This was because of the great availability of 12 volt deep cycle batteries through the RV and transportation industries. Another factor was the availability of 12 volt appliances. The wire tends to be very large which is not only very expensive, but is bulky and harder to install. These days 24 and 48 volt systems are very common. Battery banks can be wired in series to produce any voltage you wish.
Electrical power is measured in watts.
To determine how power is consumed by a given load we need to know how much time the load is on. Utility companies work with so much electricity that they skip right to measuring the power by every 1,000 watts. This is called a kilowatt. When a kilowatt is consumed for one hour, this is called a kilowatt-hour.