Harvesting Surplus Energy, Off-Grid: Page 2 of 3


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With wind and hydro systems that can’t be allowed to operate at open-circuit, dump loads, like these resistance air heaters, are necessary—but they usually don’t do useful work.
ProStar MPPT charge controllers can switch diversion loads directly (based on a fixed battery setpoint), using LVD mode with a rather high setpoint.
Morningstar TriStar can be configured for stand-alone PWM diversion using simple DIP switches that also select the battery type.
Blue Sky Energy’s charge controllers (left) mostly contain a 20 A load relay that can be used for diversion. Also, the DUO Option software upgrade converts the Solar Boost 3024 controller’s auxiliary output into a 20 A diversion-type PWM charge controller.
OutBack Power’s FLEXmax series can divert PWM current to loads via SSR based on prevailing battery-charging voltage setpoint.
The Schneider Electric C-series of PWM charge controllers can be configured for stand-alone PWM diversion using jumpers. Two potentiometers and a multimeter are used for adjusting the battery-charging setpoints.
MidNite Solar’s Classic controller has two aux outputs, one of which can divert PWM current to loads via SSR. The KID controller offers PWM diversion directly (without SSR) from its load terminals. In both cases, control can be based on prevailing battery-charging voltage setpoint.
MidNite Solar’s Classic controller has two aux outputs, one of which can divert PWM current to loads via SSR. The KID controller offers PWM diversion directly (without SSR) from its load terminals. In both cases, control can be based on prevailing battery-charging voltage setpoint.
Morningstar products can operate relays based on a variety of parameters via the optional Relay Driver module. Their MSview software has many advanced tools for customizing relay drivers and charge controllers.
Solar converters offer low-cost stand-alone circuit boards that you can build into your own control cabinet. The LDR is a PWM load controller, whereas the VCS is a mechanical relay that switches on and off based on battery setpoints configured with a screwdriver.
Mechanical relays are simple, inexpensive, and efficient, but they switch more slowly than solid-state relays. They have a limited service life, so it’s wise to buy a relay base that you can plug them into for easy replacement (as shown in the lower photo).
Mechanical relays are simple, inexpensive, and efficient, but they switch more slowly than solid-state relays. They have a limited service life, so it’s wise to buy a relay base that you can plug them into for easy replacement (as shown in the lower photo).
Solid-state relays are ideal for PWM or rapid, frequent switching, as they will not wear out, but they do generate some waste heat and need a heat-sink to prevent damage.
Specialty water heating elements often come with several subelements. Use them in series for higher voltage or for lower power. Use in parallel for operation at nominal voltage, switching as many as needed, using one or more relays.

Diversion Controllers & Relays

One way to set up an opportunity load is with a separate controller. Use a second pulse-width modulation (PWM) controller, such as Morningstar’s TriStar or the Xantrex C-40, configured for diversion mode, to do the job of controlling the battery voltage. Keep your solar controller for the sake of its maximum power point tracking to maximize energy capture. Set the MPPT controller’s charging setpoints slightly higher than the PWM unit’s setpoints, so the PWM is activated first by rising voltage. If your MPPT controller has no means of driving a relay, then adding a second controller is a good way to set up opportunity DC water heating.

Often, your MPPT controller will “know” when there is excess power. MidNite Solar’s KID charge controller can be set to “PWM Divert” and run a DC load (like a heating element) directly on its load output. Many of Blue Sky Energy’s SolarBoost controllers can also switch loads using an internal 20 A relay, while its DUO upgrade option contains a PWM diversion control function. Most other makes of MPPT controllers offer an auxiliary output or “aux port” that can produce a 12 V signal (or close some switch contacts) when a battery voltage setpoint is reached. Connect this to the coil or input of a relay and it will switch on a load to make good use of the surplus energy.

Aux ports need to be configured for a particular “mode” that determines the criteria for switching. MidNite’s Classic and OutBack’s Flexmax charge controllers offer modes that energize their aux ports when the battery voltage setpoint is reached for the prevailing stage of charging—absorption, float, or equalization—just like a dedicated diversion controller. If you plan to use diversion whenever possible, then you should use these modes. In other MPPT controllers (and inverters), the aux port modes offer only fixed voltage setpoints. A fixed voltage setpoint for diversion will either be too low to allow proper absorption or too high to be activated during the float stage of charging. It may work well for relatively low power loads or for heating a small water tank with a thermostat that opens after an hour or so, but otherwise it will prevent your RE system from properly charging the battery.

Some controllers offer Aux port modes that signal when charging has reached the float stage. Several offer modes for a certain percentage state of charge (SOC). These modes may be worth considering for operating motorized opportunity loads, such as irrigation pumps that can only work at full power. But they will miss out on the gradually rising surplus of power that occurs during the absorption stage.

