Hi ah ok. I don't have one of those. My concern is that when we switch over to DV and there's a power cut as our phone mobile signal is often is poor we'll loose ability to make calls. the mobile signal is so poor we rely on wifi calling when at home! BT's recommended approach is to provide a battery backup for the SH2 so that it continues to work in a power cut. Just hoping to get BT to do what their policy is!!
If there was a power cut you would have lost Wifi calling in any event even before you move to DV because your router would stop working and as such you would have no wifi signal.
Also if you are presently using a wireless phone with a base station (DECT) you would also loose the ability to use that at present if there was a power cut so it appears to me that you are really no worse off if you were to move to DV and there was a power cut.
The actual running time would be much longer, as the SH2 and a DECT phone are unlikely to need less than 30W.
There is a post on this forum from someone using this arrangement, and they said how long it lasted.
You may find the whole thread of interest.
If power cuts are an issue consider a generator. That way you will have more than a phone line working during a cut.