Which is better for a 9 years experienced electrical engineer, online masters degree in project management, or online masters degree in electrical power?

It all depends upon the nature of the task or the problems in hand at that time !

If you have specialty experience in your field then choose a degree in Electrical Power with the specialty you have

If you are doing general jobs and you are targeting managerial positions then choose Project Management or any other MBA degree

you should try Project Management exams its better than online degree .

it depends on the career path you are targeting , if you're going for construction management go for the PMP , if you're going for the the engineering path then a masters in the electrical power is your best choice although an online masters in engineering won't be taken seriously .

of your choices, managment field is better one because you have experience already in power i think. but now we have renewable energy as a new field and i think you must looking for it