High-array-voltage triggers are another possible mode to use for diversion. If the controller is rejecting surplus PV power, then the array voltage will rise beyond the maximum power point. You can choose an Aux port mode to trigger diversion as the array rises a little above its normal, observed MPP voltage. This should not interfere with the battery’s ability to achieve absorption voltage setpoint, but your trigger point may be a moving target. This mode is ideal for hydro turbines, but less so for PV arrays, as the energy capture drops fast as the array voltages rises above MPP voltage and the MPP voltage will vary with array temperature.


A relay is a switch for high current that is operated by a tiny current. Older mechanical relays use an electromechanical solenoid to close physical contacts. They can open and close many thousands of times but will eventually wear out. Choose these relays for modes that cycle on/off over periods of minutes—for example, driving motorized loads. Use a “plug-in” relay that is easy to replace every couple of years.

For more rapid cycling (many times per second) conditions, choose a solid-state relay (SSR) that uses semiconductor technology. SSRs are more costly and they need a heat sink, but they can be driven fast enough to implement pulse width modulated (PWM) switching that ramps the average diverted current up and down smoothly.

Use a solid-state relay (SSR) when you need frequent, rapid switching. For example, an OutBack or MidNite controller’s Aux port in PWM mode linked directly to the input terminals of a Crydom D1D40 SSR can reliably modulate a DC load up to about 25 A at 60 V (2.4 ohms, 1,500 W). Mount it on a heat sink rated for less than 2°C temperature rise per watt. You can also use an AC SSR to switch AC loads via the inverter, but a heavy load may cause your lights to flicker as it pulses.

When choosing a relay, make sure the is rated well above the working voltage and current. As with most products, they have a higher failure rate when pushed to their limits, so good safety margin, such as a factor of two, is wise. Some relays are designed for AC, and some for DC—which require heavy-duty contacts due to arcing potential. Aux ports typically provide 12 V, which will work for most SSRs. If you only have aux contacts switching 48 V battery power, it would need to somehow be stepped down below 30 V to be usable for SSR input. Mechanical relays can be found with many different coil voltages, including 48 V DC.

You can set your relay to operate a DC heater that draws current directly from the battery. But if your inverter is large enough, you may prefer to divert to an AC load, which has the same effect. The Aux ports in OutBack inverters have an “AC diversion” mode that prevents overloading the inverter. Irrigation pumps and air conditioning are among the possible loads, along with AC water heating elements. (Note that 120 V or 240 V heaters are easier to find than battery-voltage ones, and most thermostats are not designed to switch high DC currents.)

Comments (6)

Mark S's picture

Is there a reason that the "View article as a single page" does not function and that these articles are not available as downloadable PDFs?

Michael Welch's picture
Hi Mark. The single page view does work, providing the entire text of the article. It is strange and potentially confusing, though, that over to the right it still leaves the option to choose which page you are on.

As a digital subscriber, you can download that entire issue, but we don't have a setup for downloading single articles.
dave@atomicsolar.biz's picture

I like using a 120V (or 240V depending on inverter) water heater element(1000-2500 watts), placed in the top element location in a conventional electric water heater, to dump excess. I use a relay and aux 2 output from a Midnite Classic charge controller. It is a bit easier than dumping DC, as you can use smaller wire and cheaper AC elements. This applies to places where NEM is not available, or self consumption is a priority.

solarKings's picture

Enphase's AC Battery is a good residential storage solution which allows you to build out a solar battery farm in your home; each storage battery is independent of the others, uses safe Lithium-Iron-Phosphate, has a long longevity, is scalable up to 14 units on a single DP 20A circuit breaker, and can be installed into any existing or new solar PV system. Because it it is an integrated AC solution combining inverter with charge-controller and battery, it can be installed in under and hour and connect right to a home's utility lug panel. It is an intelligent battery that can be easily monitored from your computer, cell phone, tablet, etc. Cost is about $1,000/kWh installed.

fowlerrudi's picture

Has anyone ever tried a geothermal "invertor series" heat pump that would directly modulate with available surplus power to store hot water in a storage tank? I am considering trying this using by pulling data from Schneider Electric's Combox (or a solar insolation sensor combined with relay from Combox) to adjust load on a variable speed compressor geothermal heat pump off grid. Storing hot water in 1000 gallons of insulated storage for low-temp infloor heating system. Thoughts?

Secundius's picture

@ fowlerrudi.

I don't know if this will answer your question or not?/! But consider placing "SharkBite" PEX/XLPE (Polyethylene) Tubing under the Flooring or Foundation of you're Homestead. Pressure Rated at ~160-lbs/in. and Temperature Rated at ~200F. Comes in 1/2-inch by 500-foot lengths. Some Universities use it as a "Poor Mans" method of Shoveling Snow, to keep the Walkways clear of Heavy Snow and/or Ice. Try "FarmTek.com" for better Information or their Zendesk (Customer Services), their Seasonal so some Info Digging might be required...

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