries added later will have their power drawn as the batteries equalize. More batteries mean more available power or power available longer before recharging.

Wind

Windpower requires a turbine mounted far enough above the ground to avoid turbulence caused by obstructions. They require some maintenance, potentially under less-than-ideal weather conditions. They also involve moving parts. Keep this in mind if you want to use this resource. Average windspeed for a day is not a good indication that you have adequate wind. A gust followed by hours of very light or no wind will give a false impression of what is available. A steady wind 12-25 mph is a good wind resource. Ideally you would have one year's worth of windspeed data at your site before investing in a system. If the trees in your area exhibit flagging (more branches and a bit of a lean in one direction) you probably have a good resource. When shopping for systems, pay attention to how much wind is needed before generation occurs and what happens during excessively high winds.

Wind-related websites to check out are:

Also, Paul Gipe writes excellent books on the topic, like Wind Power for Home & Business.

Generators

Generators serve every day to provide the only electric power at construction sites and remote facilities, and stand-by at many other facilities as emergency back-up power. They are common in recreational uses as well and come in many sizes. They burn gasoline, natural gas, propane, or diesel fuel, each of which has unique storage considerations. They may produce single or three-phase power. Many, but not all, have a battery charger, which is essential if you are using it to back-up a renewable system's battery bank. If weather conditions conspire against you, you can still charge your batteries if you have fuel. Check with your insurance company and fire marshal regarding approved fuel storage. It is possible to void your fire insurance with improper storage.

Large semi-portable generators are commonly used on job sites and farms. Small, portable generators are popular for recreational and emergency household applications.

Tips For Using Generators:
  • Portable generators may be attractive to thieves and their noisy operation will advertise their presence. Consider the noise rating when locating your generator, so it is not too disruptive for you or your neighbors. If you use it only to charge batteries and then run off the stored power, the noisy times can be chosen.

  • Generators are most efficient running near capacity, so know your loads and manage them accordingly. You do not want a large generator running to power a couple of lights.

  • Be sure the generator will run your critical equipment such as well pumps; both wattage and peak amperage draw are important.

  • Generators need weather protection and excellent ventilation. Do not operate them inside attached garages, especially if your furnace is also in that area or you may end up circulating exhaust gas through the dwelling.

  • Electric starts, with or without remote operation, are a big plus as physical condition and strength of the operator could be a factor in pulling a starter cord.

  • Protect your equipment with properly sized fuses or circuit breakers and have some spares on hand.

  • Generators should usually not run continuously at more than 80% of their rated capacity. Be sure you have a meter to indicate how much of a load you are drawing at any time to avoid damaging the generator or your equipment. It can be tempting to just keep turning things on because you have open plugs, or because incorrect assumptions are made about how much power an appliance will use.

  • Motorized equipment uses extra power to start up, and only one should be started at a time. Whatever the source of homemade power, expect to be intimately involved in monitoring and maintaining the system.

Alternatives To Using Electricity

There are several alternatives to using electricity for common household task-heating, water heating, refrigeration, cooking, lighting, and laundry. The following sections discuss some of these alternatives.

Heating

Natural gas or propane furnaces with high efficiencies are available. Since you have natural gas heat already, your biggest problem is solved. They do require some electrical energy to power the fans. Check to see that you can light the appliance in a power outage and know how the thermostat will operate. Programmable thermostats are excellent devices for controlling the climate in the house and save energy by reducing the heat whenever the house is scheduled to be unoccupied.

Some of the freestanding heating stoves have fans optional—they work fine without them, but fans improve distribution of heat. Cracking open a window at the opposite end of the house will draw the heat toward it.

Homes can be designed or modified to capture solar heat in cool months while avoiding overheating in the summer months. South wall greenhouses can capture heat, even in winter on sunny days and also provide gardening space. Using the "free" heat from passive solar heating strategies will reduce the demand on your heating system.

Water Heating

It's great that you already have natural gas water heating. Hot water tanks should be well insulated. The bigger the temperature difference between the contents of the hot water tank and the surrounding air, the faster the rate of heat loss. Setting the temperature down from say 130 degrees to 120 degrees will save substantial energy, especially if the unit is operating a lot, and provide you with useful hot water.

You may want to supplement your system with a solar thermal system. Solar hot water heating systems may produce all your hot water during warm weather and act as a pre-heating system the rest of the year. Some systems require electric power for circulating the water, but thermosiphon systems do not. Freeze protection is a must if the system is operating in cold weather, and adds to the cost of the system. Some people just use a simple system in warm weather to reduce their hot water expense, and drain the system during cold weather. The DOE Energy Efficiency & Renewable Energy Office Solar Water Heaters website provides basic information on the functioning, selection, equipment, and installation of the systems .

