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My goal is to regulate loads in my offgrid inverter-powered system.

I moved to a fully offgrid house with batteries and a pure sine inverter. I would like to control how much I dump of my solar production into my water heater, like when I was grid-tied: meter the house net and use a basic zero-crossing TRIAC to regulate the heater anywhere from 0 to 100% according to my will.

Eg. with a 2.1kW production and a 3kW heater set at 30% I would have 1.1kW left for other appliances.

However I am not knowledgeable enough to make my mind regarding the impacts on the off-grid inverter and/or on the noise on the mains, without the backing of an external electricy supply (and pretty long cables, ie. using the grid as a regulating flywheel...)

I keep on fearing worst case scenarios. Say, with a 2.6kW inverter, I could theoretically have two 2.6kW heaters at 50% duty cycle (one cycle on, one cycle off, the average being 2.6kW). But how true is it with an off-grid inverter? Because if both of them fire in sync, it would be 5.2kW half of the time and 0W the other half. Instant peak vs fast average...

My swiss-made inverter seems robust (studer compact 2600-24) but I fear it is not designed to work like this (I have RTFM twice). Or it might reduce its lifespan, or switch to protection mode, or make other devices fail all around the house with severe voltage spikes and drop downs.

As a final remark, I obviously favor zero-crossing because of possible inductive loads like pumps. May be I might better focus on resistive-only loads like heaters and opt for higher frequency PWM switching (like, 500Hz or more?). But would it really be kinder to the inverter and overall noise, how much would such PWM interefer with the inverter correct operating mode?

All in all:

  • feasability of PWM zero-crossing regulation on an offgrid inverter?
  • impact of higher frequency PWM?
  • other? (eg. use an expensive, slow motorized variac, or try to add some inertia/smoothing via kers or motor+alternator seem all overkill). Bypass the inverter and try to connect directly to the low-voltage batteries (unsuitable existing cable size) ?

Nb: When using conventional means to control a heating element, how does it affect the peak required VA? is pretty close but does not answer my need to regulate smoothly from ~0 to 100%

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    \$\begingroup\$ Heaters generally do not need phase control - simply on/off control via a thermostat is more common. Obviously a 2.6kW rated inverter will not appreciate a 5.2kW load. As for how the inverter would cope with full load, read the datasheet for the inverter. It might be rated at 100% load indefinitely at full temperature range, or there might be some derating. The specs for the inverter should outline this. if you have specific questions about the inverter, then you'd best be asking the manufacturer. \$\endgroup\$
    – Kartman
    Jan 25, 2022 at 12:00
  • \$\begingroup\$ @Kartman sorry may be I was unclear, but this is not about the max load of the inverter neither what the heaters can accept. I precisely that I do not want them to be either off or on. Ie. if there are 760 watts left "unused" from my solar panels after all other higher priority appliances, then I want to put exactly 750 watts in my heater. What I want to know is the consequences and possible risks of using zero-crossing pwm to do anything between 0 and 100% power, and how to do it if there is a better way. \$\endgroup\$
    – MoonCactus
    Jan 25, 2022 at 13:47

2 Answers 2

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Good question! You are correct to be concerned because in the unlikely event of the controller's overload detection being cycle-by-cycle, you would theoretically have a problem, especially in the scenario you have described. But I would lean toward phase-controlled switching on your regulator rather than zero-crossing to insure you have a symmetry on your inverter's positive and negative halves of the waveform. I wouldn't worry too much about instantaneous overload from your regulator because the inverter's design probably has some overhead to handle surges for starting motors, so it will be able to handle the high instantaneous currents as long as the average current is not exceeded. I would try to stick with one and definitely not go more than two separate regulators, however, because I think the inverter probably doesn't handle surges more than two or three times its rating, and the surge capability is still needed when you actually do start motors.

Congrats and kudos for your courage in going completely off-grid!

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  • \$\begingroup\$ I use full positive+negative cycles and some bresenham-like algortihm to even out the selected ones. I could add some randomness to avoid stacking demand. I guess I'll soon give it a try with small loads to see. As for inductive loads, so far I only have small-enough motors which nominl current surge stays within limits. And I use a power generator for arc soldering ;) Going offgrid is not that hard actually, but I like the deal, and I practiced progressively higher degrees of autonomy with my previous house ;) The critical thing is the borehole water at 130 meters! Cheers \$\endgroup\$
    – MoonCactus
    Jan 26, 2022 at 1:53
  • \$\begingroup\$ Just a gut feel, but stringing overload cycles and no-load cycles through your inverter magnetics seems like it might have some unintended consequences - I think you are wise to experiment some. Good luck and I'd be interested to know what you learn. \$\endgroup\$
    – John Birckhead
    Jan 26, 2022 at 15:50
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I have a Voltacon(Voltronic rebadged) 11kW Max Off Grid Inverter currently with 1 4kW String connected. (and a utility charger supply) I have a 3kW PWM controller supplying a 3kW immersion heater. I have CTs on the Inverter output and the utility charger input so can monitor solar shortfall. With the Inverter in Solar>Utility>Battery Mode, and the PWM running at 50% the Utility demand is never zero even when there is an excess of solar it hunts up and down. With the Inverter in Solar>Battery>Utility Mode, it cycles the PV load up and down instead which results in cyclic charging and discharging of the battery to try and match the apparent changing load caused by the PWM. I have seen suggestion of using a DC load controller direct from the PV connection may be a better option than chopping up AC. My PV Array is 400V though and don't fancy DIYing something for that.

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  • \$\begingroup\$ Mark Townshend - Hi, Welcome to the site. You posted this as an answer, but from a quick read, I don't see a solution contained in it. Instead, it seems like you are saying that you have a problem too. Is that correct? || Stack Exchange has different rules than typical forums and something posted as an answer must answer the original question at the top of the page. Otherwise it may be deleted. Please see the tour & help center for the main site rules. You can also edit your answer if you want to clarify it. Thanks. \$\endgroup\$
    – SamGibson ♦
    Jun 26 at 10:55
  • \$\begingroup\$ This does not really answer the question. If you have a different question, you can ask it by clicking Ask Question. To get notified when this question gets new answers, you can follow this question. Once you have enough reputation, you can also add a bounty to draw more attention to this question. - From Review \$\endgroup\$
    – Greenonline
    Jun 26 at 11:25

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