Energy in buildings: Science and limits

Why a gas flame warms the kitchen more than the pan

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

Homes and offices turn electricity, gas or sunlight into heat for cooking and washing, motion in fans and pumps, and stored energy in batteries. The chain is short, but losses at each step add up: a gas flame wastes most of its heat around the pan, while an induction hob heats the pan itself.

The conversion, step by step

Gas hob: flame heats the pan
Chemical (LPG or natural gas) → Heat in the food · About 32% on the US test method¹Induction hob: coil field heats the pan
Electrical to magnetic (alternating field) → Heat generated inside the pan base · About 85% (resistance hobs about 75-80%)¹Solar water heater: collector to tank
Sunlight → Heat in stored water · Not quantified in the sources used; a 100 litre per day system needs about 4 m2 of roof²Ceiling fan: motor turns blades
Electrical → Kinetic (moving air) · BLDC fan draws 28-32 W against 70-75 W for an induction-motor fan of the same 1,200 mm size³Rooftop solar: module then inverter
Sunlight to DC electricity to AC electricity → Electrical (AC, into the home) · Modules average 22.7% (Q4 2024); a performance ratio of about 80% is a standard modelling assumption for system and inverter losses⁵Home inverter/UPS: charge, store, invert
Grid AC to chemical (battery) to AC → Electrical (backup supply) · Lead-acid round trip 70-85% and inverter about 80-90%, so about 56-77% overall (calculated; figures from an industry advocacy letter)⁷Phone or laptop charger
Mains AC → Low-voltage DC into the battery · EU minimum about 85% for a 20 W standard charger and 88% above 49 W (calculated from the legal formula)⁸

The science

Cooking and water heating are about getting heat into a target with as little as possible going elsewhere. A gas flame heats the air around the pan as much as the pan. An induction hob passes high-frequency alternating current through a copper coil under the glass; the changing magnetic field induces eddy currents (circulating currents) inside an iron or steel pan, and the pan's own electrical resistance turns them into heat, so the glass stays relatively cool. Heating water takes a fixed amount of energy: 4.19 kJ per litre per degree, so warming 100 litres from 20 C to 55 C needs about 4.1 kWh (calculated), whether the heat comes from a resistance element, the sun or a heat pump. Fans and pumps follow the affinity laws: the power needed rises with the cube of speed, so running a fan at half speed takes roughly one-eighth of the power. Brushless DC (BLDC) motors use permanent magnets and electronic switching instead of a rotor that must be magnetised by induced current, which removes much of the rotor loss. Solar panels and batteries make or store DC, so every link to the AC grid needs a power-electronic inverter or rectifier.

Limits set by science

Limit	Value	Why it exists	Best today
Energy to heat water	4.19 kJ per litre per degree C; 100 litres by 35 C is about 4.1 kWh (calculated)²	Set by the heat capacity of water. An electric element already turns almost all its electricity into heat, so savings must come from solar heat, heat pumps or using less hot water.	Solar water heaters supply the heat from sunlight with an electric element as backup
Shockley-Queisser limit for a single-junction solar cell	About 33.7%¹¹⁶⁵	Photons with less energy than the band gap pass through and those with more waste the excess as heat.	Silicon's practical limit is about 29.4%; average commercial modules 22.7% (Q4 2024)
Induction heating ceiling	Close to 100% of the field energy can end up in the pan; measured hobs reach about 85%¹	Losses are in the coil resistance, the power electronics and heat from the pan to the air.	About 85%, against about 32% for gas on the same test
Charger standby loss	No-load limit 0.10 W for chargers up to 49 W in the EU, which is under 1 kWh a year if left plugged in (calculated)⁸	A charger draws a little power even with nothing attached, to keep its control circuit alive.	Modern gallium nitride chargers are small and efficient, but the limit is set by law, not physics
Battery round trip	Lead-acid 70-85%⁷	Charging and discharging lose energy as heat through internal resistance and side reactions; lithium-ion cells lose less.	High-frequency home UPS designs reach about 90% inverter efficiency

Sources

2021 ENERGY STAR Emerging Technology Award: Residential Induction Cooking Tops US EPA ENERGY STAR, 2021
FAQ: Solar water heating systems Energy Efficiency and Renewable Energy Management Centre, Government of NCT of Delhi, 2024
EESL unveils 5 star rated ceiling fans with BLDC technology in India Energetica India, 2022
Photovoltaics Report (updated 14 July 2026) Fraunhofer Institute for Solar Energy Systems ISE, 2026
Reassessment of the limiting efficiency for crystalline silicon solar cells (Richter, Hermle and Glunz, IEEE Journal of Photovoltaics 3(4)) IEEE, 2013
An open letter: BEE, make inverter efficiency standards mandatory (Kunwer Sachdev, founder of Su-Kam; advocacy, not independent data) kunwersachdev.com (personal blog), 2026
Commission Regulation (EU) 2019/1782, Annex II: Ecodesign requirements for external power supplies legislation.gov.uk (retained EU law text), 2019
Tabulated values of the Shockley-Queisser limit for single junction solar cells (Ruhle, Solar Energy 130, 139-147) Elsevier, 2016