That is exactly what i say when a customer asks me for a price over the phone,
Customer- how much to clean my windows/solar panels please.
Me- how longs a piece of string, ill have to come later and give you a quote or i could charge you too much or not enough.
Customer- ok that will be great, thank you see you soon.
Only a fool would quote work without viewing it unless you can guarantee what the job entails. Call that thick but i call it having my head screwed on!.
Some houses i do work out £1 per panel and some £5 all depending on access etc, if a custy rung me and i said £1 each panel but when i got there it should be more £5 a panel id feel a right old tit!.
Id have thought you would know how much to charge for solar panels with the amount of work you have anyway.