It strikes me that the actual costs of extending the warrantee might be considered.
These may be nil if the delay in the project delays the in service date, and thus the risk of fault.
The onus might be put on the supplier to prove the materiality of additional warrantee costs whatever they might be.

As warranty effort is provisioned to fix bugs of contractor's defects, since the delay is caused by the Employer, the Contractor is entitiled to claim additional cost. Not only is this instance 'due to failure of the employer (customer)', which is not due to cause by the contractor, there could also be a delay in the service start date, which means the plans for service could be impacted. Therefore, I believe the additional cost is justified.

Our service department has actually been able to provide a number for us by unit of what an additional year of warranty costs us. Do you know how much the first year costs you? You could always just submit that as an estimate for year two.

If the project is delayed by the Employer... first make sure that there were no delay causes by the Contractor on the previous agreed milestones, for you to be fully eligible for a compensation. Now the tricky part to kick in, is the delay resulting to only warranty extension, which means that all construction, installation and pre-commissioning done? Just ensure that you've all signed-off documents that it's pre-commissioned. if so, your service team will provide an additional cost for extra-warranty coverage and you can lever it as a scope-creep. This is not an issue, if all of your works are completed and no more to come back to do additional works, then you need to add remob to commissioning cost plus the extended warranty cost and get a CO prior to agreement of extension. Don't forget to have your insurance company informed on this extension, as they were notified of previous warranty commitment, not the new one. Hope this serves.