A company is going to contract out some technical writing to me, just a few SOPs and policies to get them started. There's already an agreeable rate in place, but I was wondering if there was a good standard for units (hours, etc.) billed. For example, would I just bill for the exact hours spent on the project, per deliverable, or is there some other method (e.g. per diem, pre-defined project hours, etc.)? Are there pros/cons to any particular method that would make it beneficial to try and push that particular method?
This question is difficult to give a specific answer to, because there really is no single specific method. You should agree on a billing method with your client and then include that in your contract. Some clients or consultants may prefer hourly billing, others may want a lump sum for an entire project, while others may want a piece rate for each task you complete.
I'd probably go with per day or half day. For example if you finish at 3pm on the last day, are you going to be able to productively use the remainder of the day on another job?
It follows, therefore that any estimates are in those units (days).
It really depends on how much work there is, and as @dwizum says, it's whatever you agree with the client.
If you're billing for time, make sure that they can't withhold payment for being dissatisfied with the documents; with some clients, you'll forever be locked into a cycle of changes before getting paid.
[Related - agree what's expected to be delivered]
- [Related - offer a limited amount of free rework / minor amendments]
- Make sure you have a clearly defined window or date by which payment must be made (usually referred to as "terms", e.g. 30 days from invoice).
- If it's a really big company, make sure you get a purchase order before starting, if small a letter of intent or email saying "please do xyz".
- Make sure there's a late payment penalty clause and that the later it gets, the harsher the penalty.