Learning Zone · Practical guide
Light bulbs compared: brightness, energy use and the right colour temperature
From old incandescent bulbs to LEDs: a plain-language guide to lumens, watts, Kelvin and running costs. Then we calculate how many hours of light a 150 Wp SunPower photovoltaic kit can support in energy terms.
- Compare bulbs by lumens first, not watts.
- A roughly 9 W, 800 lumen LED can replace a traditional 60 W incandescent bulb.
- At 3 hours a day, our LED example uses about 9.9 kWh a year; the incandescent uses about 65.7 kWh.
- Colour does not depend on watts: choose it using Kelvin.
A short history of the light bulb: from hot filament to semiconductor
The electric light bulb was not born from a single flash of genius. In the late nineteenth century several inventors worked on filaments, vacuum and service life. Joseph Swan and Thomas Edison helped make incandescent lighting reliable and commercially practical: current heats a filament until it glows. It works, but most of the energy becomes heat.
Halogen bulbs followed in the twentieth century. They still use a hot filament, but are somewhat more efficient and longer-lived. Compact fluorescent lamps became widespread in the 1980s and 1990s, cutting energy use sharply while introducing warm-up time, electronics and a small amount of mercury that requires proper end-of-life handling.
The LED breakthrough came from semiconductors. Efficient blue LEDs enabled modern white light and earned Akasaki, Amano and Nakamura the 2014 Nobel Prize in Physics. A good modern LED turns far more electricity into useful light and far less into heat than an incandescent bulb.
Lumens and watts: the difference that prevents most buying mistakes
Watts (W) tell you how much electrical power a bulb draws. Lumens (lm) tell you how much light it emits. Two bulbs with the same lumens give similar brightness even when their wattage is very different. Read the package rather than relying on memories of old bulbs.
Incandescent, halogen, CFL and LED compared
The table uses representative bulbs close to 800 lumens, the brightness traditionally associated with an approximately 60 W incandescent bulb. These are typical ranges: model, quality, temperature and fixture can change them.
Energy and efficacy
On a smartphone: swipe the table horizontally to view all columns.
| Technology | Typical power (W) | Efficacy (lm/W) | Energy per 1,000 h (kWh) |
|---|---|---|---|
| Incandescent | 60 | 10–15 | 60 |
| Halogen | 42 | 15–25 | 42 |
| Compact fluorescent (CFL) | 14 | 50–70 | 14 |
| LED | 9 | 80–120 | 9 |
Life and everyday use
On a smartphone: swipe the table horizontally to view all columns.
| Technology | Indicative life (h) | Start-up | Colour rendering | Note |
|---|---|---|---|---|
| Incandescent | 1,000 | Instant | Excellent | Very hot; inefficient technology |
| Halogen | 2,000 | Instant | Excellent | Hot; only modestly better than incandescent |
| Compact fluorescent (CFL) | 6,000–15,000 | May warm up slowly | Good | Contains mercury: use WEEE collection |
| LED | 15,000–50,000 | Instant | Good to excellent | Less radiant heat; electronics depend on quality |
Practical choice: LED is almost always the most efficient home-lighting option. Incandescent and halogen bulbs are useful historical benchmarks, not energy-saving choices.
How much does it cost to run a bulb?
Transparent example: 3 hours a day, 365 days and an assumed electricity price of €0.30/kWh. Replace €0.30 with the unit price on your bill.
Annual energy = watts × hours per day × 365 ÷ 1,000
On a smartphone: swipe the table horizontally to view all columns.
| Technology | Power (W) | Annual use (kWh) | Annual cost (€) | Saving vs incandescent (€) |
|---|---|---|---|---|
| Incandescent | 60 | 65.7 | 19.71 | — |
| Halogen | 42 | 46.0 | 13.80 | 5.91 |
| Compact fluorescent (CFL) | 14 | 15.3 | 4.60 | 15.11 |
| LED | 9 | 9.9 | 2.96 | 16.75 |
In this example, one 9 W LED saves about 55.8 kWh and €16.75 a year compared with a 60 W incandescent bulb. Ten equally used light points raise the order of magnitude to 558 kWh and €167.50 per year.
What to check on the box
- Lumens: amount of light.
- Watts: power drawn.
- Kelvin: warm or cool appearance.
- CRI or Ra: colour fidelity; 80 is common, while 90 or above is preferable where colour matters.
- Rated life and switching cycles: important in corridors, stairs and bathrooms.
- Dimmability: it must be stated; not every LED works with every dimmer.
- EU energy class: the current scale runs from A to G; a new label can look lower than the old A++ scale without the product being worse.
Colour temperature: what do 2,700 K, 4,000 K and 6,500 K mean?
Kelvin describes the appearance of the light, not physical heat and not brightness. Lower numbers look warmer and more yellow; higher numbers look whiter, bluer and more alerting.
