They are likely to charge the same anyway. HVAC guys are among the trades known to shyster people. Another 5 mins isn't likely to matter.
And if upon showing up you said that to me, I'd say what that poster here says all the time: Get off my lawn! If you have an issue like that right off the bat, I don't want you working on my house. I've had the occasional trades folk over many times, never found one that had an issue with a place to plug in an extension cord or where to get power. Only one time in 25 years was powered needed for the AC service. And if they need power or water, they typically just ask where it's located.