I don't know the number, but it's more than one. Remember though, we're not talking heat here. There are other ways of starting a chemical reaction, including vibration and electrical and magnetic fields (eg nitroglycerine). There is a reason that cellphones are not allowed on service station forecourts- they can dissipate up to five watts through the aerial (though it's usually much less) and that gets radiated into the surrounding air, which contains a near-perfect air/fuel vapour mix.

Anyway, we're getting way off topic here, and I think I've made the point I set out to...

Cheers
Jon