Diane Warren is pop's go-to woman for power ballads – and the writer of this year's UK Eurovision entry. But why do her own songs make her feel sick?

A year ago, Diane Warren knew almost nothing about the Eurovision song contest, something most Americans (if they have even heard of it) consider a bizarre rite best left to nations the other side of the Atlantic.

But this weekend, Jade Ewen will be singing one of Warren's songs as the UK Eurovision entry - My Time, with music by Andrew Lloyd Webber. Warren is one of America's most successful songwriters: she wrote Toni Braxton's Un-Break My Heart and Celine Dion's Because You Loved Me. Earlier this year, she took a week out of a busy schedule to write her Eurovision song in London, and we meet in an absurdly chic hotel soon afterwards.

"We did it quickly, a couple of days," she says of My Time. "Well, the meat of it. There are still things that need fine-tuning." Would she describe the song as a power ballad, the sort of quintessentially American song she is famous for? She balks. "I don't just do power ballads. I have a lot of uptempo stuff, too - like Can't Fight the Moonlight [the LeAnn Rimes hit]. I do Latin, I incorporate some rap rhythms. It's funny how people tend to bag you as one thing."

But her ballads - massive, emotional and usually delivered by singers who sound on the verge of tears - are Warren's most memorable compositions. And even the uptempo numbers she's referring to - Starship's Nothing's Gonna Stop Us Now, Cher's If I Could Turn Back Time - have the same heroically huge quality. Warren, who had her first hit in 1983 with Laura Branigan's Solitaire, doesn't do subtle.

Nine of her songs have been nominated for Grammys and six for Academy awards, and while it's hard to quantify Warren's exact sales, it's enough to earn her up to $20m a year in royalties. "That's in a good year," she adds. In a bad year - well, she can still afford the expensive cars she has a passion for, and run two LA homes.

A native Californian despite her east-coast Jewish mindset ("I'm totally an east-coast person, energetic and sarcastic. I'm not a nice LA person"), she writes using piano and guitar in a Sunset Boulevard office she calls "the cave". "It's very dirty, so I'm probably immune to every disease. I thrive on squalor."

Warren is very funny, not at all what you expect from the author of so much racked balladry. She is even a fan of the English songwriter Brian Higgins, whose sparkling hits for Girls Aloud are the polar opposite of her widescreen work. Does she have a theory about why Americans are such fans of we-made-it-through-the-storm epics - while the Brits are more partial to quirkiness, to new sounds?

She looks dubious. "The British like ballads, too. Leona Lewis?" Yes, but Lewis was groomed to sound as American as possible, so that she stood the best chance of making it there. (Inevitably, Warren is slated to work with Lewis in the nearish future.) "Mmm. I don't know. I guess it's because the UK charts are much more open, and you play lots of different stuff, and our radio is more rigidly formatted. But a big song is international. It doesn't have boundaries."

Amazingly, Warren says she has never been in love: she hasn't had a relationship in 17 years. "I've never been in love like in my songs. I'm not like normal people. I'm no good at relationships. I draw drama to me - it's the Jew in me." So how can she write about it? "I have a good imagination. Look, I know what it feels like to have a broken heart. I know what it feels like to feel something for somebody. I'm just too weird to be in a relationship."

She has never felt the need to be close to another human being? "If I do feel the need, it passes - thank God. Why do I want to wake up with someone and talk to them? Yeuch. It's like when I wrote for Aerosmith [on their No 1 single I Don't Want to Miss a Thing], 'I could stay awake just to hear you breathing ... '" She makes a disgusted face. "If someone was listening to me breathing all night, I'd throw them out the window. Preferably a high-rise. Why would I want someone to listen to me breathe?"

Instead, she shares her Hollywood Hills home with a cat and a parrot - "both aloof animals". But does the lady protest too much? Isn't she actually a romantic at heart? "I am an incredible romantic. That's why I'm always so cynical and sarcastic. I am a bruised romantic." So the songs are an outlet for that side of herself? "Yeah. Probably."

She is travelling with a friend, the vice-president of her music publishing company, who reminds her that she has a meeting with Lloyd Webber to get to. "Don't be a buzz-kill," Warren says, grinning.

• Read tomorrow's Film & Music for a comprehensive guide to the Eurovision song contest main contenders. The second semi-final is on BBC3 at 8pm tonight; the final is on BBC1 on Saturday 16 May at 8pm