Its not a gamble I would take. If the unit is rated for 120 Amps, then you should budget that into your electrical planning. Heating domestic hot water requires alot of energy. In cold weather, incoming water is colder and the energy requirement goes up. Run a load of laundry, dishwasher or shower at one time and the energy draw will go up. Not sure what else you are running but that leaves you with 80 amps of reserve after the wh which is a tad low for a typical house nowadays.
I think the only benefit that I have here is that I live in the south-east. So, the incoming water is a bit warmer by default, but it'll still be somewhat cold during the winter months. I do agree with you that it's definitely not ideal... especially given we're on a tech forum, and a good deal of us probably have plenty of power-thirsty toys. I think the other benefit is that I rarely use warm water, which is part of the reason why a tank isn't that ideal for me -- apart from the immediate cost difference. The only time I use warm water is for a shower in the morning, and with the dishwasher. The latter is only maybe used once every 2-3 days. I
usually do my laundry with cold water instead of hot.
Although, I do have a rule for modifying things on my house... and that's to avoid doing something that I may prefer but others wouldn't like as much, or that I may be fine with, but it won't work for others. So, that's why I'm not as keen on considering my usage as being the normal usage for a system.
Maybe look for an electrician to give you his evaluation. Unless the service entrance cable was run in such a way that it restricts access to the knockouts on the side of the panel, or prevents you from installing a new breaker, this shouldn't be that great of an issue. Cable is flexible, you make it work. It can be moved over or even unfastened and relocated to make room.
It would probably be a lot easier if it were moved to the top, large knock-out (the one I see used with service wires a lot).
Here's a picture of my panel prior to when the kitchen lines were added:
https://imgur.com/a/a9tnA
The one thing that bugged me about when I paid to have the kitchen lines installed is that I asked to have lines removed that weren't being used, but they didn't do it. It's really just a couple lines from the tandems on the right. They also tossed in two normal breakers at the top to remove the 50A breaker.
There is also another factor to consider. electrical is considered 100% efficient (or very close to it) where you consume it. Gas boilers/appliances etc... are not 100% efficient so lets say you get a non-condensing unit with 85% efficiency. So we need 15% more gas. Your new figure is $0.89. Using a condensing unit with 95% efficiency, this costs you $0.81
Thank you for the numbers! So, if we're generous and say that I save $2 per month using gas, that means I have $24 per year. So, I could use that factor to find out how many years it would take to recoup the difference in cost.
Quick update: Called around the area including homedepot's recommended vendors, they all came back around $1600 for a 6 year warranty-50Gallon Tank, it goes up for additional $200 for years of warranty, e.g $1800 for a 9 year and $2000 for a 12year....
I was lucky to find a HVAC company that was actually honest, straight forward and reasonable. I went to homedepot, picked up the 9year-50Gallon for $550+Tax, came home, drain my old tank out. They came on time and replaced & removed my old tank for $250(parts and labor). Saved me over Half!
This probably isn't super surprising, but Home Depot adds a hidden fee onto everything if you go through them. If it's a simple job, it's
far cheaper to find someone yourself. When I was dealing with some contractors through Home Depot and asked about an extra task, they told me that it would be $x if I went through them, but if I contacted Home Depot about it, expect it to be another 30%. But only $250 sounds pretty good... like I mentioned, the HVAC company that I was talking to wants $1300 just to remove the old water heater. To be fair, I don't know all of what that includes, but capping off water lines isn't very hard. Although, I've found this HVAC company's work to be less than stellar in the past, so they'll have to be rather willing to negotiate on price.