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Storage of electricity is inherently difficult and expensive. Batteries are required when grid independence is desired for grid outages or off-grid situations. Such grid-independent systems can be designed and customized for many different situations and for a given budget.
The simplest battery based systems are DC (direct current) systems that operate on 12 or 24 Volt DC. Appliances that work in cars, RV’s and boats can be used with these systems. This can involve lighting, refrigeration, pumps, entertainment, mobile phone equipment and computers. These systems can be used in off-grid cabins or they can be integrated in new homes as an alternative system to augment regular AC wiring. Cost for this type of installation is $1,000 and up. This may include a 100 watt solar panel and a charge controller.
If an inverter is added to a DC system it becomes an AC (alternating current) system which in principal can then power all the standard house appliances. Typically a back-up system for a home would only power critical appliances during a power outage as for example a water pump, furnace, pellet stove, refrigerator and a few lights and outlets. These so called critical "loads" are powered from the inverter by using the power stored in the batteries during a power outage.
The batteries can be charged by the grid and are always kept ready and fully charged by a charge controller. During an outage the inverter instantly goes into inverting mode and powers the critical loads as needed and without any interruption even if no one is home. If the power comes back after a few hours the inverter/charger recharges the batteries and keeps them fully charged until the next outage. As a rule of thumb, a modest but adequate backup system costs about $5,000 with a battery pack for one day. Each additional day adds about $2,000 in batteries.
However if an outage lasts for days the batteries will have to be recharged from a different source. The most common source is solar panels. Wind turbine and water turbines are less common but many work in certain situations better than solar. Generators can also be used to recharge the batteries as a last fall back solution.
Although back-up systems like this are far more expensive than cheap generators their advantages are significant. They are:
An existing or new grid-tied PV system can be expanded to become such a power source to refill batteries and power your critical "load" during an outage. In this way your PV system will provide power to your own home during an extended power outage.