To charge this battery would require a dynamo capable of delivering 10 amperes of current for 9 hours. The generator should be of 45 volts pressure (allowing 2-1/2 volts in the generator for each 2 volts of battery) and the capacity of the generator would therefore be 450 watts. This would require a 1-1/4 horsepower gasoline engine. At 1-1/4 pints of gasoline for each horsepower, nine hours work of this engine would consume 14 pints of gasoline--or say 16 pints, or two gallons. At 12 cents a gallon for gasoline, lighting your house with this battery would cost 24 cents for four days, or 6 cents a day. Your city cousin, using commercial current, would pay 5-1/2 cents a day for the same amount of current at 10 cents a kilowatt-hour; or 8-1/4 cents at a 15-cent rate. If the battery is charged by the farm gasoline engine at the same time it is doing its other work, the cost would be still less, as the extra gasoline required would be small. This figure does not take into account depreciation of battery and engine. The average farmer is too apt to overlook this factor in figuring the cost of machinery of all kinds, and for that reason is unprepared when the time comes to replace worn-out machinery. The dynamo and switchboard should last a lifetime with ordinary care, so there is no depreciation charge against them. The storage battery, a 30-volt, 80-ampere hour installation, should not cost in excess of $100; and, if it is necessary to buy a gasoline engine, a 1-1/4 horsepower engine can be had for $50 or less according to the type. Storage batteries of the lead type are sold under a two-years' guarantee--which does not mean that their life is limited to that length of time. With good care they may last as long as 10 years; with poor care it may be necessary to throw them away at the end of a year. The engine should be serviceable for at least 10 years, with ordinary replacements; and the storage battery may last from 6 to 10 years, with occasional renewal of parts. If it were necessary to duplicate both at the end of ten years, this would make a carrying charge of $1.25 a month for depreciation, which must be added to the cost of light. _Figuring by Lamp Hours_ If all the lamps are to be of the same size--either ten, fifteen, or twenty watts, the light requirements of a farm house can be figured readily by lamp hours. In that event, the foregoing table would read as follows: Lamp hours Kitchen, 1 lamp, 4 hours 4 Sitting room, 3 lamps, 4 hours each 12 Dining room, 2 lamps, 2 hours each 4 Bedrooms, 3 lamps, 1 hour each 3 Halls, 2 lamps, 4 hours each 8 Bathroom, 1 lamp, 2 hours 2 Pantry and cellar, 2 lamps, 1 hour each 2