While tankless systems may seem more efficient, it's when they are not heating water. During use the efficiency goes way down.
I would look into an electric unit. Less complicated install, no drain, no exhaust needed and virtually silent. However you do need sufficient space in your breaker panel. Depending on KW size may take four or six slots in the panel. Those with gas water heaters seem to be terrified of electric units but they do run nearly 100% efficient. I been living in Florida since 1989 and have had only electric WH. Gas is limited in Florida.
This simply is not accurate.
1st - your “efficiency in use” comment applies equally to your electric heater.
2nd - you didn’t do much research or are trying to sell electricity because gas fired on demand heaters have pretty much the same efficiency ratings as electrics. 90-95% for both. Sure , some entry gas units are in the 80+ % range but the bigger condensing units are 95%. And , rinnai has a 98% efficient unit that no electric on the market that I know of can touch.
3rd - Complicated install is user subjective. True , an electric doesn’t need venting. But as you point out - open spaces paired / opposite each other in an electric panel is almost as rare as unicorn tears. And for a whole house unit we‘re really talking 6 spaces minimum and 8 for anything approaching whole house use for a family. Furthermore, the 150amp draw an elect. whole house unit would draw on full tilt would tax most 200amp residential electrical services to failure. Even with 400amp service and multiple panels - a retrofit that size would would involve a lot of panel reorganization ($$$). 8AWG wire is also $5+ a foot vs. 50 cents a for CSST gas pipe. And don’t forget we’re talking about running up to 4 separate 8awg lines from the panel to the heater vs.
one gas line from the meter or suitable trunk line.
An additional consideration for some people would be generator power. You’re not running an electric on demand heater in a sustained power outage - but a gas unit could be activated with the cheapest battery backup power supply or show a nuisance load to a whole house generator. So, venting is not really the critical decision point - especially with a condensing unit that uses 2” pvc pipe.
4th - even the largest resi. elect. unit still falls waaaaaaay short in terms of capacity/volume vs. a fossil fuel unit. A large electric may be fine for a single person or even a couple that has low use , but even the largest electric will not keep up with a family of four using 3-4 showers at the same time. Or, 2-3 + a washer or dishwasher. And lots of PoCo’s are starting to roll out demand pricing during peak use time (when you want to shower and wash clothes and dishes) which means you’ll be paying x times more for that Kw of electricity your heater uses when it is running. Most nat. gas co.’s have stable yearly rates per therm and those with propane can buy it in summer when it’s dirt cheap to save even more $$$.
At the end of the day what’s best for a given project first boils down to demand. If you have lots of demand - elect. can easily be crossed off of the list. If your needs are more moderate or low - elect. may make sense but you‘ll need to now look at infrastructure costs and to a lesser extent fuel costs per btu. The new infrastructure cost will usually reveal what best. 500-1500 gallon propane tanks ain’t cheap but neither is adding 400amp elect. service. If you already have nat. gas service , re-piping or even upgrading that will usually be cheaper than upgrading your electric service.
