TR Forums

Hey dudes. Not sure where I should post this so maybe here's a good place.

See, at home I usually use my tablet or phone to surf the web. Guess I'm too lazy to sit in front of my PC. Using a low power device like a phone is quite obviously far less power hungry than using your PC, fans and platters spinning and separate display and all. Of course PCs are undeniably faster but I reckon surfing with your phone is fast enough assuming you've got a good phone.

So I've done some math to see just how many watt-hours my different devices probably pull. You obviously can't do this at the wall like Scott does with their hardware testing because they get power from their batteries so I think a different approach may be appropriate. I usually charge my devices with 5V/2A (10w) chargers and, depending on how long a device takes to charge and how long I usually use it before plugging it to charge again, we can easily determine how many watt/hours the device effectively pulls every hour (assuming 5V/2A chargers pull 10w off the wall too and disregarding battery inefficiencies).

For example:

Samsung Galaxy Tab S2 = 10w x 3 / 7hours = 4.28w/h
Acer Liquid Zest Plus = 10w x 3 / 15 hours = 2w/h

Of course the hours figures are rough figures only and the final w/h numbers are approximates but so are PC power draw numbers because that's the current nature of modern computing. For PCs though, at idle I reckon they would pull in something like 80w including the display, so that's straight up 80w/h, without gaming. As we can see, even at 5w/h a tablet is far cheaper to run.

Am I missing something here?

The ampere rating of chargers doesn't mean it's using that much. Devices may consume less, and definitely less as it slows down for the final stages of charging.
I also assume it's the output rating, so input will be more because of AC/DC conversion loses.

A modern PC consumes much less than 80W for light tasks. I don't think modern LCD monitors are that power hungry (also depending on size), so I expect overall less than 80W.

But still, sure, phone < tablet < laptop < desktop.

Yup... if you really want to figure it out you will need to measure the power draw of the charger at the wall over time as the device charges from empty to full, then track the run time you get for a typical task (say, web surfing) on a full charge.

The watt rating of the charger doesn't really tell you anything, since the device probably doesn't draw the rated output of the charger the whole time it is charging, and there are conversion losses as well.

Power usage is also going to be highly dependent on ambient lighting levels. Under brightly lit conditions the display backlight is a large -- if not the largest -- consumer of power in a mobile device, while in a dimly lit environment it is likely to be negligible.

To bypass the problem of metering the total power drawn by the charger during a charge cycle, you can make a rough guess that the battery's rated capacity in mAh, plus 20% for losses, is about what the charger will need to draw for an "equivalent billable hour" of electric use.  Example: If your tablet has a 5000 mAh battery at 3.7V, then assume that a full charge requires drawing an equivalent 6000 mAh, or 6A for one hour at 3.7V. Using the simple equation for power, P=V*I, then  6*3.7=22.2W.

If your utility bills by the kWh, then divide those Watts by 1000 to get kW, and multiply by the rate.  Ignoring the fixed-fee portion of my bill, which I will pay regardless simply for having a meter, I pay about $0.12 per kWh, so (22.2/1000) * 0.12 = a fourth of a cent.  Of course, this same math says that even if my PC somehow DID consume 80W of power (your number) to browse the Internet, a three-hour evening would cost me slightly less than three shiny pennies.  As long as the PC is set to sleep when not in use, I don't worry about it too much.  Your local cost of power will determine whether your usage would be noticeably expensive.

The simplest way is to just divide the battery capacity by the run time. Notebookcheck does web browsing battery life on phones and PCs. A quick check on some popular models suggests 1.2-1.5W for a smartphone and 6-8W for a laptop. I'd guess a large tablet is somewhere mid-way between them.

Desktops are rather inefficient by comparison. 80W is a high figure these days but I'd certainly say 40-50W was still typical unfortunately.

EndlessWaves wrote:

The simplest way is to just divide the battery capacity by the run time. Notebookcheck does web browsing battery life on phones and PCs. A quick check on some popular models suggests 1.2-1.5W for a smartphone and 6-8W for a laptop. I'd guess a large tablet is somewhere mid-way between them.

Desktops are rather inefficient by comparison. 80W is a high figure these days but I'd certainly say 40-50W was still typical unfortunately.

Spot on keep it simple.
Don't know if much has changed lately,but generally you have to put in X1.4 to get a full charge.
Reminds me of a conversation had earlier this year.
Thro phones and tablets use tiny amounts of power-their constant use,and billions of them in use must
have some effect on global warming.
Bit of a sidespin for you there Ronch