The question now to be considered differs materially from that involving merely the variations of water-power in the several months, or even the possible yearly output from water-power. The ratio of output from water-power to the total yearly output of an electrical system includes the result of load fluctuations in every twenty-four hours and the variable demands for electrical energy in different months, as well as changes in the amount of water-power available through the seasons.
In order to show the combined result of these three important factors curve No. 4 has been constructed. This indicates the percentages of total semi-yearly outputs of electrical energy derived from water-power in two supply systems. Each half-year extends either from January to June, inclusive, or from July to December, inclusive, and thus covers a wet and dry season. Each half-year also includes a period of maximum and one of minimum demand for electrical energy in lighting. The period of largest water supply usually nearly coincides with that of heaviest lighting load, but this is not always true.
Electrical systems have purposely been selected in which the water-power in at least one month of each half-year was nearly or quite sufficient to carry the entire electrical load. The percentage of energy from water-power to the total energy delivered by the system is presented for each of five half-years. Three of the half-years each run from July to December, and two extend from January to June, respectively. The half years that show percentages of 66.8, 80.2, and 95.6, respectively, for the relation of energy from water-power to the total electrical output relate to one system, and the half years that show percentages of 81.97 and 94.3 for the energy from water-power relate to another system.
For the half-year when 66.8 per cent. of the output of the electrical system was derived from water-power, the total output of the system was 3,966,026 kilowatt hours. During the month of December in this half-year more than 98 per cent of the electrical energy delivered by the system was from water-power, though the average for the six months was only 66.8 per cent from water.
In the following six months, from January to June, the electrical supply system delivered 4,161,754 kilowatt hours, and of this amount the water-power furnished 80.2 per cent. For the six months just named, one month, May, saw 99 per cent of all the delivered energy derived from water-power.