On 16 April 2015 manufacturers have to comply with the new DOE standards. (But you can install current WHs as long as you can find one.) All residential electric water heaters over 55 gallons will have to have a heat pump. WHs 55 gallons and less will have more insulation. If you need more than a 55 and do not want a heat pump, buy two 40s or 50s and plumb in series and wire in series.
Rewire the WH closest to the point of use. Unwire the top element from the top thermostat and reconnect the lower element in its place. Then connect the second WH where the lower thermostat was previously connected at the top thermostat. Now when the first WH’s top thermostat is satisfied, power shifts to the second WH. This eliminates having to pull a second circuit and minimizes the draw on the utility.
3 phase commercial WHs do not have to comply with the new rules. So you could buy a 3 phase, 480 volt, 18 kW WH and wire all 3 elements at 240 and get a 4500 watt WH.