A solar regulator (also known as a charge controller) is used in conjunction with a stand alone (off grid) system, or a grid connect solar power system that incorporates a backup battery bank. For a grid connect solar power system that doesn’t use batteries, a solar regulator is not needed.
A solar regulator is a small box consisting of solid state circuitry that is placed between a solar panel and a battery. Its function is to regulate the amount of charge coming from the panel that flows into the deep cycle battery bank in order to avoid the batteries being overcharged. A regulator can also provide a direct connection to appliances, while continuing to recharge the battery; i.e. you can run appliances directly from it, bypassing the battery bank; but the batteries will continue to be charged.
Modern solar regulators are very efficient and will outperform just about any 3 stage mains power battery charger.