Running this plant at only half-load--that is, using only 18 lights, or their equivalent--would reduce the price to about 3 cents an hour--since the efficiency decreases with smaller load. It is customary to figure an average of 3-1/2 hours a day throughout the year, for all lights. On this basis the cost of gasoline for this one-kilowatt plant would be 16-1/2 cents a day for full load, and approximately 10-1/2 cents a day for half-load. This is extremely favorable, as compared with the cost of electric current in our cities and towns, at the commercial rate, especially when one considers that light and power are to be had at any place or at any time on the farm simply by starting the engine. A smaller plant, operating at less cost for fuel, would furnish ample light for most farms; but it is well to remember in this connection plants smaller than one kilowatt are practical for light only, since electric irons, toasters, etc., draw from 400 to 660 watts each. Obviously a plant of 300 watts capacity would not permit the use of these instruments, although it would furnish 10 or 12 lamps of 25 watts each. CHAPTER XI THE STORAGE BATTERY What a storage battery does--The lead battery and the Edison battery--Economy of tungsten lamps for storage batteries--The low-voltage battery for electric light--How to figure the capacity of a battery--Table of light requirements for a farm house--Watt-hours and lamp-hours--The cost of storage battery current--How to charge a storage battery--Care of storage batteries. For the man who has a small supply of water to run a water wheel a few hours at a time, or who wishes to store electricity while he is doing routine jobs with a gasoline engine or other source of power, the storage battery solves the problem. The storage battery may be likened to a tank of water which is drawn on when water is needed, and which must be re-filled when empty. A storage battery, or accumulator is a device in which a chemical action is set up when an electric current is passed through it. This is called _charging_. When such a battery is charged, it has the property of giving off an electric current by means of a reversed chemical action when a circuit is provided, through a lamp or other connection. This reversed action is called _discharging_. Such a battery will discharge nearly as much current as is required originally to bring about the first chemical action.