Off-grid homes powered by solar energy can operate just like normal, grid-connected homes. During sunny days, the solar cells will provide all the power needed and, when it's cloudy or during the night, the house runs on batteries. When you have unusually high power requirements, or for extended periods of heavy cloud, a backup generator kicks in. For remote cottages such a system is often substantially less expensive to install than to bring in the grid connection. Once your system is in, its operating costs are minimal.
Before you purchase solar power equipment, you have to develop a concept for the overall system. In addition to the solar panels themselves, you need a charge controller to prevent overloading the solar cells and an inverter to change the direct current output from the solar panels to normal 120 V AC household current. Several types of deep-discharge batteries are available to power the home during the night. The backup generator could be gasoline, diesel or propane. It's a good idea to check with neighbors who might have similar systems already installed, and with local authorities who can supply information and guidance about what kinds of systems are common in the area.
The most important consideration for a solar power system is its capacity in kilowatt hours, or kWh. The amount of power generated by the solar cells during a typical day has to be more than the amount of power you use during that day. To calculate your power requirements, list each electric load in your home and multiply its power draw in watts times the number of hours you normally use it in a day.
Typical off-grid systems do not use solar power for heating or cooking because you can fulfill these functions more efficiently and at lower cost by using another fuel, such as propane. Once you have added up all your other loads to get a total kWh figure, add a load factor of 40 percent to cover inefficiencies in the inverter, batteries and wiring. This total is the load your solar panels have to supply.
A second characteristic of a solar power system is the amount of peak load it can deliver. A large load like a pump for a well or a dishwasher might need more power than is available from the solar cells. In this case, the inverter will draw power from the batteries as well, but it must be sized to supply this extra power.
To determine the peak load, go down the list of loads you used for calculating the capacity and add up all the loads in kW that might be running simultaneously. If you start with the largest load and add any that might also be running, you will get a total kW load. The inverter has to be rated to deliver this peak load.
If you are experienced in construction and electrical installations you may be able to install your own solar power system. Most people prefer to let professionals do the work and look for a local contractor specialized in the field. Important considerations are whether the contractor will complete all the work and guarantee not only the equipment but also that it will produce the load that you have calculated. Ask for references and for any certifications. Some contractors will also handle the paperwork required for subsidies from local utilities or from the state.
Before selecting the supplier and purchasing the solar system, verify the installation details. The solar panels are usually installed on a roof so your roof has to be able to support the weight. The batteries may produce gases, depending on the type, and may have to be installed with ventilation. The generator usually has to be installed outside the house and may require a separate shed. Check how the supplier is going to run cables to make sure you agree with his installation plans.
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