Sky is to launch its own 'piggyback' mobile network next month with Sim-only data plans that let you roll over unused data for up to THREE years.

Sky Mobile's trio of flexible plans based on data usage start from £10/month for 1GB, with the option to add texts and calls at extra cost – unless you're an existing Sky TV customer where they'll be free. The network will piggyback (use the signal of) O2.

Every plan includes data rollover where unused data each month accumulates and can be used to top up your allowance in future – a new feature for the UK's mobile market.

Sky TV customers and those who have pre-registered in time will be invited to join Sky Mobile from mid-December, with everyone else able to sign up from January.

We take a look at how it works, what it costs and whether it's any good. (See our Cheap Sim-only guide for our top-pick deals.)

Martin Lewis
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How it works and what it costs

For now you'll only be able to get Sim-only plans through Sky, though handset contracts are planned for the spring. Their plans work a little differently from traditional tariffs in that they don't come with any minutes or texts as standard – only data.

If you want to make calls or send texts you'll be charged 10p/min and 10p/text, or choose to pay £10/month – on top of the data plan – for unlimited calls and texts. Sky TV customers get this, among other perks, included with their data plan for no extra charge.

Three plans will be up for grabs; all come with 4G data and include tethering (data sharing via a hotspot):

Sky Mobile Sim-only plans

Data plan Non-Sky TV customers Sky TV customers
Data only (1) Data + unltd mins/text Data + unltd mins/text
1GB £10/mth £20/mth £10/mth
3GB £15/mth £25/mth £15/mth
5GB £20/mth £30/mth £20/mth
Plans available from January (December for existing customers and those who have pre-registered). (1) Minutes and texts charged at 10p/min and 10p/text.

Data 'Roll'

Unused data each month accumulates in a so-called Sky Mobile 'piggy bank', which can be used to top up your data allowance if you run out in future months. Data is stored for up to three years and can be redeemed in 1GB increments.

Unlike other operators' data-rollover plans which only carry over one month, the unused data compounds month after month. Say you sign up for a 5GB plan and only use 3GB in your first month, the 2GB shortfall carries over, giving you a total of 7GB to use the next month. Only use 4GB in this second month and you'll add a further 1GB to your piggy bank, giving you a total of 8GB to use in your third month – and so on.

Alternatively, if you run out of data you can choose to pay the extra to move to a higher plan for the rest of the month, or buy a 1GB add-on for £10 (though this costs more than just moving up a plan).

'Mix'

Although a Sky Mobile plan is a 12-month term minimum, you can upgrade or downgrade between plans as you see fit. This can be done at short notice – to take effect from the start of the following monthly billing cycle.

Perks for Sky TV customers

For as long as you are a Sky TV subscriber you get a few perks for signing up to Sky Mobile (cancel Sky TV and you'll lose them).

  • As mentioned, unlimited calls and texts are included in your data plan without extra charge.

  • If you have Sky+ you also get Sky Go Extra – a feature that lets you download and watch content on other devices – free of charge (normally £5/mth, currently on offer at £2.50/mth).

  • Sky+ customers can also take advantage of 'Sync', a new feature that will allow you to stream or download your TV recordings on up to four mobile devices anywhere via a personalised playlist. Sky Q customers will be able to do this from next year.

Sadly, there are no Sky Mobile discounts or perks currently available for Sky Broadband customers without TV.

Sky Mobile is coming – here's how it works and what it'll cost
Sky Mobile is coming

Family friendly

Families will be able to take up to five Sims, all to be itemised on a single bill. The lead account holder can choose how much control they have over the plans in the family and monitor their allowance via the Sky website.

All unused data goes into a shared 'piggy bank' each month. If one of the family uses up their allowance the lead account holder can decide to give them saved up data from the pot, purchase another 1GB add-on or upgrade the plan of the user in question.

Sky says this will prevent any large bills being run up as you can't use any more data beyond your allowance without first deliberately choosing to do so.

So is Sky Mobile worth it?

On the face of it, the plans don't provide the most data you can get for the price. For example, Plusnet Mobile's* launch-offer gives you 1,500 mins and 4GB of 4G data for £10/month on a 30-day contract. The People's Operator* are currently flogging 600 mins and 2GB for only £6.99/month on a 30-day plan.

These are better-value deals on the face of it, though Sky's proposition becomes more attractive if you have Sky TV – or don't tend to call or text much – while its data rollover effectively puts more data in your pocket every month.

Sky Mobile could also suit families, putting control in the hands of the lead account holder and keeping all plans under a single bill. Having said that, BT's Family Sim* discounts each additional plan you take (and without requiring a 12-month commitment for each).

Ultimately compare what's out there and what best fits your circumstances. For more advice on choosing a top deal including our own top picks, see our Cheap Sim-only guide.

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