I'm just in the middle of setting up and was thinking about what to charge. Do you charge hourly, half hourly or 15 minutes. I thought I would charge hourly but after doing some research on google theres a fair few that charge for 15 minutes.
Charge hourly, or part there of. If you charge by 15 min slots you will spend all your day logging who you are working for, as opposed to doing their work!!
At the moment, I only have one client so I charge per every 15 minutes that I work, although I will review this once I get busy (hopefully :D) as like Liz says it would be time consuming if you were working 8 hours a day for different people.
I have charged per every 15 minutes since I started my business in August 2006.
It has always worked well and clients have been happy that they are not being charged more than actually worked on their books.
Even at times worked for many different clients in a day did not matter, have duplicated manual system to record the time (in case I lose one) which is a very small cheap notebook that I write times in when I start client work and when I finished that client which I keep in a different location to invoicing diary. Then I can transfer this information to my invoicing diary at the end of the day. Works well for me.
I do kind of the same thing as you Alison, because I have the boys running around and the phone rings, answering the door etc I have an online stopwatch which I start and pause so I know exactly how long I am spending on my clients work. Every day I record the amount of time that I work into an A5 diary which I use for nothing else and then at the end of the month, I transfer the diary amounts into a monthly time record so that I can work out how much to invoice for. Because I charge £8 an hour, I invoice at £2 per every 15 minutes and like you say it is better than overcharging your client or undercharging for your time.