I'm considering installing an electric tankless water heater in my house, but need to know what amp service I currently have. I read that you should look at the amps on the main breaker to the house to determine this. So I inspected the main panel on the outside of my house, and found the main breaker that feeds to the subpanel inside the house. It is a 2-pole breaker that says "100" on each side. Does that mean it's 100 amps total or 1100 on each pole (=200 amps?):
A lot of what I am about to answer is personal opinion so there may be quite a bit of comment on this answer.
It appears that you have a 200A service being fed to a Zinsco Panel which is being used as a service panel utilizing the 6 switch disconnect rule, NEC Article 230.71 (A). The 100A breaker you are referring to appears to be servicing the subpanel.
The Bad news: You cannot by code add a new disconnect to your main panel.
The panel is a Zinsco and it appears to be in bad shape. Also Zinsco's are no longer manufactured and are considered to be the second worst panel in the industry (FPE being the worst). They have a tendency to overheat and damage the bus and breakers. I would not recommend putting a high stress load like a tankless heater on them.
Recommendation: Replace your main panel with and install a 200A main breaker. Then I would add a second main breaker for the tankless water heater which would keep it from creating extra stress on the existing electrical system. If possible in your jurisdiction you might upgrade the entire service to a 320A service. You will need to contact your utility provider to check availability.
Final comment: In respect to your question about the 100A breaker. With different voltages (120 vs 240) it's not about amperages but available power. In other words a 120V 100A circuit would be 12,000VA on a single phase, and a 240V 100A circuit would be 24,000VA or 12,000VA per phase. That should clear things up or just muddy the water a little bit more.
Hope this helps and stay safe.
It takes a very high current to power an electric tankless whole house water heater.
For comparison, our 2000 sq ft 2-bath house in the relatively warm climate of Dallas, Texas originally had a 40 gal NG fired tank (I think a 40,000 BTU/h NG burner). We replaced it with a Bosch Aquastar NG tankless with a 117,000 BTU/h burner. Works for two undemanding people aged 75. Not sure it would work for a family with teenage children. Most houses in the US this size would be fitted with a larger tankess heater: 150,000 BTU/h or 180,000 BTU/h or even 199,000 BTU/h.
Due to our low needs we operate the Bosch tankless on the lowest burner setting which may be only 30,000 or 40,000 BTU/h. We try not to run the clothes washer with hot water or run hot water from the kitchen sink or the utility sink in the garage while someone is showering. I think both showers can be operated at the same time, but we don't usually do it.
What size electric tankless would we need to replace our NG fired tankless?Conversion factor 1 BTU/h = 0.293 W.
On high burner 117,000 x 0.293 = 34,300 W or 34.3 kW. At 240 V the current would be 34,300 / 240 = 143 so ~ 140 A for a 34 kW heater.
On medium burner 70 A for a 17 kW heater
On low burner 35 A for an 8.4 kW heater