ing room; and 10-watt lamps
in the halls, bathroom, and bedrooms. His requirements may be figured
either in lamp hours or in watt-hours. Since he is using two sizes of
lamps, it will be simpler to figure his requirements in watt-hours.
Thus:

Number Size of Hours Watt-
Room of lamps lamps burned hours

Kitchen 1 20 4 80
Dining room 2 20 2 80
Sitting room 3 20 4 240
(3) Bedrooms 1 (each) 10 1 30
Bathroom 1 10 2 20
(2) Halls 1 (each) 10 4 80
Pantry 1 10 1 10
Cellar 1 10 1 10
----
Total 550

Since amperes equal watts divided by volts, the number of ampere hours
required in this case each night would be 550 / 30 = 18.3 ampere
hours; or approximately 4-1/2 amperes per hour for 4 hours.

Say it is convenient to charge this battery every fourth day. This
would require a battery of 4 x 18.3 ampere hours, or 73.2 ampere
hours. The nearest size on the market is the 80-ampere hour battery,
which would be the one to use for this installation.

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 ave