A common question I get asked is
“Can I use a standard 120/240 AC heating element or hot water heater element as a DC dump load for my wind turbine/water turbine/solar system?”
The answer is yes but don’t expect it to consume (dump) as much electricity as it is rated for. The lower the voltage of the electricity you want to dump (the voltage of your battery system), the less electricity the heating element will consume.
The first thing to stop worrying about is whether the element is rated for AC or DC. Both will work fine and there will be no difference in performance.
If the heating unit/element has some type of fan for circulation, thermostats, electronics or remote control devices it will not be suitable for a dump load as these add-ons will not work on DC electricity.
Now let’s look at an example:Let’s pretend your solar system is 48 volts (which is really 60 volts) and your heating element is 2000 watts at 120 volts as in the photo above. At 120 volts the amperage of this element is 16.67 amps.
At 48 volts the amperage will only be about 8.33 amps or 500 watts which is is only 1/4 of the element’s rated output.
In a 48 volt system the actual voltage you will be dumping power at will be closer to 60 volts, the bulk charge voltage of your batteries.
First we need to find the resistance of this heating element. A heating element is a resistor.
Now to calculate how much electricity (in watts) this element will consume at 48 volts (which is really 60) we use the following formula:
WATTS=VOLTAGE X VOLTAGE/OHMS
At 60 volts, your 120 volt heating element will consume 500 watts (8.33 amps x 60 volts). You may have noticed this is exactly 1/4 of the power at 120 volts and you are correct.
If you were to use a 240 volt heating element in a 48 volt system, the power dumped would only be 1/16 of the element’s rated power output.
As the voltage difference gets larger between the heating element and the battery system, the amount of dumped power is less and less.
Fortunately it is very easy to calculate how much electricity will be dumped with the following charts.
Here are the numbers:
120 VOLT HEATING ELEMENTS
12 VOLT BATTERY SYSTEM DIVIDE BY 64 (2000W@120V= 31W@15V)
24 VOLT BATTERY SYSTEM DIVIDE BY 16 (2000W@120V= 125W@30V)
48 VOLT BATTERY SYSTEM DIVIDE BY 4 (2000W@120V=500W@60V)
240 VOLT HEATING ELEMENTS
12 VOLT BATTERY SYSTEM DIVIDE BY 256 (2000W@240V=8W@15V)
24 VOLT BATTERY SYSTEM DIVIDE BY 64 (2000W@240V=31W@30V)
48 VOLT BATTERY SYSTEM DIVIDE BY 16 (2000W@240V=125W@60V)
Using the above chart let’s imagine we found a water heating element with the following rating:
2400 watts at 240 volts AC
Our battery bank is 24 volts so we will be dumping power at about30 volts.
How many watts will our new found water heating element be able to dump?
The answer is 37.5 watts!
Using our chart we found that to convert an element from 240 volts to 24 volts we need to divide by 64.2400 WATTS DIVIDED BY 64 = 37.5 WATTS
AC Heating Element Considerations and Tips
Water heating elements are made to be immersed in water. If you use one in the open air it will almost always burn out and be ruined. However, if you are using a 240 volt element on a 12 volt battery system it will only consume 1/256th of its rated wattage. That tiny amount of heat is not likely to damage the element even if in the open air.
Do not use any AC heater that has a built in fan. The fan will not operate at low voltage DC and could even catch fire over time.The fan is made to distribute heat in your room as well as keep the heater cool. If you disable the fan the heater might overheat causing a fire. But if you are only producing a tiny amount of heat disabling the fan will probably not cause any problems.
Polarity is not relevant. As a heating element is nothing more than a resistor it does not matter how it is wired. There is no positive or negative terminal on a resistor.
Always, always use a DC rated fuse or breaker on your dump load. Fuses are not optional. Not installing a fuse (or breaker) may result in severe damage to your system or even fire. Using an AC rated fuse or breaker will also result in damage or fire.
Only use elements rated for continuous usage. Although you could use a burner from an electric stove, keep in mind a burner is not really designed to operate continuously for days and days. Some good examples of dump load heating elements are water heating elements, electric baseboard heaters, electric space heaters (that have no fans).
Seriously consider using your extra electricity to heat water. Dumping excess electricity into an air heater does not make much sense. When you need the heat in the winter, it is likely you have no extra power to dump. In the summer when you have lots of extra electricity, heating the air just adds to your cooling costs.
Do not use AC heaters that have built in digital thermostats or electronics or switches of any type. All of these electronics will be damaged by DC and could possibly start a fire. Only use heaters that have an on-off switch or nothing at all. Electric baseboard heaters generally have no extra controls making them suitable.
There are four different heating elements shown in the photo near the top of this page.
- 1500 watt 120 volt baseboard heater
- 1800 watt 240 volt water heating element
- 2000 watt 120 volt water heating element
- 2200 watt 240 volt space heater
If you had all four of these heating units in your basement, which would be the best for your 48 volt, off the grid electrical system dump load?
My first choice would be the 3rd one, the 2000 watt 120 volt water heater.
First, it is usually preferable to make hot water with our extra power instead of hot air.
Second, the #3 element in the list will be able to dump up to 500 watts (2000 divided by 4) which is more than any other the others.
- #1 will dump 375 watts
- #2 will only dump 113 watts
- #4 will only dump 138 watts
It is almost always best to use a 120 volt heating element instead of a 240 volt heating element as the power output at 120 volts will be multiplied by 4 compared to the same wattage 240 volt element.
A 1500 watt 120 volt heater will consume 375 watts while a 240 volt element will consume only 94 watts at 48 volts. This is a factor of four.