WildSilo

Good to hear. I have looked at a few combinations of AC/DC and AC elements. For example, the spec sheet shows 8-20 Ohm resistance for the 1100W PTC element at 25C (rated at 72V). But that cannot be the typical/stable resistance, as you would need about 15A at 72V to get to 1100W. And that means the steady state resistance is at about 4.7 Ohm. Same story for the AC elements. I have two geysers in series (I heat up both in the day, staggered). The reason is some shower in the evening, then the main geyser is still fed with hot water, and next morning others can shower again, still with hot water. Works almost 100% of the time without needing any Eskom. The second geyser was like a cheap battery for me. To make a meaningful difference to the hours that I heat these two geysers off my inverter, I need about 10kWh (5 hours at 2kW currently). In winter, I get about 3.5 full hours equivalent from my panels on average. Maybe I can get 4. So I would need 2.5-3kW PV to get to this target. So I need a 3kW element. I can then circulate the water between the two geysers right next to each other) to spread the warm water as if it is a large 300L geyser I am heating. A cheap 10L/min circulation pump will mix them 50/50 in under 10 minutes a day, and fully cycle one geyser through the other one in 15 minutes. So that is very easy. Now the theory. If I put 6x 470W (43V Vmp) panels in series (I will get less than 2.8kW from them, but let's assume I do), then I get to 260V/11A/2.8kW/24Ohm. 3kW element at rated 230V is 13A/18Ohm. Do you think this will work without an MPPT/controller? The PV string's resistance is higher than the element's, to avoid the element resistance blocking current (18<24), PV Volt is about at the max Volt of element (265V), PV power is within element Watt limit (3300W). I can get the PC for about R11k + delivery it seems. R11k, say R13-14k with install to sort out two geysers winter and summer and give me back 2kW on my inverter capacity most of the day sounds like a good plan without any lengthy new pipework and associated risk. I just want to test the theory with someone who has done it. What do you think? Enjoy the weekend.

Hi Pieter I realise it is quite a while since you posted, but I am considering replacing my 2kW AC element with a PTC AC/DC element. I currently run 3x geysers on 2kW via my inverter (staggered). Has been working well for a couple of years, but when there are extended outages, I need to watch out for overloading the inverter (As there is no Eskom backup). So dropping 2kW load from the inverter for a few hours a day will help. So I was thinking to add 2 new PV panels to the DC side (keep the AC side connected to my inverter). With PV prices as low as they are, I can get anything between 2x330W and 2x550W for a decent price. I think this is similar to what you have done. No MPPT, jsut straight to the DC side. The Geyserwise MPPT is way more than a panel, wo it starts to destroy the ROI. I understand that I need to match the resistance of the PV panels (e.g. 2 in series) to the DC element. But the PTC resistance is stated strangely (as it is a variable resistor). For normal elements, the calculation is quite straightforward. If I decide to go with 2x550W panels at Vmp/Imp of 83V and 13A, that works out to about 6.2 Ohms on the panels' side. I see the 900W element (which had a max of 1100W) has seemingly been replaced by an 1100W DC element now (with a max of about 1200W). So 2x 550W panels is as high as I want/need to go. If I get 900W for 3-4 hours a day out of it, I have covered most of my required output for one geyser. The 1100W PTC element is rated at 72V DC. That gives about 15A/4.7 Ohm. But the spec states between 8-20 Ohm resistance at 25C. (https://www.geyserwise.com/2kw-ac-1100-dc-72v-ptc-element/) The variable Ohm I assume is as temperature rises. So I am struggling to match the PV Ohms, and I know bad matching destroys the yield. Since you have done this already, I wanted to know what you found the PTC resistance to be (or how you calculated the PV Vmp/Imp combination to match)? Would you mind sharing? Thanks Gerhard

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