16 hours - average?

Hi, I am in the support group for ESA. I have read mixed information online regarding if the under 16 hours permitted work is each week or an average. I invigilate exams so it mostly involves sitting and not doing a lot and works well with my health condition that fluctuates.

Many weeks I do no work at all but other weeks I have invigilate under 16 hours but sometimes and offered the opportunity to do a bit more. If I am feeling well enough to I can agree to invigilate another exam but am worried this could push my time up over 17 hours in a given week. But the average over a month would still be below 16 hours a week in a month. And well below 16 hours a week in a year. Is this ok? It seems really inconclusive from what I have read. Thank you.

Thank you. Do you know where I can find the documentation to support this? Everything i Google still comes up with mixed answers. Non of which i can find more official documentation about 'average' hours and wage. It all seems to just be websites that summarise or indeed say the contrary of it just being under 16 hours per week rather than an average. Thank you very much.