Question on Wattage / Kwh calculations (or wattage vs consumption)

Louiscar

Hi,

Can someone help me understand the relationship between wattage and consumption.

For instance - my desktop computer has a  750w PSU but this does not by any means help me calculate what it costs to run it.

I don't have a smart meter but I do have a monitor and note  the following in relation to my desktop 'puter:

When browsing doing normal tasks this might show as anything between 180 - 250 Kwh. Play a game and that will jump another 150.

So with this in  mind I'm trying to assess where I can make savings but the question is how do I do that when I don't know this relationship.

I can calculate wattage > Kwh easily but I can't actually determine the consumption. Easy when I have a 100w light bulb - that is the wattage but my desktop??

Now to the think I need answers on (given my observation with the desktop): I am trying to make savings on my kitchen lighting - currently I have fluorescent lights under my cabinets. These consist of 4 x 8 w and 1 x 6w so it's easy this is 38watts.

Now I want to convert this to RGB lighting or led strips however, this isn't simple. I probably will have more than psu (DC) if rated at 19watts this is the desktop conundrum to me.

It appears that the wattage of the psu is not the calculating factor as this seems to indicate a maximum and would depend on how much current is drawn by the led strips.

So I might need 2 or 3 separate PSUs but I really cant add these up  as if I did (say  2 x 19w) I'd come to the conclusion that it'd not be cheaper to run.

But I suspect that like my desktop pure wattage is not relevant here there must be another way to figure out if RGB led lighting would be cheaper to implement and if so how do I then calculate this?

Any electricians that can help me with the calculations?

Spies

Watts is a measure of instant power, say you had a constant use of 200 Watts over 1 hour, you have used 0.2kWh.

If you pay 35p for electric, then 0.2*35=7

So 7p an hour to run based on that unit rate. 

[Deleted User]

Use the computer for a set amount of time for a representative mixture of things.  Note down the total energy used (in kWh), divide by the number of hours, and you have the average wattage.  It's only the average wattage for that particular mix of things, but if you do it a couple of times you will come up with a good average overall.

Either that, or as you have done - the wattage for normal use is 200W, but the wattage for gaming use is 350W.

[Deleted User]

Same sort of thing would work for your lights. The PSU would be rated at 19W maximum (like your computer is 750W maximum), but what actually uses the power is the lights (+ inefficiency in the PSU).

Add up the wattage of all the LED strips that you would want to put in, just like you would add up the lightbulbs.  Then add a little bit (these PSUs are pretty efficient) and you have the total.

TadleyBaggie

The 180-250 is watts not kWh, using it for an hour would result probably less than 0.25kWh used.

The 750 watt PSU is just the maximum it could provide to the computer electronics, usually it will much less than this.

Louiscar

[Deleted User] said:

Same sort of thing would work for your lights. The PSU would be rated at 19W maximum (like your computer is 750W maximum), but what actually uses the power is the lights (+ inefficiency in the PSU).

Add up the wattage of all the LED strips that you would want to put in, just like you would add up the lightbulbs.  Then add a little bit (these PSUs are pretty efficient) and you have the total.

Thanks SparkyGrad, Just managed to see this on the specs for an LED strip just now. So I'm looking at typically 4.5 - 5.5 watts per meter so now I can see it as I am probably in for around 4.5 meters so I can roughly halve the consumption of my current fluorescent lights.

I also presume that if I use RGB lighting where they can be dimmed I can expect lower wattages.

Louiscar

TadleyBaggie said:

The 180-250 is watts not kWh, using it for an hour would result probably less than 0.25kWh used.

The 750 watt PSU is just the maximum it could provide to the computer electronics, usually it will much less than this.

Hi TadleyBaggie, you are right it's wattage indicated on the monitor and I did know that but wasn't clear about it. Thanks for pointing this out.

[Deleted User]

Depends how they are dimmed - if it's just dumping the excess input energy as heat then there would be no reduction.

Louiscar

And thanks to all others that gave input. Much appreciated

Louiscar

[Deleted User] said:

Depends how they are dimmed - if it's just dumping the excess input energy as heat then there would be no reduction.

OK also useful information. I will exclude that from consideration in that case and just calculate worst case. It still appears it's worth it.

I did want to do this some time ago but seemed a bit complex as I have 3 separate zones and therefore each would need it's separate PSU as I don't want wires going from one zone to the  other especially as one would cross above the cooker hob.

I think I can reduce this to 2 zones, so 2 PSUs and still 4.5 ft of strips in total (I said meters but it's 4.5 - 5.5 watts per foot not meter).

As with everyone I'm sure desperately trying to reduce consumption and not have to live in complete darkness :-)

[Deleted User]

Saving 10 watts on your lights for 8 hours would be the same as using the computer for 25 minutes less (or gaming for 14 minutes less).  Don't panic about it.