None of these heaters comes with a plug so they need to be HARD WIRED back to an electrical panel due to their amperage requirements. A standard circuit within a home is 15 amps for a 120 volt circuit. The AE3.4 requires 30 amps, so it can not be "plugged in". The AE3.4 can only really be used in warm weather climates where the inlet water is at least 65 or warmer. The AE3.4 will raise the water temperature flowing through the heater 20 degrees at a flow rate of 1 gpm. So, if you are starting with 70 degrees the outlet temperature will be 90. If the flow rate is .5 gpm, then the outlet temperature change will be 40 degrees.
The Powerstar AE7.2 is also a 30 amp unit but utilizes 240 volts thus it has twice the power of the AE3.4. At a flow rate of 1 gpm, the unit will provide about a 45 degree change in the water temperature. This unit may still not be "big" enough for cold weather climates if the inlet temperatures are 50 thus the outlet temperature would be 90. This is good for washing hands but not much else. Please note that the AE3.4, AE7.2 and AE9.5 do NOT have a knob or dial to control outlet temperature. The AE12 does have a temperature control though.
The Powerstar AE9.5 is also a 240/220/208 unit and will provide about a 60 degree change at a 1 gpm flow rate and the heater requires 40 amps to operate. This unit is usually the best fit for most point of use applications.
The Powerstar AE12 Power Stream tankless heater is the "largest" in terms of KW in the family. Please note that all the heaters are identical in physical size. The AE12 can provide a temperature change of 40 degrees at 2 gpm and a 65-70 degree change at 1 gpm. This unit can possibly be used with low flow showers in a warmer weather location.
When dealing with electrical tankless heaters, the input voltage is very important. The heaters are rated based on a full 240 volts. If the input voltage is less than 240 then the wattage will be reduced thus the output temperatures will be reduced. This is NOT a linear function. For example, if the voltage is cut in half, then the wattage will be 1/4 of the rated kw and the performance will be reduced by 4 times. It is important to have an electrician measure your actual voltage and to review your panel capacity when installing any Electric Tankless Heater including the Power Stream Point of Use heaters.