MarkT:
Depending on the state you are in, and the company, usually the underwriters will calculate the liability premium based on some of the following criteria:
How much experience in the field you have.
What professional licenses you have - security, low voltage, electrician, etc., in addition to requirements of having all local and state business permits and licenses.
Your average annual $ billings for work you do, percentage of equipment sales vs install labor.
How much of your labor billing is through licensed subs, vs your own company.
Also considered is do you require the subs to provide you with a certificate of their insurance for each job, endorsed for the job location, they do for you.
These kind of criteria result in commercial liability rates to be very unique and different for each person and company and location - as opposed to personal lines such as homeowners and auto.
Mike, Usually a PERSONAL umbrella liability policy has major exculsions for this use. After all, it is a PERSONAL not business coverage.
Just some things to consider from an old man with almost 30 years in the Property and Casualty Insurance field