Communication loss in a power cut without a landline

[#1]

I suspect that, ultimately, there is no simple and entirely satisfactory general answer to this matter - beyond having your own standby generator*.  Which remains an inherent flaw with the digital network.

* Electricity companies can and do provide standby generators to individual homes in case of domestic power loss in some circumstances - I have seen this locally myself. This may be the only real answer for homes with vulnerable individuals in some cases.

[#2]

A simple answer may be stay put & don't panic !

Recall the storms of 1987 ?  I'm sure the OP does.

[#3]

It all comes down to probability and budget. One easy option for long term availability is to get a camping battery ‘power station’ with an add on solar panel. You don’t need power after going to bed and the solar panel should keep it going and the battery recharged during the rest of the day. You just need to make up a few leads to use the 12V output to power the router and ONT.

I have one of these units that can be recharged if required by solar panels, a USB input from the car or simply taking it to a friend who still has power and plugging it into the mains.

The thing to remember is that if you want a sensible up time from a battery inverter, not to use the mains voltage output.

[#4]

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@MisterW wrote:

I suspect your relative has been converted to 'SoTAP for Analogue' , see here for some details https://www.ispreview.co.uk/index.php/2023/11/openreach-prep-alternative-uk-analogue-style-phone-pro....

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I watched the video download in that article.  The lack of gravitas coming from many of the BT 'professionals' was disconcerting.  That was a briefing to the CP industry and the BT folks appeared to be well outside of their comfort zone.

[#5]

There are a few things you need to take into account if you are looking at keeping a phone line active during a power cut.

With the old analogue system, the only phones, that is those connected to a BTB/openreach phone jack that will work are corded, non powered phones, that is those that have no mains adaptor plugged into the mains, only a single cord that plugs into the BT(openreach) socket.

 Even then if the power lines are bought down by a storm/weather the same thing can also bring down overhead phone lines.

I ported my landline number to voipfone, i have full fibre.

 I have also had power cuts, some short some longer ( 2 hours+) mobile phone signals/reception also drops out after 15 minutes of no power , because of this i have a basic analogue non-cordless jack powered phone, previously this would work plugged into the line jack.

My set up has so far kept everything going for roughly 5 hours ( longest power cut since full fibre/voip port) and consists of the following

 Openreach ONT, input power fed from a battery back up 

Phone ported to voipfone, runs on a Grandstream HT801 - this in turn has a USB power input, which is powered from a usb port on battery back up unit 2, 

Router, connected to ONT via a long Ethernet cable, the Router and Grandstrsam HT801 are connected to a second battery back up.

 The grandsteam HT801 analogue phone adaptor has two phones connected, a cordless phone with multiple cordless handsets and the analogue line jack powered phone.

The HT801 was purchased from voipfone, it was pre-configured for their service, but is unlocked so can be reconfigured to work with another VOIP service should i wish to port the number elsewhere.

Equipment used : talentcell back up units ( amazon "TalentCell Mini UPS Uninterrupted Power Supply 27000mAh")

Grandstream HT801 - purchased from voipfone 

voice service - voipfone £6/month with 100 minutes included

Cat 7 slim 4mm diameter 15metre long ethernet cable ( between ont and router) 

[#6]

Thanks for all detailed info including the kit details. That certainly gives me something to check out.
We had yet another power cut last week so that’s about 4 in the last year. After having no such issues in the previous decades, I can’t ignore the risk.

[#7]

While a far more extreme option, a 'house battery' might be worth looking at as it can allow you to run on 24/7 cheap rate electricity - and get the backup in power cuts as a side benefit. 

It is primarily going to work financially if you are electrically heated - rather than gas or oil.

[#8]

A Tesla Powerwall 3 at almost £10,000 would run electric heating for our size of home for barely 3 hours.

I suspect folks might consider say £250 for a UPS as where they draw a line.

[#9]

Heading off topic a bit, but I have:

£8k battery system at 15kWh, runs the 3-bed bungalow all day (it has controlled electric heat on top of off peak storage / water)

I have zero peak power use, all at 9p.

it is though a Swedish 'kit' with high insulation and heat recovery air