Total Amperage and Electical Box Availability
That is very likely the "average running" current rating, not the peak inrush current when you first turn something on. However, you figure of 176A seems very high for an average home.  If that is really what you are currently using then your power charges will be quite high every month, AND you have NO EXTRA CAPACITY to add a very-high current device like a tankless water heater.

To really know how much current is being drawn from any branch circuit (breaker) or even from the total load (via the incoming service lines), you would use a "clamp on" AC ammeter.  You don't need great precision, so an inexpensive meter like https://www.amazon.com/dp/B00NWGZ4XC would give you ACTUAL numbers to work with.

You did not reveal how much current your tankless water heater needs. But they are typically VERY HIGH current devices and demand heavy wiring and separate branch circuits.