# Power of 40w Bulb Use at 120v

1. Feb 21, 2012

### kgal

How many watts should a 40w bulb use at 120v?

2. Feb 21, 2012

### Staff: Mentor

40 watts?

wikipedia has a discussion on watts and watt-hours that might clear up things.

3. Feb 21, 2012

### AlephZero

THe bulb would be marked as using 40W at a specified voltage. If you don't know that information, you can't answer the question.

4. Feb 21, 2012

### mathman

In the U.S. standard voltage is 120 volts, so light bulbs will use the wattage as marked.

5. Feb 21, 2012

### cmb

Due to what I believe is partly an historic anomaly, bulbs in the UK are rated at 240V, but 230V is the EU voltage standard which should be delivered to UK homes.

I think in part it is usual to rate the bulb at a higher voltage as the life expectancy [of filament bulbs] drops exponentially once above the rated voltage. So bulbs may be sold into a market with voltage ratings slightly in excess of the specified volts.

Electrical supply into homes is rarely that accurate, at least in UK. My own house suffered for years at variable under-voltage. We share a relatively small spur with an industrial estate. It dropped (I kid you not) to 183V on one occasion I was measuring it myself. I complained to the elec co and they did some work. I then had >250V for several months until I complained enough they installed Â£70,000 of new kit in the local sub-station.

6. Feb 21, 2012

### Naty1

That's a power use figure, like one on a frig or work tool.....it's not an indicator
of how much light it produces.....

7. Feb 21, 2012

### cmb

Hence the question "How many watts should a 40w bulb use..."

8. Feb 22, 2012

### pantaz

As previously stated, forty watts at the voltage specified by the manufacturer.

Due to a variety of factors, the measured power consumption will vary from the advertised value.

9. Feb 22, 2012

### cmb

Yes, I know that, but Naty 1 didn't appear to get the question.