It's perfectly acceptable British English.
21 occurrences of "tariff" on one of British Gas' pages: https://www.britishgas.co.uk/energy/guides/off-peak-electric... ("Your energy provider may offer time of use tariffs and cheaper night-time electricity rates.")
Good to know, so it's an American vs. British thing. Thanks!
Is there any further distinction between a "tariff" and a "rate" in British English? The example sentence you provide uses both, which makes me wonder if there's even more to the picture here.
Without checking, my feeling is a "tariff" is the whole contracted agreement, and a "rate" is a part of it.
An EV electricity tariff might have a cheap night rate, and a more expensive day rate. Another tariff might be entirely variable rate (price changes every hour).
Wiktionary defines a tariff as "A schedule of rates, fees or prices." so I think my feeling is correct.