Its hard not to talk about fees and other variables when you talk about retainers and billable hours.
When I worked with a friend of mine our “retainer” was charged on the hour block and rate.
80$ per hour, billed to you at end of month with 21 days to pay full amount. 7% late fee or 50$ whichever is greater. (7% was always scary for the client.)
If not paid after 51 days total, all services suspended and recurring interest on the total balance accrued.
If it was in blocks of hours it was to be paid on the first day of the month every month unless pending change in conditions. The hour block was covering all types of support, phone, remote, email or on-site.
If the customer required Phone or Email support, it was billed by 15 minute increments. Remote and on-site was billed for first hour and 15 minute increments after.
Anything that was referred to as a project was not included in the “retainer” and was negotiated separately. (Allows you to make sure you negotiate a price and hours. I always made sure there was a 6 to 12 hour extra included in the negotiation in case something went wrong. If no issues came up and those were not needed, the customer did not receive the charge for those in the invoice.
10 hour block was billed at 75$ per hour
20 hour block was billed at 65$ per hour
50 hour block was billed at 50$ per hour.
(I do offer roll over of the 10 hour blocks for up to 12 months.)
My customers loved knowing I offered bigger blocks of hours because it showed them I would dedicate all my needed time.
You could not just call us up and asked to be put on a retainer. These are offered only clients who had been with us 6 months.
Edit: Evenings and weekends are billed at twice the rate.