Energy Fundamentals: Resources, Units & Conversion

The big picture of energy. Learn energy vs power, units and conversions (J, kWh, toe, BTU, kcal), primary, secondary and final energy, the global and Indian energy mix, conversion chains and efficiencies, the laws of thermodynamics as limits, energy density of fuels and batteries, capacity factor, and the energy transition — with Python converting fuels and tracing primary energy behind household electricity.

Energy engineering is about producing, converting, transmitting, storing and using energy efficiently, affordably and cleanly. Every choice — a power plant, a solar farm, an efficient motor, a battery — is judged by energy flows and conversion losses.

Understanding units, conversion chains and efficiency limits lets engineers compare options fairly: a kilowatt-hour of electricity from coal may require more than three kilowatt-hours of primary energy.

An analogy

An energy system is like a water supply with leaky pipes: primary energy is the water at the reservoir, final energy is what reaches your tap, and every conversion (power plant, transmission, appliance) is a leak along the way.

Energy vs power

Power is the rate of energy flow (watts); energy is power × time (joules, kWh). A 1 kW heater running for 3 h uses 3 kWh. Power ratings describe capacity (MW of a plant), energy describes output (MWh per year). Confusing them is the most common beginner error.

Energy resources and the mix

Primary sources:

Conversion and thermodynamic limits

Heat engines are limited by Carnot (1 − T_c/T_h); real steam plants reach 33–46%, combined cycles 55–63%. Direct conversions avoid heat-engine limits: electric motors (90–97%), batteries (90%+ round trip), fuel cells (40–60%), PV cells (20–25% of sunlight, but the 'fuel' is free). Every conversion also has second-law (exergy) losses.

Energy transition

Decarbonisation strategies: efficiency first, electrify end uses (transport, heating, industry), decarbonise electricity (renewables, nuclear), use hydrogen and sustainable fuels for hard-to-electrify sectors, and CCS for residual emissions. Challenges: variability of solar and wind (storage, flexibility, grids), critical minerals, affordability and energy access. Energy security and just transition matter as much as technology.

Formulas

Worked example: Energy use

Given: 1.5 kW AC running 8 h/day for 30 days

Find: Monthly energy.

Answer: 360 kWh

Worked example: Capacity factor

Given: 100 MW solar farm produces 184 GWh/yr

Find: CF.

Answer: 21%

Worked example: Carnot limit

Given: Steam at 600 °C, condenser at 40 °C

Find: η_max.

Answer: 64% (real plants ~40–45%)

Fuel conversions and primary energy

# Energy units and a household energy balance
kWh_per_MJ = 1 / 3.6
lpg_kg, lpg_MJkg = 14.2, 46.1          # one LPG cylinder
elec_kWh = 250                          # monthly electricity
print(f"one LPG cylinder = {lpg_kg*lpg_MJkg:.0f} MJ = {lpg_kg*lpg_MJkg*kWh_per_MJ:.0f} kWh")
print(f"monthly electricity {elec_kWh} kWh = {elec_kWh*3.6:.0f} MJ")
# Primary energy behind 250 kWh of coal electricity (plant 35%, T&D losses 15%)
primary = elec_kWh / (0.35 * 0.85)
print(f"primary energy at the power plant = {primary:.0f} kWh ({primary/elec_kWh:.1f}x)")
# one LPG cylinder = 655 MJ = 182 kWh
# monthly electricity 250 kWh = 900 MJ
# primary energy at the power plant = 840 kWh (3.4x)

A 14.2 kg LPG cylinder holds as much energy as 182 kWh of electricity, but 250 kWh of coal electricity costs 840 kWh of coal energy because of plant and grid losses. This is why saving electricity at the point of use saves roughly three times as much primary energy.

Common mistakes

Applications

Key takeaways

Going further