Compatibility with UPS units and 12vds to 120vac power inverters.   Products    Environment Control       Herpstat       Herpstat ND       Herpstat II       Herpstat PRO       Herpstat Humidity       Herpstat Accesories       Night Drop Cable       Replacement Probe       USB Data Cable   BUY DIRECT       Online Store   News       What's New?   Projects       Future Development   About Us       Company Info       Reseller Info       Terms The short answer: Most computer UPS backup systems and 12vdc to 120vac inverters output a modified sine wave. This will not work with any proportional thermostat. They will work with relay based non-proportional thermostats. In order to work with a proportional thermostat the backup system must produce a "True" or "Pure" sine wave. These systems are usually double the price of a standard UPS or power inverter. The long answer: All proportional thermsotats use a Triac to control the AC output. This component gets triggered and turns off on its own once the AC voltage reachs zero volts. Common UPS systems or power inverters using "modified" sinewaves do not recreate the smooth curve sinewave that comes from your home outlet. Instead they spike the voltage 120vac positive, then drop to zero, then spike to 120vac negative. Because of this there is no way to dim the power. Also in my testing the triac typically will not turn off properly and stays stuck ON once its triggered. In order to have a proportional thermostat work properly off a battery powered (or gas generator for that matter) device it has to output a "True" or "Pure" sine wave. These systems recreate the correct smooth arced sinewave just like your home power. Because of the extra components and technology they are usually double the price. Dion Brewington Owner, Spyder Robotics The information in the article is the opinion of the author and is to be used at your own risk. Spyder Robotics assumes no responsibility for the use of this information.