The expense of maintenance is limited to gasoline and oil. Depreciation enters in both cases; and though it may be more rapid with a gasoline engine than a water wheel, that item will not be considered here. The cost of lubricating oil is inconsiderable. It will require, when operated at from one-half load to full load, approximately one pint of gasoline to each horsepower hour. When operated at less than half-load, its efficiency lowers. Thus, for a quarter-load, an average engine of this type may require three pints of gasoline for each horsepower hour. For this reason it is well, in installing such a plant, to have it of such size that it will be operating on at least three-fourths load under normal draft of current. Norman H. Schneider, in his book "Low Voltage Electric Lighting," gives the following table of proportions between the engine and dynamo: Actual watts Actual Horsepower Nearest engine size 150 .5 1/2 225 .7 3/4 300 .86 1 450 1.12 1-1/4 600 1.5 1-1/2 750 1.7 1-3/4 1000 2.3 2-1/2 2000 4.5 5 4000 9.0 10 This table is figured for an efficiency of only 40 per cent for the smaller generators, and 60 per cent for the larger. In machines from 5 to 25 kilowatts, the efficiency will run considerably higher. To determine the expense of operating a one-kilowatt gasoline generator set of this type, as to gasoline consumption, we can assume at full load that the gasoline engine is delivering 2-1/2 horsepower, and consuming, let us say, 1-1/4 pint of gasoline for each horsepower hour (to make allowance for lower efficiency in small engines). That would be 3.125 pints of gasoline per hour. Allowing a ten per cent loss of current in wiring, we have 900 watts of electricity to use, for this expenditure of gasoline. This would light 900 ÷ 25 = 36 lamps of 25 watts each, a liberal allowance for house and barn, and permitting the use of small cooking devices and other conveniences when part of the lights were not in use. With gasoline selling at 12 cents a gallon, the use of this plant for an hour at full capacity would cost $0.047. Your city cousin pays 9 cents for the same current on a basis of 10 cents per kilowatt-hour; and in smaller towns where the rate is 15 cents, he would pay 13-1/2 cents.