Energy in farming: Science and limits
Why crops catch so little sunlight, and cattle waste feed
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.Farming is a solar energy business: crops capture a few per cent of sunlight at best, and farmers add fossil and electrical energy (diesel for tractors and pumps, electricity for irrigation, natural gas locked inside nitrogen fertiliser) to raise how much of that sunlight ends up as food.
The conversion, step by step
Photosynthesis in the cropSunlight → Chemical (sugars, starch, protein) · Theoretical maximum about 4.6% for C3 crops (wheat, rice) and 6% for C4 crops (maize, sugarcane); real fields are well below this8Fertiliser manufacture (off-farm)
Chemical (natural gas) → Chemical (ammonia, then urea and other nitrogen fertilisers) · Best plants about 66% of the theoretical minimum; world average about 45% (calculated from 28 and 41 GJ/t against 18.6 GJ/t)4Electric irrigation pump set
Electrical → Hydraulic (water lifted and moved) · Existing Indian farm pump sets about 25-30%; BEE star-rated sets about 50-60%5Diesel irrigation pump set
Chemical (diesel) → Mechanical, then hydraulic · About 25% for a new, well-matched diesel pumping plant (engine about 35% times pump about 75%); small, worn Indian sets are lower, roughly 12-15% (assumption)11Solar irrigation pump
Sunlight to DC electricity → Hydraulic · About 11% sunlight to water energy (calculated: 22.7% module efficiency times an assumed 50% for controller, motor and pump)9Livestock: feed to food
Chemical (feed energy) → Chemical (milk, meat, eggs) plus body heat and manure · About 7% of feed calories end up in food on average (US study): beef about 3%, pork 9%, poultry 13%, eggs and dairy 17%12
The science
Plants use light to split water and fix carbon dioxide into sugars. Several physical steps cap the efficiency: only about half of sunlight's energy is in wavelengths plants can use, each absorbed photon's extra energy above what the reaction needs is lost as heat, and in C3 plants an enzyme mistake called photorespiration wastes part of the captured carbon. C4 plants concentrate CO2 inside the leaf to avoid much of this, which is why their ceiling is higher. Nitrogen is usually the nutrient that limits growth; plants cannot use the nitrogen gas in air, so the Haber-Bosch process (see heavy industry) does the job with natural gas, and that embedded energy is often the largest energy input to a field of cereals. Water must be lifted from wells and canals: the hydraulic energy needed is mass times gravity times height, so lifting one cubic metre of water by 10 m needs about 0.027 kWh (calculated), and a pump set at 25% efficiency draws about 0.11 kWh to do it (calculated). Animals burn most of the energy they eat to keep warm, move and grow bones and organs; on average only about 7% of feed calories become milk, meat or eggs.
Limits set by science
| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| Photosynthetic ceiling | About 4.6% (C3) and 6% (C4) of total sunlight8 | Losses from unusable wavelengths, reflection, conversion of photon energy to chemical energy, photorespiration and the plant's own respiration. | Breeding and genetic research aim to cut photorespiration and improve canopy light use |
| Energy to lift water | 0.027 kWh per cubic metre per 10 m of lift (calculated: 1,000 kg x 9.81 m/s2 x 10 m)5 | Fixed by gravity; deep wells cost proportionally more. Losses come from the motor, pump, pipes and foot valves. | Star-rated pump sets reach about 50-60% overall, roughly double the existing fleet |
| Minimum energy for nitrogen fertiliser | 18.6 GJ per tonne of ammonia net, about 23 MJ per kg of nitrogen (calculated, ammonia is 82% nitrogen)4 | The energy is mostly in the hydrogen taken from methane. Today's average of 41 GJ/t is about 50 MJ per kg of nitrogen (calculated). | Best gas plants 28 GJ/t net, about 34 MJ per kg of nitrogen (calculated) |
| Solar cell limit for solar pumps | Silicon practical limit about 29.4%109 | The same limits as any solar panel; the pump and motor then lose part of the electrical output. | Average commercial modules 22.7% (Q4 2024) |
Sources
- Ammonia Technology Roadmap IEA, 2021
- Agricultural Demand Side Management (Ag-DSM) Program in India: Adopting technologies to boost efficiencies (white paper) (2016 data) ICF International, 2016
- What is the maximum efficiency with which photosynthesis can convert solar energy into biomass? (Zhu, Long and Ort, Current Opinion in Biotechnology 19) University of Illinois / Elsevier, 2008
- Photovoltaics Report 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
- Basics of on-site pumping plant evaluations (Pub. 3241-F) LSU AgCenter, 2014
- Energy and protein feed-to-food conversion efficiencies in the US and potential food security gains from dietary changes (Shepon et al., Environmental Research Letters 11, 105002) IOP Publishing, 2016