Formulas

Electrical

Volts = Amps x Ohms. Watts = Volts x Amps. Amps = Watts / Volts. Ohms = Volts / Amps.

Amps from watts: at 12 V, divide watts by 12. At 120 V, divide by 120. A 1500 W load draws about 12.5 A at 120 V and about 125 A at 12 V.

Element ohms from watts: ohms = 120 x 120 / watts. A 1440 W element is about 10 ohms. A 300 W fridge element is about 48 ohms.

Voltage drop percent = drop / source x 100. Aim for 3 percent or less.

Watt-hours = amp-hours x volts. A 100 Ah, 12 V battery holds about 1200 Wh. Lead-acid: use about half. Lithium: about 80 to 100 percent.

Run time (hours) = battery amp-hours usable / load amps.

Delta-T = return temperature minus supply temperature. 12,000 BTU/hr = 1 ton. 13,500 BTU = about 1.1 ton. 1 watt = 3.412 BTU/hr.

F = C x 1.8 + 32. C = (F - 32) / 1.8. Examples: 32 F = 0 C, 68 F = 20 C, 77 F = 25 C, 100 F = 37.8 C.

1 gallon of propane is about 91,500 BTU. A 20 lb cylinder holds about 4.7 gallons, about 430,000 BTU. 1 psi = about 27.7 in W.C.

A one gallon jug filling in 85 seconds is about 0.7 GPM. In 60 seconds it is 1.0 GPM. GPM = 60 / seconds to fill one gallon.

14 AWG 15 A. 12 AWG 20 A. 10 AWG 30 A. 8 AWG 40 A. 6 AWG 55 A. 4 AWG 70 A. 2 AWG 95 A. 1/0 AWG 125 A. Smaller number = thicker wire.