This 8-hour rate also determines the rate at which a battery should be re-charged, once it is exhausted. Thus, this battery should be charged at the rate of 7-1/2 amperes for 8 hours, with another hour added to make up for losses that are bound to occur. A battery of 120-ampere hour capacity should be charged for 8 or 9 hours at the rate of 120 ÷ 8, or 15 amperes, etc. To determine the size of battery necessary for any particular instance, it is necessary first to decide on the number of lamps required, and their capacity. Thirty-volt lamps are to be had in the market in sizes of 10, 15 and 20 watts; they yield respectively 8, 12, and 16 candlepower each. Of these the 20-watt lamp is the most satisfactory for the living rooms; lamps of 10 or 15 watts may be used for the halls, the bathroom and the bedrooms. At 30 volts pressure these lamps would require a current of the following density in amperes: Candle Power 30-volt lamp Amperes 8 10 watts 0.33 12 15 watts 0.50 16 20 watts 0.67 Let us assume, as an example, that Farmer Brown will use 20-watt lamps in his kitchen, dining room, and sitting room; and 10-watt lamps in the halls, bathroom, and bedrooms. His requirements may be figured either in lamp hours or in watt-hours. Since he is using two sizes of lamps, it will be simpler to figure his requirements in watt-hours. Thus: Number Size of Hours Watt- Room of lamps lamps burned hours Kitchen 1 20 4 80 Dining room 2 20 2 80 Sitting room 3 20 4 240 (3) Bedrooms 1 (each) 10 1 30 Bathroom 1 10 2 20 (2) Halls 1 (each) 10 4 80 Pantry 1 10 1 10 Cellar 1 10 1 10 ---- Total 550 Since amperes equal watts divided by volts, the number of ampere hours required in this case each night would be 550 ÷ 30 = 18.3 ampere hours; or approximately 4-1/2 amperes per hour for 4 hours. Say it is convenient to charge this battery every fourth day. This would require a battery of 4 × 18.3 ampere hours, or 73.2 ampere hours. The nearest size on the market is the 80-ampere hour battery, which would be the one to use for this installation.