Efficiency Comparison
Part of Lighting
Comparing the luminous efficacy, lifespan, and practical tradeoffs of candles, gas lamps, arc lamps, incandescent bulbs, fluorescent tubes, and LEDs.
Why This Matters
Light is one of the most important products of energy. The efficiency with which a civilization converts energy to useful illumination determines how much fuel, wood, or electricity must be generated to maintain a given quality of life. An LED light provides 100 times more light per watt than a candle. This 100× efficiency gain means that 1% of the firewood a pre-electric household burned for candles, burned to generate electricity, can illuminate the same space more brightly with LED lights.
For a rebuilding civilization, understanding lighting efficiency enables rational decisions about technology investment and resource allocation. The question is not just “does this light source work?” but “how much of our limited energy budget does this type of lighting consume, and is there a better option available to us?” These comparisons inform the priority sequence for developing different lighting technologies.
Luminous Efficacy: The Key Metric
Luminous efficacy is the ratio of luminous flux (lumens) to power consumed (watts). Its unit is lm/W (lumens per watt). A higher number means more light per unit of energy.
Efficacy values across technologies (approximate ranges):
| Light Source | Efficacy (lm/W) | Lifespan (hours) |
|---|---|---|
| Candle (tallow) | 0.1 | — |
| Candle (beeswax) | 0.2–0.3 | — |
| Oil lamp (flat wick) | 0.2–0.4 | — |
| Oil lamp (Argand burner) | 0.8–1.5 | — |
| Gas mantle (coal gas) | 1–2 | — |
| Gas mantle (pressure kerosene) | 2–4 | — |
| Carbon arc lamp | 3–8 | 1–4 hrs/electrode |
| Carbon filament incandescent | 1.5–4 | 200–300 |
| Tungsten incandescent (60W) | 10–15 | 1,000 |
| Tungsten halogen | 15–25 | 2,000–4,000 |
| Mercury vapor | 25–50 | 8,000–15,000 |
| Fluorescent T12 (old type) | 40–60 | 7,000–10,000 |
| Fluorescent T8 (modern) | 60–100 | 15,000–25,000 |
| High-pressure sodium | 80–130 | 15,000–25,000 |
| LED (current, 2020s) | 80–200 | 25,000–100,000 |
Pre-Electric Lighting: Setting the Baseline
A standard tallow candle produces approximately 12 lumens from burning 7.5 grams of tallow per hour. The tallow energy content is about 37 kJ/g, so: power = 7.5 × 37 / 3600 = 77 W of thermal power. Efficacy: 12 / 77 = 0.16 lm/W.
A room requiring 200 lumens for basic task lighting (reading) needs 17 candles. Each candle burns through 7.5 g/hr, so 17 candles consume 127 g/hr of tallow. A year of 5 hours/night illumination: 127 × 5 × 365 = 232 kg of tallow per household per year — a significant agricultural burden.
The Argand oil burner (1780s) improved over flat-wick lamps with a cylindrical wick and glass chimney that increased combustion air supply. It achieved 6–15 times the light output of a candle from similar fuel consumption, bringing efficacy to ~1.5 lm/W. This was a significant improvement that changed household economics.
The gas mantle (Welsbach mantle, 1890s) achieved 2–4 lm/W from coal gas or kerosene vapor — a further improvement. Gas street lighting in the 19th century was a genuine technological advancement: a single gas lamp produced as much light as 8–12 candles at one-quarter the operating cost.
Electric Lighting: The Step Change
The first practical incandescent lamps (Edison’s carbon filament, 1879) produced only 1.5–4 lm/W — comparable to or only slightly better than gas mantles. Their advantage was not efficiency but safety (no open flame) and convenience (switch on/off instantly).
Tungsten filament lamps (introduced around 1906) improved efficacy to 8–12 lm/W. The gas-filled tungsten lamp (1913) reached 10–15 lm/W by slowing evaporation of the filament in nitrogen/argon fill gas. This was the standard household incandescent for most of the 20th century.
Fluorescent lamps (developed commercially 1940s): a quantum jump in efficacy. The mercury vapor discharge produces UV, which excites fluorescent phosphors coating the tube interior, producing visible light. Total efficacy 40–100 lm/W — 4–10 times better than incandescent. Fluorescent lamps require a ballast and are more complex to manufacture, but their efficiency advantage is decisive for any application where they work.
LEDs (light emitting diodes): the current state of the art for solid-state lighting. Modern LED lamps achieve 80–200 lm/W in commercial products, with laboratory devices exceeding 300 lm/W. They also offer very long life (25,000–100,000 hours versus 1,000 for incandescent). LED manufacturing requires advanced semiconductor fabrication, but salvaged LEDs from existing infrastructure provide an immediate bridging supply.
Comparing Total Lighting Energy Cost
For a rebuilding community making decisions about technology investment, the comparison should include total energy cost for equivalent illumination — not just efficacy.
Scenario: illuminate a 20 m² workshop to 300 lux (adequate for assembly work). Required lumens: ~6,000 lm.
Candles: 500 candles (6000 lm / 12 lm per candle). Power equivalent: ~39 kW thermal. Cost in firewood for 8-hour workday: enormous — clearly impractical.
Kerosene pressure lamp (3 lm/W): need 2,000 W of fuel energy. About 0.3 L/hr of kerosene per 1,000 W. So 0.6 L/hr. In 8 hours: 4.8 liters of kerosene per day — significant recurring cost.
Incandescent (12 lm/W): need 500 W electrical. In 8 hours: 4 kWh. With a diesel generator at 30% efficiency: about 1 liter of diesel per kWh → 4 liters per day.
Fluorescent (80 lm/W): need 75 W electrical. In 8 hours: 0.6 kWh. With same generator: 0.6 liters per day.
LED (150 lm/W): need 40 W electrical. In 8 hours: 0.32 kWh. With same generator: 0.32 liters per day.
The ratio from incandescent to LED is roughly 12:1 in energy cost for the same illumination. This is why switching to LED lighting is one of the highest-leverage efficiency improvements possible in the electrical use of any early-recovery community.
Technology Sequencing for a Rebuilding Civilization
Practical sequence based on manufacturing accessibility and impact:
First available: candles and oil lamps. Make immediately from local materials. Low efficacy but no electricity required. Focus on improving wick design (braided cotton) and lamp design (Argand type) to maximize output per unit of fuel.
As electricity becomes available: salvage incandescent bulbs from existing infrastructure. They are simple to use, work on AC or DC, and require no ballast. Low efficacy but acceptable for priority applications (medical, security, workshop).
Next priority: salvage or produce LED lighting. LED lamps operate on 12–24 V DC (with LED driver) or on 230/120 V AC. Even if local manufacturing cannot produce LEDs, salvaged LEDs from abandoned buildings provide enormous efficiency gains. A single building’s LED lighting inventory could supply a community for years.
When manufacturing capability exists: produce LED drivers from available components. An LED driver is an electronic circuit that converts supply voltage to the constant current LEDs require. Simpler than other electronic manufacturing, it enables production of new LED lighting from salvaged LED components.
Fluorescent lamps: useful when LEDs are unavailable. Require ballasts, starters, and mercury-containing tubes. More complex than LEDs but still 5–7× better than incandescent. Appropriate bridging technology.