At the beginning of the 21st century, the policies supporting solar energy development in many parts of Europe and North America initially preferred household distributed photovoltaic systems, although the cost of roof photovoltaic systems was relatively high compared with large ground solar photovoltaic power stations. Until recently, these regions have shown strong interest in household photovoltaic and energy storage integrated systems. The factors driving the rapid development of energy storage include policies, utility tariff design, and concerns about power outages in some areas. The decline of battery energy storage cost constitutes a prerequisite for this development. The price of lithium-ion batteries for residential energy consumption has been steadily declining, reaching 1100 euros / kwh in 2019, but there is still a large premium compared with vehicle batteries, indicating that there may be further room for price reduction.