Wow, 18 to 22kW! That makes my old 6kW shower unit seem like a weakling! Those sort of figures sound more like the equivalent power output of the gas-fired "multi-point" heaters like Tom described.
Here's sthe manufacturer's data for the British Triton TW10i, recommended for up to 2 outlets. I've converted the metric & centigrade measurements to U.S. units, hence the non-round flow figures:
Power: Selectable 6kW or 9.5kW
Dimensions: 7.1 x 11.6 x 3.4 inches
*
Water in at 50 deg. F, 9.5kW setting:
0.53 U.S. gal/min., 158 F.
0.79 U.S. gal/min., 133 F.
1.06 U.S. gal/min., 111 F.
*
Water in at 77 deg. F, 9.5kW setting:
0.53 U.S. gal/min., 185 F.
0.79 U.S. gal/min., 160 F.
1.06 U.S. gal/min., 138 F.
*
Water in at 77 deg. F, 6kW setting:
0.53 U.S. gal/min., 167 F.
0.79 U.S. gal/min., 138 F.
1.06 U.S. gal/min., 120 F.
*
The cost of this unit by cmparison is £122.50 (about $180) plus tax.
