From their Spreading Warmth page:
You are a new customer, or currently on one of the qualifying tariffs chosen by npower from time to time. Qualifying tariffs include our standard tariffs and some fixed price tariffs, but others may be added.
If you are on a non-qualifying tariff but you otherwise qualify, you can if you wish change to a Spreading Warmth tariff but you would lose any benefits specific to your tariff. You would be advised about this if relevant to you.
You have to study very carefully the prices of the tariff they may put you on.
Where I am, 5475 units of electricity (15 x 365) would cost £538 from British Gas Click 6 whereas nPower would charge £707. That's £169 more expensive. And that is even if they let you stay on an online tariff. If they bumped you off on to the standard tariff it would cost £795 - that's £257 more. And the Spreading Warmth discount is only £125 per fuel.
And then there's your gas.
It could work out to be a reasonable offer but you will have to keep your wits and your calculator about you. It may be a lot poorer value than it seems.