Very often I hear the statement – don’t leave your phone registered to the UMTS (3G) or GPRS (2G) network, as this will decrease its battery life and you’ll need to charge it more often.

Today I decided to bust this myth and also to compare if UMTS (3G) uses more battery power than the slower GPRS (2G) transfer standard.

The theoretical speeds of the GSM standards in question follow:
– GPRS (2G) – up to 114 kbps (~14 Kbytes/s)
– UMTS (3G) – up to 2 Mbps (~250 Kbytes/s)

The following was tested with a Nokia 5800 XpressMusic via a Bluetooth connection to a computer. The Bluetooth connection itself uses some power but my measurements show that its usage is about 0.15W, so we will consider it negligible for our benchmarks.
The power usage was measured using Nokia Energy Profiler.

So here are the end results about power usage in different scenarios:

• 0.40W – no connections at all.
• 0.40W – no Bluetooth connection, just registered to 2G or 3G network.
• 0.52W – Bluetooth and 2G or 3G connections established, but connections are idle.
• 1.27W – 8 Kbytes/s (64 kbps) download via 2G.
• 1.69W – 65 Kbytes/s (520 kbps) download via 3G.
• 1.73W – 120 Kbytes/s (960 kbps) download via 3G directly from mobile phone, no Bluetooth. If we add the Bluetooth power usage, this would sum to up to 1.90W.

The myth is busted: Leaving your phone registered to the 2G or 3G network shouldn’t drain up your battery faster, if you make no traffic to the 2G or 3G network.
Update: Maybe I spoke too early. As “varnav” commented below, the 3G mode may make your talk time 2+ times less. At least that’s what the technical specifications of iPhone show. I haven’t tested this but I doubt it that Apple haven’t, so they are probably right, at least for their iPhone models. 🙂
Conclusion: You should use 3G if you are about to make data transfers and probably it is better to turn it off after that. Nokia 5800 has an option to enable the 2G/3G data connection only when data is about to be transferred, and this is what I currently use as settings.

Let’s analyze if 2G or 3G uses more power if you start making data transfers. First impression is that 3G uses more power. However its transfer speed is much greater, so the overall Watts usage for the same downloaded size will be smaller, which makes 3G more efficient and thus less power consuming in practice.

Here is an example to illustrate the above statement. Let’s assume that we want to download 1000 Kbytes:

• 2G: 1000 Kbytes at download speed 8 Kbytes/s will take 125 seconds. At the power usage of 1.27W this will take about 159 Watts-seconds (125 seconds * 1.27 Watts).
• 3G: 1000 Kbytes at download speed 65 Kbytes/s will take 15 seconds. At the power usage of 1.69W this will take 26 Watts-seconds (15 seconds * 1.69 Watts).

Therefore, using 3G the consumption has decreased to 16.3%! Yes, in this case 3G would use six times less energy from the phone battery, if we downloaded the same amount of data using 2G, at maximum speeds.

One interesting power usage pattern by 3G is that when you stop the data transfer, the power usage doesn’t drop immediately to the full idle state. Instead, about 0.80W is drained for about 30 seconds, and then the connection is put in total idle state. Review the power usage images below, in order to get a better idea of what I mean. Even if we add up these 24 Watts-seconds (30 seconds * 0.80 Watts) to the above calculations, 3G would still use 318% less energy.