Shift premiums are quoted as percentages, which is the least useful way to understand them. The percentage applies to some hours and not others, so the headline figure and the annual effect rarely resemble each other.
Why the percentage misleads
A twenty per cent night premium does not raise annual pay by twenty per cent. It raises the pay of the hours classed as unsocial, which on most rotas is a subset of the week — often the hours between a fixed evening and morning boundary, not the whole shift.
Someone working a rotating pattern may find only part of their week qualifies. The premium is real, but it applies to fewer hours than the advert implies.
This is why two people on nominally identical rates at the same site can be several thousand apart by year end.
The number to ask for
Skip the percentage and ask for the enhanced hourly rate and the number of qualifying hours in an average rota cycle. Two numbers, and the annual figure follows directly.
If a recruiter cannot produce them, that is worth noting on its own — it usually means they are quoting from a job spec rather than from payroll.
What it costs you
The premium exists because night work is genuinely harder to recover from, and the research on long-term shift work is not reassuring. That is a real cost and it does not appear on a payslip.
The honest framing is that a night premium is compensation for something, not a bonus. Whether it is adequate compensation is a personal calculation — but it should be made against the correct annual number, not the percentage in the advert.