Let us assume, as an example, that Farmer Brown will use 20-watt lamps in his kitchen, dining room, and sitting 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:
| Room | Number of lamps | Size of lamps | Hours burned | Watt-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 4½ amperes per hour for 4 hours.
Say it is convenient to charge this battery every fourth day. This would require a battery of 4 × 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 2½ 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 1¼ horsepower gasoline engine. At 1¼ 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 5½ cents a day for the same amount of current at 10 cents a kilowatt-hour; or 8¼ 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 average 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 1¼ 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, 17½, 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.