Almost every policy involves an amount the insured pays before cover applies. Raising that amount lowers the premium, and the mechanism behind the reduction is not the one most people assume.
The excess removes claims entirely
Small claims are far more frequent than large ones, so an excess set above the typical small claim eliminates most of the claims a policy would otherwise receive.
Each avoided claim saves the insurer not only the payment but the cost of handling it, which for small amounts can approach the payment itself.
The premium reduction therefore comes mainly from the volume of claims that never happen, not from the fixed amount deducted from the ones that do.
Behaviour changes as well
An insured facing the first portion of any loss has a direct interest in avoiding losses, which reduces both carelessness and marginal claims.
This is the standard response to the problem that cover itself reduces caution, and the excess is the simplest instrument available for it.
It also discourages claims that would technically qualify but are not worth pursuing, which removes a category of cost that is difficult to price.
Why the saving is not proportional
Doubling the excess does not halve the premium. The reduction is steep at first and flattens as the excess rises past the point where it is eliminating frequent claims.
Beyond that point the insured is absorbing more of each large claim while removing few additional claims, so the insurer saves little and offers little.
Identifying where the curve flattens for a particular policy is the practical question, and it varies by cover type because claim frequency distributions differ.
The excess must be affordable to be useful
A high excess is a commitment to fund the first part of a loss at the moment it occurs, which is often a moment of financial disruption.
An excess that would have to be borrowed converts an insured event into a debt event, which defeats the purpose of holding the cover.
The sensible upper limit is therefore what can be paid from accessible savings without difficulty, rather than the highest figure the premium table rewards.
Where multiple excesses apply
Policies frequently carry different amounts for different causes, with specific perils or circumstances attracting a separate and higher figure.
Compulsory and voluntary components may also stack, so the amount payable is the sum rather than the voluntary figure alone.
Terms, names and regulatory requirements for these arrangements differ between markets, so the policy wording governs rather than any general expectation.