Wikipedia:Reference desk/Archives/Science/2021 September 27

= September 27 =

Why bulbs are sell in terms of watts instead of amperes or voltage?
Why bulbs are sell in terms of watts instead of amperes or voltage?

If laptop chargers are sell in terms of voltage and ampere: here, but why bulbs are sell in terms of watts like hereinstead of amperes or voltage? Rizosome (talk) 04:29, 27 September 2021 (UTC)
 * The wattage tells you the relative brightness of the light. ←Baseball Bugs What's up, Doc? carrots→ 04:31, 27 September 2021 (UTC)
 * Brightness, or, rather, luminous power, is measured in lumen (unit). A 25 W incandescent light bulb will typically deliver something like 375 lm. A 25 W LED bulb will deliver more like 2000 lm. The wattage tells you the energy consumption, which for incandescent bulbs is mainly wasted on heat. --Lambiam 09:54, 27 September 2021 (UTC)
 * Sure. But experience tells us that the higher the wattage of an ordinary incandescent bulb, the brighter it is. And we select wattage based on how bright we want it. 60 watts is typical. ←Baseball Bugs What's up, Doc? carrots→ 10:55, 27 September 2021 (UTC)
 * And how much electricity it uses. Chargers have many different output voltages, so it is important to be able to select the correct one for your appliance.--Shantavira|feed me 08:31, 27 September 2021 (UTC)


 * Light bulbs use the main supply, which is a fixed-voltage, as-is; while its exact source depends on the country it usually is roughly 230V of AC current at 50 or 60Hz. As a consequence, in a given country, rating light bulbs by intensity or by wattage is equivalent, and there is no voltage rating to give since you have no choice as to what voltage you use at home.
 * There is also another complication linked with the different technologies of light bulbs. The power a bulb consumes is always equal to voltage times intensity (root mean square-d for AC current) whatever the technology, but the efficiency by which they convert this into light depends on whether they are LEDs, incadescent bulbs, phosphor, halogen lamps etc. I used to see many bulbs marketed as "X watts, but equivalent to Y watts of incandescence" (with the Y value given much more prominence than the X value on the packaging) five years ago - I assume the manufacturers thought that a consumer trying to replace a 60W incandescent bulb would not buy an LED rated 10W "because I do not want something six times dimmer".
 * Laptop chargers, or any appliance that uses DC voltage from the AC supply, are a different thing entirely. They modify the current supply from AC to DC and also change the voltage. As a consequence, they have to specify their output more fully, so that (for instance) you can check if the charger you got by buying laptop A is compatible with laptop B. Taking as an example the image you linked to, INPUT: 100-240V~2.34A50-50Hz means in order
 * 100-240V: "will accept voltages between 100V and 240V" (i.e. almost every grid system around the world)
 * ~: "of alternating current"
 * 2.34A: "might draw up to 2.34A from the supply"
 * 50-60Hz: "will accept frequencies between 50 and 60 Hz"
 * and similarly OUTPUT: 19.5V⎓9.23A means
 * 19.5V: "will output 19.5V"
 * ⎓: "of direct current"
 * 9.23A: "from which you can draw up to 9.23A"
 * In this case, the wattage of that charger is about 180W (19.5V times 9.23A), if used with a compatible laptop (that draws the maximum power). Tigraan Click here for my talk page ("private" contact) 09:18, 27 September 2021 (UTC)
 * Note that in some parts of the world where incandescent bulbs are no longer available, their LED equivalents are sold with a lumen rating rather than unhelpful wattage. E.g. Bazza (talk) 09:44, 27 September 2021 (UTC)
 * So, as with wattage, you have to learn by experience, how bright a given level of lumens is. ←Baseball Bugs What's up, Doc? carrots→ 15:34, 28 September 2021 (UTC)
 * Currently in the UK the packaging usually indicates the bulb's actual wattage, its lumen output, and what the wattage of a traditional incandescent bulb of the same brightness would be. That of any incandescent bulbs still on sale often also quotes their lumen values, so there are various available comparisons. {The poster formerly known as 87.81.230.195} 90.200.67.3 (talk) 13:43, 29 September 2021 (UTC)