The maintenance rate is the cost per hour that the customer currently pays to replace burnt out lightbulbs. We suggest interviewing each customer at the start of the site-visit to find out who handles light replacements and how much they charge.
There may be hidden maintenance charges that your customer doesn't always consider. For example, in an office building amount of time to change one lamp can take 40 minutes or more for a maintenance person, office manager, and an office worker. The maintenance rate should be the blended average of all of the people it takes to make the repair.
If the replacements are handled by in-house employees payroll taxes may be applicable here too.