With gasoline power, the cost of the engine offsets the cost of the water wheel. The engine is more expensive than the ordinary gasoline engine; but even this item of cost is offset by the cost of labor and materials used in installing a water wheel.
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 | 0.5 | ½ |
| 225 | 0.7 | ¾ |
| 300 | 0.86 | 1 |
| 450 | 1.12 | 1¼ |
| 600 | 1.5 | 1½ |
| 750 | 1.7 | 1¾ |
| 1000 | 2.3 | 2½ |
| 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 2½ horsepower, and consuming, let us say, 1¼ 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 13½ cents.
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 3½ hours a day throughout the year, for all lights. On this basis the cost of gasoline for this one-kilowatt plant would be 16½ cents a day for full load, and approximately 10½ 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