I have been offered consultancy on-the-side of my full-time startup. However, I have no idea how to set my rate.
The work is software-based but niche/scientific. I have perfectly-suited experience and skills.
Client is a tech company based in a US city. I would work remotely from Europe. I had a very similar gig 10 years ago and charged US $100 per hour.
Can someone help me value my time? Even a range of rates would help, or a view on consulting fee inflation over the last 10 years.
Try having a search on the job boards for similar contract/freelance jobs. This should give you an idea of ranges. If you have specialist or niche skills then you could probably charge a bit more.
Also consider what you're time is worth to you. If you already have a full-time startup job then what is an hour away from that worth?
There are 2 "vectors" you can test it by:
What will I do it for?
If you include your Expected earnings for the year / hours worked for the year (maybe you want 3 months holiday which would change this expectation). Remembering to include your insurance, your taxes, your expenses of running your business like accounting and a risk factor for not being in work 100% of the time.
This usually comes up with a conservative but good minimum "walk away" rate.
What will the market stand?
This says look at the salary surveys, look on job boards, talk to similar profile companies to your target market ... these are the best and they don't have anything to loose by telling you.
Typically the answer you get from asking the "like companies" are the highest values because they are thinking of the long term "great performers" rather than the "newbie/generalist" that you see advertised on job boards (the last resort if you can't find someone).
If your skills are very targeted and you have a lot of experience you can often ask a higher rate and people will take it.
Before you do all that Have a listen to Deep Fried Bytes Episode 67: Tips on how to be an Independent Software Consultant with Michael Eaton and BayCHI podcast: How to Fire Your Boss and Start Your Own Consulting Business. These are the best 2 discussions and 2 very different approaches to the same concept.