Refrigeration

The refrigerator is the single biggest power consumer in most households not using electricity for heat, hot-water, or cooking. Even a new Energy Star refrigerator is a very large load for homemade power to generate. More expensive, but energy efficient, alternatives would be the Sunfrost or Vestfrost products that come in AC or DC and use closer to 300 kWh a year. Other refrigeration options include propane units. If you want to be fully independent from the utility grid at some point, you probably need to consider one of these options.

Cooking

Again you are well prepared with your gas, pilot-light model stove. Many modern gas and propane stoves require electricity to light burners and keep the oven operating.

A microwave takes a lot of power, but for a short time. Pressure cookers shorten cooking times with no additional energy requirements. Woks have a small surface area of burner contact so require only a small flame, which saves some energy. Lids on any pot conserve energy.

Lighting

Electric lamps provide instant, high-quality, bright light. The standard incandescent lamp is actually a heater, producing light as a byproduct representing 10% of the total energy used. They are very inexpensive, last about 1000 hours each, and use a lot of energy for what they give. If you calculate the life-cycle cost of these lamps against the substantially more expensive compact fluorescent lamps (cfl) you will find the cfls greatly outperform the incandescents. If you are generating your own power, you can light a whole house with cfl's, whereas only a room or two with equivalent light (lumens) could be done with incandescent lamps. The wattage of a cfl will be about 1/4 to 1/3 of an equivalent incandescent.

When shopping for cfl lamps you want to compare lumens to those on the incandescent lamp you are replacing. Also be aware of the color quality: high color-rendering lamps approximate incandescent lamps, while lower color-rendering lamps can be unacceptably bluish or greenish. The size and shape will determine which fixtures they will fit into. Best performance is often realized from a fixture designed for a compact fluorescent lamp. Compact fluorescents are also available for 12-volt DC systems.

There are affordably priced compact fluorescent torchiere lamps using 30-72 watts. They provide great area lighting, as well as reading light, at low wattage, cool temperatures, and have a long life as well.

Of course, when available, daylight is wonderful. Even overcast days produce large amounts of light. It is important to use windows that insulate, and not to overheat in the summer months. Skylights and the smaller, but very effective tubular daylighting devices such as solartubes, also bring in tremendous amounts of light during daylight hours with a much smaller hole through your roof.

Laundry

For daily, long term use, a frontloading washing machine like those in the WashWise program saves energy by using substantially less water than traditional units (less to pump, less to heat). Dryers should be natural gas powered, and of course the "solar dryers" (known as clotheslines and clothes racks) can be used whenever possible.

Other Appliances

The ENERGY STAR website lists many household appliances that will save you substantial energy over standard products; however, products developed for off-grid applications are not likely to be found there.

In summary, the first step in sizing a back-up power system or a renewable energy system is to carefully look at your current energy needs. Reduce your use and convert to more efficient forms of energy wherever possible. Determine your potential for power production and choose your products carefully. Beyond short term emergency products, go for long-term sustainability. Understand your system and manage your load wisely. Be aware of safety issues and environmental impacts of the technologies you choose. Become familiar with using your system and alternative practices such as new cooking styles. Keep generator batteries charged and fuel fresh.

Hybrid Wind and Solar Systems Resources

  1. U.S. DOE. Hybrid Wind and Solar Systems.

  2. O. A. Soysal, H. S. Soysal. A Residential Example of Hybrid Wind-Solar Energy System: WISE.

  3. Mohammad Shadmand and others. Implementation of Photovoltaic-Wind Hybrid Systems with Battery Back-up in the State of Texas.

Additional Resources

  1. U.S. DOE. Planning for Home Renewable Energy Systems.

  2. U.S. DOE. Energy Efficiency & Renewable Energy Office. Buying and Making Electricity .

  3. Puget Sound Energy. Standby or Backup Generators..

  4. "Electric Bill Too High?," Backwoods Solar Electric Systems.

  5. "Combustion Pollutants in your Home," California Environmental Protection Agency, Air Resources Board

  6. Carbon Monoxide Safety Tips. National Fire Protection Association.
Topic: Onsite Power--Storage
Topic: Onsite Power--Standby Generators
Topic: Renewables--Photovoltaic
Topic: Renewables--Wind
Topic: Environment--Sustainability
Sector: Residential
Sector: Utility
Content Type: Q&A
Keywords: green power, photovoltaics, renewables, wind, appliances, generators, backup power, back-up power
ID:  912