Chaka Khan, Shepherds Bush Empire, London

She may be a 55-year-old grandmother, but Chaka Khan can still keep everyone warm on a cold night

Speak the name Chaka Khan to a bunch of men of just the right age, and their mind's eye will fix on the same remembered image. One Friday night in the bleak mid-1980s, at the peak of her commercial comeback, the soul diva pitched up in a Newcastle television studio to perform live on The Tube, and the Channel 4 cameraman beamed low-angled pictures of her plentiful figure into living rooms across the nation, a startling and formative sight for the British adolescent male. In school playgrounds on Monday, the phrase "thunder-thighs" was whispered, with admiration and awe.

Chaka Khan, no question about it, always had a certain something. She seemed to embody all the fruits of the tree of life, ripe and bounteous. It was as if, when she sang "I'm every woman, it's all in me", it was to be taken literally – not something that could ever be said, of course, about the inferior 1993 cover version by Phasmatodea Houstonis.

At the age of 55 and now a grandmother, Chaka Khan has still got it, whatever that "it" may be. Resplendent in a black corset and rattling gold jewellery, her cerise-tinted hair billows in the wind-tunnel blowers she's had installed on stage and, when she starts singing, it's a wonder the hair of the punters in the front row doesn't billow in the opposite direction.

In a discussion about African-American music in the late 1970s, a facile but perhaps useful dichotomy can be set up between Chaka and a rival diva, Donna Summer. Each singer represented a different archetype. The slender, pale-skinned Summer embodied aspirational, white-friendly values. Donna was disco; Chaka was funk, with all the lubricious richness that word implies.

And with all the unapologetic blackness, too. Chicago-raised Yvette Marie Stevens changed her name to Chaka (meaning "fire") while working with the Black Panthers on their Free Breakfast for Children programme, a philanthropic interest she's maintained to this day with her Chaka Khan Foundation (which offers assistance to inner-city schools), even if she has hedged her political bets in recent years by playing both the Democrat and Republican conventions.

It helps her retrospective legacy, of course, that Chaka has never dropped a clanger like Donna's – you know the one I mean – although she is, sad to say, a creationist, and furtive roll-eyed glances are exchanged when a large part of tonight's show, around a distressingly overblown "Angel", is given over to tedious sermonising about how she was once "a bad, bad, bad girl" who was "drinking so much that my children were writing suicide notes", too intoxicated to remember making some of her finest records and who "didn't party for hours, or days, I partied for weeks", until God saved her life.

Even an atheist can forgive her this sort of indulgence, however, when she unleashes a voice which, as producer Arif Mardin once put it, can hit "notes that aren't in the book". In front of a crowd made sparse by snow-induced travel chaos (no such problems for Khan herself, who has since 2006 been living a short cab ride away in West Hampstead), she kicks straight into her biggest global hit, "I Feel for You", which, even without Melle Mel on hand for the rapped introduction and Stevie Wonder to provide the harmonica breaks, reminds you that she achieved the impossible back there in 1984: she improved on a Prince song. (Seriously, have another listen to the impish one's original on 1979's self-titled Prince album, and hear how flimsy and weedy it sounds in comparison to Chaka's rap-funk reworking.)

Nominally, this is a show to promote her most recent album, the Jam & Lewis-produced, double Grammy-winning Funk This, but she's in nostalgic, crowd-pleasing mood, running straight from "I Feel for You" into the almost supernaturally funky "Ain't Nobody", her lungs warming up the Empire with some of that Chaka-fire.

Other classics from the Khan catalogue include the famous cover of Stevie Wonder's "Tell Me Something Good" from her days fronting Rufus, and "Through the Fire", as sampled by Kanye West on "Through the Wire" (itself not the only time a snatch of Chaka has given birth to an iconic record: a tiny guitar lick from "Fate", one of her album tracks, was the basis for Stardust's "Music Sounds Better with You").

The encores verge on the ridiculous, as she's joined for the inevitable "I'm Every Woman" by – wait for it – Lulu, but by that point we've had enough of the sublime to insulate us. Despite her ecstatically wailed protestations, Chaka Khan isn't everywoman at all. She's a one-off.