Connecting dumpy load to sunny boy

MarangujonMarangujon Registered Users Posts: 2
Dear sir/madam,
Am John a renewable energy engineer based in Nairobi Kenya. I have a min hydro system which I installed a year ago an now I want to incorporate water heating element as the dump load. I wanted to enquire how do I connect it to the system. The element is AC.


  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    AC vs DC power to a water heating element does not matter to the element.

    The bigger issue is that it is relatively "easy" to switch AC power on and off. Switches need to be designed much larger/heavier  to turn on/off DC power (DC current "sustains" arc much better than AC current).

    To pick the right element--Need to know more about your system. What working voltage will you be using? Will it be AC or DC?

    You can use a 230 VAC (and VDC) rated heating element at 120 volts, but it will have about 1/4 the wattage:
    • Power = V^2 / R = I^2 * R
    (1/2 the voltage or current, the square of 1/2 = 1/4).

    Another issue with dump loads and water heating... If the water heater gets hot and turns off the element (or the dump load fails electrically)--What will happen to your Hydro system (will the turbine over speed)?

    And--Is this a domestic hot water system? You can heat the water to >~165F (74C)--But the danger of the very hot water can cause scalding (burns) on your family/workers. Typically, you do not want to go much over 120F (49C) at the faucet for safety.

    With solar/dump load water heating, typically a "tempering" valve is used to mix very hot+cold water to a constant maximum set temperature.

    Also, with any solar/dump load application, you want to protect against failures. For example, a failure with switches is that the (for example) the contacts can weld together and raise the temperature to over boiling in the tank (if pressurized water system). A T&P safety valve (temperature and pressure) is used.

    Exploding water heaters (and boilers) is an old and very dangerous problem. Water heaters can take off like a rocket or level a house if they overheat and no T&P valve.

    What you want can certainly be done--But you really need to very careful with the safety and failure issues.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MarangujonMarangujon Registered Users Posts: 2
    Thank you. Am using 240v heating element. Then the overheating scenario may not arise because the hot water will constantly be in use.
    Then I need clarity on where to connect the element on sunny boy to take up the excess power when the batteries get fully charged.
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    What exact Sunnyboy inverter model are you using (link to model is OK). And which battery charge controller (if any) are you using?

    Generally, you have a choice of connecting a heating element to the 240 VAC output of your inverter (if you have enough extra inverter capacity). Or, you can run from your 48 VDC battery bank (or what ever voltage).

    You can use a 240 VAC element at 48 VDC--But the amount of heating will be much less:
    • (54 volts nominal/240 volts rated)^2 = 0.051 or ~5% of element rated power
    • 2,400 Watt AC element * 5.1% DC bus voltage derate = 245 Watts
    If you can run at 240 VAC--You will get much more heat and not have the DC switching issue (you should be able to get different Watt AC rated elements- pretty cheaply).

    Then you have the issue of when to turn on. Some solar charge controllers have programmable outputs... For example, they may turn on a signal when the batteries are floating. Or turn on when batteries are "gassing" (for running vent fan).

    You can get "dump load" controllers from Schneider/Xantrex (so you can have a "real" charging cycle. Or you can use a voltage controlled switch (say turn on at ~54.4 volts and turn off at  ~51,2 volts) to control a 240 VAC relay to the heating element.. (DC dump controller) (voltage controlled switch)

    I realize that you are not in the US--Just using above links to give you an idea of what to look for.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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