On a smartphone: swipe the table horizontally to view all columns.
| Temperature | Appearance | Good uses | Watch out for |
|---|---|---|---|
| 2,200–2,700 K | Very warm / amber | Bedroom, lounge, hospitality | Relaxing, but can shift cool colours |
| 2,700–3,000 K | Warm white | Homes, hotels, reading lamps | Closest common choice to incandescent |
| 3,500–4,000 K | Neutral white | Kitchen, bathroom, office, workshop | Balanced; also check CRI |
| 5,000–6,500 K | Cool white / daylight | Garage, precision tasks, some work areas | Can feel harsh or less relaxing at night |
The background colours are illustrative only: a screen cannot reproduce a lamp faithfully. Kelvin, lumens and CRI describe three different qualities.
From the light bulb to photovoltaics: producing the energy we use
Cutting demand comes before generating energy. Replacing 60 W with 9 W asks the solar system for about 85% less power for similar light. That is why efficiency and photovoltaics work so well together.
Three units, three meanings
- 9 W: power drawn while the bulb is on.
- 45 Wh: energy used by a 9 W LED over 5 hours.
- 150 Wp: the photovoltaic generator’s rated peak power under test conditions. It does not mean a constant 150 W all day.
The 150 Wp SunPower kit in our example
The MR WATT product contains 40 SunPower Maxeon Gen III cells rated at 3.72 Wp each: 40 × 3.72 = 148.8 Wp, marketed as 150 Wp, plus connection accessories. It is intended for building a custom module.
Important: this is a cell kit, not a ready-to-use lighting system. Safe use requires electrical design, soldering, backing and encapsulation; energy storage also requires a charge controller and battery. A 230 V bulb additionally needs a suitable inverter. For a small system, a DC LED with the correct driver is often more efficient.
Calculation: how long can a 9 W LED stay on?
We use a cautious, repeatable estimate: daily energy = 150 Wp × peak-sun-hours × 0.75. The 0.75 factor represents temperature, wiring, controller, battery and conversion losses together. It is not a production guarantee.
On a smartphone: swipe the table horizontally to view all columns.
| Peak-sun-hours | Estimated useful energy (Wh/day) | Equivalent 9 W LED hours | Equivalent 5-hour nights |
|---|---|---|---|
| 1.5 | 169 | 18.8 | 3.8 |
| 2 | 225 | 25.0 | 5.0 |
| 3 | 338 | 37.5 | 7.5 |
| 4 | 450 | 50.0 | 10.0 |
| 5 | 563 | 62.5 | 12.5 |
With 3 peak-sun-hours, the balance gives about 338 useful Wh: on paper, enough energy for 37.5 hours of a 9 W LED or 7.5 five-hour evenings. This does not mean the bulb can run for a week without sun: actual battery capacity decides that.
What if we want it on for 24 hours?
A 9 W LED running for 24 hours uses 216 Wh a day. With the 0.75 performance factor, the kit needs an average of about 1.92 peak-sun-hours a day to balance that energy: 216 ÷ (150 × 0.75). Winter, shade and long cloudy spells require margin and storage.
Battery example for five evening hours
The bulb uses 45 Wh. In a 12 V system, assuming 80% usable battery energy and 90% circuit efficiency, the theoretical minimum capacity is 45 ÷ (12 × 0.80 × 0.90) = 5.2 Ah. A nominal 7 Ah battery can cover the example in ideal conditions; chemistry, temperature, desired autonomy and manufacturer limits may require a larger size.
Correct flow: assembled and encapsulated cells → charge controller → protected battery → compatible LED driver → lamp.
Five things the simplified calculation must not hide
- Peak-sun-hours change with location, season, tilt and shade.
- A DIY module may not reach 150 Wp if cells, joints or encapsulation introduce losses.
- Battery capacity limits how much daytime energy reaches the night.
- Running an inverter only for one small bulb can waste a meaningful share of the energy.
- Voltages and currents must match: never connect cells, battery and lamp by guesswork.
Frequently asked questions
Does a 9 W bulb always equal a 60 W bulb?
No. It is a typical equivalence near 800 lumens. Always check lumens, beam angle and fixture.
Does a 6,500 K lamp use more energy than a 2,700 K lamp?
Not necessarily. Energy use depends on wattage and efficacy; Kelvin mainly describes colour appearance.
Do LEDs produce no heat?
They produce less radiant heat than filament lamps, but their electronics and heat sink still warm up. An enclosed fixture can shorten life and reduce performance.
Does the 150 Wp kit power a bulb directly?
Not as sold. The cells must be built into a protected module with compatible electronics. Night-time use also needs a battery.
Why discuss lumens before solar power?
Because the panel must supply the load. For equal light, an efficient load reduces the panel, battery and cost required.
Sources and further reading
- European Union — Ecodesign and energy labelling for light sources
- U.S. Department of Energy — Consumer Guide to Energy-Efficient Lighting
- Nobel Prize — Efficient blue LEDs and white light
- Tsao et al. (2010) — Solid-state lighting: an energy-economics perspective
- MR WATT — 150 Wp SunPower photovoltaic kit
Equivalences and calculations are educational examples. Always check the bulb label, component datasheets and applicable electrical rules. Use qualified personnel for grid connections or battery systems.