FORMULA SHELF
12 of 370
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
Electrical
12 formulas
Sizing an off-grid battery bank for the days you expect poor weather.
3 kWh a day for 2 days at 50% DoD on 24 V needs 500 Ah.
Rough runtime for a battery bank before depth-of-discharge limits.
100 Ah at 12 V and 80% efficiency running 100 W lasts about 9.6 h.
A first check on cable size. Always follow local wiring rules and derating factors.
20 A at 4 A/mm2 suggests at least 5 mm2 before derating.
How a capacitor resists alternating current. Used in motor starting and filters.
10 uF at 50 Hz has 318 ohm reactance.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Stored energy in a capacitor, from motor start cans to smoothing banks.
1000 uF at 400 V stores 80 J.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Matching a generator to a pump, welder or house load without nuisance trips.
4 kW at 0.8 PF needs 5 kVA plus surge margin.
Total opposition to alternating current in motors, pumps and long cable runs.
R 30, XL 40, XC 0 gives Z 50 ohm.
Choosing an inverter for a shed or house. Size for surge, not just running watts.
A 1 kW pump with 3x surge needs at least 3 kVA.
A low power factor means more current for the same useful work. It matters for pumps and motors.
8 kW from 10 kVA is a power factor of 0.8.
Parallel circuits, battery banks and multiple loads on one supply.
Two 10 ohm resistors in parallel = 5 ohm.
Adding resistors, cable runs or heating elements end to end.
10 ohm and 15 ohm in series = 25 ohm.
Sizing cable for a shed, pump or welder so the far end still gets enough volts.
10 A over 50 m of 4 mm2 cable drops about 4.4 V.
Convenience only. Double-check the result against the worked example or a second method before you use it.
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.