If a person is a renter, they may choose to purchase two Power Perfect Box Plug-Ins, one for each Phase, as it can be taken with them when they move or to identify and treat specific hot spots, in home or office as the Power Perfect Box Plug-In does a superior job of treating specific hot spot or problem areas of excessively high THD or EMF. However, as a Rule of Thumb, a general assessment can be done as to what is the right solution (including possibly the Power Perfect Box Wire-In HD) using the following rules:  Square Footage of Residence Homes/Apts. with area of 500SF to 1000SF = 2 Plug-Ins Homes/Apts. with area of 1000SF to 3500SF = Wire-In or 2-4 Plug-Ins Homes/Apts. with area of 3500SF or greater = Wire-In H.D. or 4-6 Plug-Ins Monthly Consumption Average (Rule of Thumb) Homes consumption is less than 500 kWh per month = 2 Plug-Ins Homes consumption is between 500 and 1000kWh per month = Wire-In or 2-4 Plug-Ins Homes consumption is greater than 1000kWh per month = Wire-In H.D. or 4-6 Plug-Ins Utility Service Size (Rule of Thumb) Homes with less than 60 Amps service = 2 Plug-Ins Homes with 60 Amps - 200 Amps of service = Wire-In or 2-4 Plug-Ins Homes with greater than 200 Amps of service = Wire-In H.D. or 4-6 Plug-Ins