Understanding Energy Conversions

Energy conversion is essential for understanding electricity bills, nutrition labels, heating systems, and scientific calculations. Energy comes in many forms - electrical, thermal, chemical, mechanical - and can be measured in various units depending on the application and region.

Common Energy Units

Unit	Common Uses	Typical Applications
Joules (J)	SI unit, physics, science	Standard scientific measurement
Calories (cal)	Nutrition, food energy	Food labels (actually kcal)
Kilocalories (kcal)	Food Calories (capital C)	Nutrition labels worldwide
kWh	Electricity billing	Home energy consumption
BTU	HVAC, heating/cooling	Air conditioner capacity
Watt-hours (Wh)	Battery capacity, small devices	Phone batteries, solar panels

Quick Conversion Reference

From	To	Factor	Example
Calories	Joules	× 4.184	100 cal = 418.4 J
kWh	Joules	× 3,600,000	1 kWh = 3.6 MJ
BTU	Joules	× 1,055.06	1 BTU = 1,055 J
kcal	kWh	× 0.001163	2,000 kcal = 2.33 kWh
Wh	BTU	× 3.41214	1,000 Wh = 3,412 BTU

Practical Applications

Electricity Bills and Home Energy

Electric companies charge per kilowatt-hour (kWh). One kWh is the energy consumed by a 1,000-watt appliance running for one hour. Average US home uses 900 kWh/month. At $0.13/kWh, that's $117/month. A 100-watt bulb running 10 hours/day uses 30 kWh/month ($3.90).

Food and Nutrition

Food labels show Calories (capital C), which are actually kilocalories (kcal). One food Calorie equals 1,000 small calories or 4,184 joules. A 2,000 Calorie diet equals 2,000 kcal or 8.4 million joules (8.4 MJ) of energy per day. This equals about 2.33 kWh - enough to power a 100W bulb for 23 hours.

Heating and Cooling (HVAC)

Air conditioners and heaters use BTU ratings. BTU (British Thermal Unit) is the energy to heat one pound of water by 1°F. A typical window AC is 5,000-12,000 BTU/hour. Central AC systems are 24,000-60,000 BTU/hour. Higher BTU = more cooling power but also higher electricity use.

Battery Capacity

Phone and laptop batteries rate in watt-hours (Wh) or milliamp-hours (mAh). A typical smartphone has a 3,000 mAh battery at 3.7V = 11.1 Wh. An electric car battery is 60-100 kWh, which could power a home for 2-3 days.

Energy Equivalents

Description	Energy	Equivalent
Food Calorie	1 kcal	4,184 J = 0.00116 kWh
Daily food intake	2,000 kcal	8.4 MJ = 2.33 kWh
100W bulb (1 hour)	100 Wh	360 kJ = 86 kcal = 341 BTU
Small window AC (1 hour)	5,000 BTU	5.28 MJ = 1.47 kWh
Gallon of gasoline	33.7 kWh	121 MJ = 115,000 BTU
Tesla Model 3 battery	75 kWh	270 MJ = 256,000 BTU

Power vs Energy

Don't confuse power and energy:

• Power: Rate of energy use (Watts = Joules/second)
• Energy: Total amount consumed (Joules, kWh, BTU)
• Example: 100W bulb uses 100 Joules per second
• Running 10 hours: Uses 1,000 Wh = 1 kWh of energy
• Formula: Energy (kWh) = Power (kW) × Time (hours)

Electricity Cost Calculations

Calculate appliance costs using kWh:

Common Appliance Energy Use

Appliance	Power	Daily kWh (typical use)	Monthly Cost ($0.13/kWh)
LED bulb (60W equivalent)	9W	0.054 (6 hrs)	$0.21
Laptop	50W	0.40 (8 hrs)	$1.56
Refrigerator	150W	3.6 (24 hrs)	$14.04
Space heater	1500W	12.0 (8 hrs)	$46.80
Central AC	3500W	28.0 (8 hrs)	$109.20

Tips for Energy Efficiency

• LED bulbs: Use 75% less energy than incandescent for same brightness
• Phantom power: Devices on standby still consume 5-10W each
• Energy Star appliances: Use 10-50% less energy than standard models
• Time-of-use rates: Some utilities charge less for off-peak electricity
• Solar panels: Typical home system produces 10-15 kWh/day