rage 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

To determine the ampere hours from this table, multiply the total
number of lamp hours by the current in amperes required for each lamp.
As 10, 15, and 20-watt tungsten lamps require .33, .50 and .67
amperes, respectively at 30 volts pressure, the above requirements in
ampere hours would be 12, 17-1/2, or 24 ampere hours, according to the
size of lamp chosen. This gives the average current consumption for
one night. If it is desired to charge the battery twice a week on the
average, multiply the number of lamp hours by 4, to get the size of
battery required.

The foregoing illustration is not intended to indicate average light
requirements