FORMULA SHELF
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The arithmetic behind the work: what to measure, what to type in, and what the answer comes out as. Every formula says what the symbols mean and gives a worked example, so you can check it against a job you have already done.
SHELVES
Energy
9 formulas
For sizing a solar array for the electric fence, the lights or a small freezer. Multiply the panel area by the irradiance you can count on, not the best case.
2 m2 of panel at 800 W/m2 and 20% gives 320 W.
The starting point for any electrical fault finding. If the voltage is right and the lamp is dim, the current is low, so the resistance is too high. See Electrical Fault Finding.
12 V across a 6 ohm lamp draws 2 A.
Convenience only. Double-check the result against the worked example or a second method before you use it.
For working out what a heater, a motor or a charger really uses. 1 hp is about 746 W, so a 1.5 kW heater is 2 hp of heat going into the room.
A 2,300 W kettle on 230 V draws 10 A.
For choosing a battery, and for working out how long a generator or a mains charger will take. A 12 V 100 Ah battery on a 10 A charger is about 10 hours, less the losses.
A 12 V 100 Ah battery charged at 10 A takes about 11 hours.
What the meter on the wall measures. Everything electric in the farm is counted in kilowatt hours, and a 1,000 W heater running for ten hours uses 10 kWh.
A 2 kW heater for 5 hours a day costs 10 kWh a day.
How many units of heat you get for each unit of electricity. A COP of 3.5 means the heat pump heats three and a half times more than the resistance heater it replaced.
A 500 W heat pump with a COP of 3.5 gives 1,750 W of heat.
For a string of lights, a trailer board, or when you are adding a second battery. Parallel adds power and series adds voltage.
Two 6 ohm bulbs in series give 12 ohm; in parallel they give 3 ohm.
For the three-phase supply to a barn, a grain dryer, a milking parlour or a welder. The root 3 is the bit people forget.
415 V at 20 A and a power factor of 0.9 = 12,950 W.
For a welder, a battery charger, a lamp on a long lead, and any step-down or step-up. Power is roughly conserved, so a step-up gives you less current for the same watts.
Stepping 230 V down to 24 V gives about ten times the current for the same power.
SOURCES & PROVENANCE
1 listed
This page is an AI-organised index. The label means the grouping and cross-links were compiled with AI; the information itself comes from the public-domain books, reports and manuals listed below. Open any source to read the local copy or follow the link to the original publication.