How much difference is made on battery life when keeping mobile data switched on along with WiFi ?

There is no reason why you have to turn off mobile data while you're connected to Wi-Fi. Your phone will automatically switch over when it's no longer connected to Wi-Fi and it does not use both at the same time. I have never heard that turning off mobile data while connected to Wi-Fi saves battery life. However, turning off Wi-Fi when you're using cellular data may increase battery life (but not the other way around) and this is pretty convenient because you can do this in the Control Center.

I've never seen the recommendation to turn off cellular data when connected to Wi-Fi. Cellular data is an important feature of the iPhone, so I wouldn't recommend turning it off for regular use. It is true that 3G and 4G connections will consume more power than 2G, but they are also much faster. When you're connected to a Wi-Fi network, your phone will use that and not cellular data, so it shouldn't have any noticeable impact.

kshikhar wrote:

Are you sure?

Because I've heard that 3G/4G, when turned on, keeps consuming battery in the background.

If you're phone auto locks, it will switch back to cellular unless you have it plugged in. However, as long as your connected to WiFi, cellular data won't use much battery life.

When your phone is not in use, it will switch over to a cellular connection only. Maintaining a connection to Wi-Fi when it's not in use will consume power, so your phone automatically disconnects from Wi-Fi when you're not using your phone (unless it's charging). It still needs a network connection so you can receive notifications, calls, etc. and may temporarily use mobile data for this, but it has minimal impact on battery life.

I don't know what else to offer, Meg and Original said it pretty well. Yes if your phone goes to sleep while not plugged to a charger it will revert to cellular but that is a very small amount of battery usage. I never turn my mobile data off while on wifi and my phone lasts 1-2 days without charging.

You are welcome. My wife and I both have iPhone fives (not 5s) and iPad Airs. We like the size of the 5 and if I was to replace it today I would get an SE. When we travel we take our iPads as well and we find the combination of a 4" phone plus a 9.7" iPad pretty convenient.