Generators, Battery-Backups, and Thermostats; What You Need to Know.

[The Serpent Merchant]

This thread will cover the basics of why some generators and battery-backups won't work with certain thermostats.

Full Disclosure: In this thread I will be making some oversimplifications/fudge a few things to make this simpler to understand.

AC vs DC Power:

The first thing you need to know is what the difference between AC and DC power is.

AC or Alternating Current is the type of power that comes from the wall socket. AC power periodically switches the direction it flows (thus alternating) depending what country you live in AC power switches directions 50-60 times per second or 50-60 Hz. This causes the power to flow in a sinusoidal waveform which look like this:



Many electrical devices rely on this precise alternating pattern to work properly including proportional thermostats, but more on that later.

DC or Direct Current is the type of power that comes out of a battery. DC power has a Positive wire and a Negative wire.

Direct current is used in many electrical devices. Semiconductors (computer chips) can only work with DC power.


It is very easy to convert AC power to DC power, but converting DC power to AC power is more difficult/expensive.


AC Power from the Wall vs AC Power from Battery Backups/Generators:

AC Power coming from the Wall has a nearly perfect sinusoidal waveform. This means that sensitive electrical devices will work without any issues.

AC power from most battery-backups and generators is "dirty" Its waveform isn't as perfect. Instead of the nice curves it looks more jagged, like so:



The reasons why they do this isn't important, but if you want to know just send my a message and I'll explain it for you.

On/Off Style Thermostats vs Proportional Thermostats:

On/Off Style Thermostats are very simple. They are basically electrically operated switches. They run on DC power (the wall AC power is converted to DC) You can think of an on/off style thermostat as a gate. When the thermostat is in the off position the gate is closed. Power cannot flow through to the heat source. When the thermostat is in the on position the gate is open and power can get to the heat source. This simple operation means that a no matter how "dirty" the power going into the thermostat is, the thermostat is unaffected as the dirty power is just converted back to DC power.

On/Off style thermostats are fine to use with battery-backups and generators.

Proportional Thermostats are much more complex. Instead of acting like a gate they actually alter the amount of power being sent to the heat source. This is done by a component called a TRIAC (Triode for Alternating Current) The TRIAC is used to "Phase Control" the power going to the heat source. Simply put the TRIAC only allows a certain percentage of the AC power through. To do this the thermostat relies on the sinusoidal waveform of AC power to generate known values of output.

So the thermostat knows at point "x" of the sinusoidal wave it can produce a certain output level to the heat source. When the sinusoidal waveform is "dirty" the thermostat cannot reliably produce a known output. It's like blindfolding the thermostat. The problem is that the thermostat doesn't know that it's been blindfolded. It continues to work as if nothing is wrong. This is why proportional thermostat shouldn't be used with most battery-backups or generators. While they will appear to be working correctly they won't be.


For more information on why proportional thermostats don't work properly with dirty power check out these links, or send me a message:

http://en.wikipedia.org/wiki/TRIAC
http://en.wikipedia.org/wiki/Phase_control

Final Thoughts:

Some battery-backups and generators are better than others. You will see some generators labeled as "inverter generators" and some battery backups being labeled as "pure sine wave"

These are much better, and will probably work with a proportional thermostat, but be warned. Some lesser quality products are labels such for marketing reasons. True inverter generators and pure sine wave battery backups are very expensive. So expensive they won't be worth the cost unless you have another reason to own one.

There is a much cheaper option:

Get a cheap battery backup or standard generator and a $50-$75 on/off style thermostat to use with it. This option will often be hundreds of dollars less than buying an inverter generator or pure sine wave battery backup, and is guaranteed to work.

I hope this post has been helpful. If you have any question feel free to post them below or send me a PM.

[Xcruciating]

Do you have a recommendation for an on/off style thermostat to use?

[bcr229]

Do you have a recommendation for an on/off style thermostat to use?

If you have a large collection it would be easier to get a Ranco t-stat that can handle up to 1500 W, hook it up to a space heater, put all your snakes in one enclosed room, and just use the heater to keep the room at an appropriate ambient temperature for the duration of the outage.