Or find a way to determine what annual cost will average plus what you want for profit, and make a contract based on that. Some times you will be ahead, sometimes you will be behind but in general will still be profiting. You can have exceptions for devices that are more than just worn out - like say someone bashed it with a forklift, those should be replaced as cost of new on top of the contract as this is not normal wear and tear, or even damage from a surge or lightning should be an exception to the contract.
Many power companies have contracts with cities/towns for street lighting like this. So much per year per light, to maintain them regardless of what happens except for when a motorist crashes into a pole and destroys the light and pole. Rate for them may also include energy charges based on estimated annual energy usage - which will be somewhat consistant each year, cloudy days will make some difference but they make sure they are profiting in the first place so it is not too significant.
