If you want to install a source of emergency power for use during power failures, solar power is a viable alternative to buying an emergency generator. Each emergency power source has advantages, but solar power may cost you less over the long term and is more convenient and reliable in many cases. Which works better for you depends on many factors, but if you have a south-facing sunny location for placing solar panels, it’s a good idea to evaluate the cost of a solar installation.
Design to Replace Emergency Generators
To make your solar panels work as an emergency power source you have to use the right design. You need enough power to charge your batteries during the day and enough battery capacity to let you run the electric loads you need during the day and at night. Such emergency power is equivalent to that from an emergency generator and you don’t have to worry about fuel.
If you are in an area where the power company allows you to connect your installation to their system, you can install a switch that lets you sell the solar power to the utility when you don’t need it. With the right design and a grid-tie, you can earn a small amount on your investment and live indefinitely off the grid during a power failure.
In addition to the solar panels themselves, you need to store the power, convert it to the electricity needed in your home and get the grid connection equipment specified by your utility. If you have limited space, you need high-efficiency solar panels, but you may be able to get lower-priced panels if you have lots of room to install less efficient ones.
The panels deliver low voltage direct current that can be used to charge the batteries, but you need 120 V alternating current for your home. A typical installation uses the solar panels to charge the batteries through a charge controller that protects the batteries from over-charging. The DC power is fed into an inverter that produces 120 V AC for your household use. The grid tie equipment includes a switch that automatically isolates your system from the grid in case of a power failure. This is important to avoid feeding your solar power into the dead grid and endangering utility workers who assume there is no power in the system. The grid-tie equipment also includes meters that record how much power you used and how much you sent into the grid. Many utilities issue credits for power you produce.
You can use different types of batteries to store the power, but you have to be careful where you place them. Industrial systems often use nickel-cadmium batteries while some home systems use golf cart batteries. You can use deep-discharge lead-acid batteries, but in each case you have to make sure the location is enclosed and vented, if there is a chance the batteries may give off fumes.
Sizing the System
Determining the solar system power you need is slightly different from the way you would size an emergency generator. You have to size a generator depending on how much peak power you need. If you have a load that runs for two hours per day and uses 2.5 kW, you need at least a 2.5 kW generator. A solar system supplying the same load would have to store the power to run the 2.5 kW load for two hours or 5 kWh. This is equivalent to a 12 V battery bank supplying a current of just over 200 amps for two hours or 400 ampere-hours. To size your batteries, you have to add the loads multiplied by how long you expect each load to run. The voltage and ampere hours lets you select the batteries to give you the storage you need.
Once you have the required storage, you need solar panels to charge the batteries fully each day. The government’s National Renewable Energy Laboratory has maps giving the amount of solar power you can expect to generate in your area. As a rule of thumb, you can calculate that an average panel generates power equal to four times its rating in one day. A 2 kW panel will generate about 8 kWh per day and, to generate enough power for your 5 kWh load, you need 1.25 kW solar panels.
Your panels and batteries make sure the power you need is available, but your inverter delivers it to your household system. It has to be sized for the full power. If you have a 2.5 kW load, you need a 2.5 kW inverter. In each case, it\s a good idea to size the component slightly larger than you need to take into account possible losses and overloads.
You may be able to carry out the installation of many of the solar system components yourself, but some require the help of licensed professionals. Mother Earth News has some guidelines for a DIY emergency solar power system, but utilities generally require the grid tie and some other electrical work to be carried out by, or at least be inspected by, licensed electricians.
If you are comfortable working with heavy equipment and have electrical expertise, you can probably save some money by installing the solar panels, the different components and by running the cabling. Connecting the system together, testing it and putting it into service requires more extensive knowledge and may best be left to a professional.
Photo Credit: Skytech Solar