Lead batteries and lithium-ion batteries are the two options currently available for privately operated PV systems. Lead-acid batteries, and the more recent gel variety, are cheaper than lithium-ion technology, but they don’t last as long. Lithium-ion batteries are more expensive but can store more electricity in a smaller space and promise better performance when it comes to durability and depth of discharge. However, while we know a lot about lead batteries because they have been in use for a long time, the durability of lithium-ion batteries has only been tested in relatively short ageing tests. This means there is a high degree of uncertainty in any statement about how long lithium systems can last.
In May 2013, the German government introduced a new funding scheme designed to encourage people to use more of the electricity their PV systems produce. Under the new rules, anyone who buys a new storage system with a new PV system will receive up to €600 per kilowatt-peak installed. Those who buy a new storage system for their existing panels (which went into operation after 31 December 2012) will receive up to €660 per kilowatt-peak installed. But despite the funding, lead batteries will still cost operators an additional 10 to 15 cents per kilowatt-hour of stored PV power. Users of lithium-ion batteries should expect to spend even more: between 20 and 25 cents per kilowatt-hour.
Manufacturers are therefore now developing smaller lithium-ion batteries, where the primary aim is not to increase personal consumption to the absolute maximum, but to make a system that operates in the most economical way possible. So although the new systems only promise private consumption of 50 to 60 percent, they cut the cost per kilowatt-hour of stored PV electricity. If you are interested in purchasing a solar storage system for use in Germany, you might find this (German-language) site helpful: http://www.solaranlagen-portal.com/photovoltaik/solarspeicher-rechner. It’s a calculator that helps you work out how economically viable a storage solution is.
This question was answered by Robert John Doelling of www.energie-experten.org, an online magazine specialising in matters related to energy.