How come one panel is $1,500?

Tenants won't pay more just because the water heater is new or because you replaced Federal Pacific electrical panels.  

I am currently looking at a property that has solar panels on the roof.

Clean 1970s wood paneling does not mean “updated”.Seems like a lot of people are very proud of their ugly house, too.

This overload has also left the panel extremely damaged. 

Then you have to factor in that the cost of building is sky high and likely going up due to inflation and government regulations (such as requiring solar panels or rain barrels in California).We can't freak out about the future because we don't have control over it.

.- Electrical Service: Since you are adding a new unit, you will need to priovide for a new electrical panel dedicated to that unit.

That's a problem because a.) it misleads people on how safe the outlet is for plugging in 3 prong devices (3 prong devices are better grounded which can save the device in the case of a short but they have to have a third wire in the wall for ground) and b.) by adding extra outlets to an existing 2 prong that has been inappropriately changed to a 3 prong, the outlet can overheat and cause a fire.We'd need to know more, is it knob & tube, is it aluminum, is the panel 60amp, 100amp, 200amp, are there other panels, what condition is the sheathing around the wire to know where to go from here.

You can power them via solar panels, they have alarms you can trigger and you can also speak/hear through them. 

If you add square footage, increase bed/bath counts, or do a panel upgrade, 100% get a permit.