For when the sun doesn't shine...
A battery stores electrical energy in a reversible chemical reaction. The renewable energy (RE) source (PV, wind, or hydro) produces the energy, and the battery stores it for times of low or no RE production. Most batteries employed in renewable energy systems use the same electro-chemical reactions as the lead-acid battery in your car. But, unlike your car battery, they are specifically designed for deep cycling. And most renewable energy systems have batteries which store between ten and hundreds of times more energy than a car battery. This doesn’t guarantee you will have a consistent performance with batteries. One should consider backup power in case your batteries become discharged due to lack of renewable energy in the RE system or an over consumption of energy.
There are many brands and types of batteries available for RE systems. It is important to find the right battery for your situation and wallet. The two most common batteries are the L-16 and golf cart sizes. With proper care, RE system batteries have a lifetime of five to ten years, but there are more expensive batteries that are warranted to last ten to twenty years.
Battery capacity is rated in amp-hours. 1 amp-hour is the equivalent of drawing 1 amp steadily for one hour, or 2 amps steadily for half an hour. A typical 12 volt system may have 800 amp-hours of battery capacity. This battery can draw 100 Amps for 8 hours if fully discharged and starting from a fully charged state. This is the equivalent of 1,200 watts for eight hours (watts = amps x volts), or about the same power consumed as running a small hair dryer for eight hours.
However, completely discharging your battery decreases its longevity, and can ruin it in short order. Most home power users will only tap into a portion of available capacity to keep their batteries alive longer. Opinion varies as to the appropriate depth of discharge, but most agree that 50% (and many say 30%) is the maximum a battery should be routinely discharged. Never go below 80% depth of discharge. 50% means that the above 1200 watt hair dryer would only be used for 4 hours instead of the 8 indicated by the maximum capacity of the batteries.
Batteries typically are encased in plastic and need to be wired together in series and parallel strings by the installer. Some larger batteries are pre-wired and encased in steel containers.
Batteries do not belong inside your living space. They have dangerous chemicals in them, so they must be contained to avoid spills. They also put out hydrogen and oxygen gas while being charged, so they should be vented to the outdoors. Their tops and connections must be periodically cleaned to avoid energy losses. Batteries must also be routinely topped off with distilled water. Finally, they need to be "equalized" with an occasional controlled overcharge to keep the individual cells at equal states of charge.
Using the power directly from the source...
Low voltage DC appliances (mostly 12 VDC) can be operated directly from batteries or photovoltaic modules. For many years good inverters to power the standard 120 VAC appliances common to most modern homes did not exist. Many DC appliances were developed to accommodate these systems, including DC incandescent and fluorescent lighting, televisions, stereos, refrigerators, and even vacuum cleaners and washing machines. These are mostly 12 volt, though some appliances are available in 24 volt models.
Inverters are greatly improved now, making 120 VAC appliances the standard, but many off-the-grid homes still use low voltage DC appliances. Using DC loads is a more efficient use of energy because inverters have a 50 to 95 percent efficiency, depending on the amount of power being consumed through the inverter and the make/model of the inverter. Due to declining demand, fewer DC appliances are being manufactured. Most of these are being used in third-world applications where inverters and other sophisticated electronics are still beyond the financial means of the users.