Building on the post of @Steve Babiak above, gas is the best option for a whole-home solution, as gas units have a faster ramp-up and greater temperature lift that electric.However, electric in-line heaters have their place:We have a building with solar DHW and a 100 gallon holding tank.  

And they weld authority like their a king and the other residents won't grow a pair and stand up.The last board I delt with had a guy that would send you a "friendly" letter and a picture of you trash cans if they were on the street for more that 2 hours after the truck went by. 

They can be installed in various methods whether is be fully adhered or mechanically attached but the best part about them is they have heat welded seams which are the strongest point of the sheet vs. an EPDM seam which is an adheared seam and can fail if water ponds around it.

Well with all the global warming the government needs to control peoples breathing. that's what's causing the earth's temperature to rise. we can't have all these people breathing so much. 

The temperature swings are just too dramatic.

Utilities not onInoperable heat supplyMissing or insufficient weather-strippingLoose or torn carpetMissing or cracked electrical outlet cover platesCracked or peeling paint on wallsCracked or broken window panesMissing refrigerator kick platesEvidence of roof leaks/pipe leaksDirty stoves, ovens or walls and unkempt perimeter surrounding houseInsufficient bath ventilation and inoperable smoke detectorsLeaking faucets or plumbingNo temperature/pressure relief on water heatersYou screen your own tenants, then you submit the paperwork to have a tenant approved by the Housing Authority.  

That will eliminate any costs savings you might expect from reduced electric bills.The 9RLS system has a serious temperature control issue, as well.

I am John Rockefeller making sure every one of my barrels has three welds instead of six.

He said he couldn't go any higher, as the coil was limiting him.Well, previous to installing the AC unit  the temperature in the home was 78 degrees when it was 95 degrees outside.

Just ask the landlords that got sued for having, putatively, set their water heater on  too high a temperature and were held financially liable for tenant burns for perhaps query the landlord held liable for his contractor raping their tenant.