Limit water heater power based on solar production - Page 1
Forum section: electronics / general

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 09, 2025, 11:00:33 am
content: I have a water boiler with a 2500W resistive heater. I would like to limit the power consumption of the boiler to whatever it is produced by the solar system. For example, I can only produce 1000W so the rest of 1500W are pulled from the grid.I initially thought of a DC-DC converter (the resistive element doesn't care if it's AC or DC) that I could wire to a microcontroller, fetch the data from the inverter via mqtt and control the power output.Has anyone thought of something like this before?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 09, 2025, 11:57:50 am
content: You are on-grid?Your purpose is to maximize self-use of PV and avoid buying energy unnecessarily? (e.g., by heating water for 3 hours at 1000W using PV, instead of heating for 1 hour at 2500W out of which 1500W comes from grid). - am I correct?If that, then the start would be to Google around / ask your grid company what is the billing netting time period (for example, here it is 15 minutes). Then just pulse width modulate the heater to keep the grid power close to zero. If the netting period is long (such as 15 minutes or even an hour), then a relay/contactor does fine. In some countries you may need triac control on every half-cycle of mains.Quote from: bogdan2014 on February 09, 2025, 11:00:33 amCareful! The element doesn't care, but the heater would have a thermostat (probably two - a normal one and extra safety one preventing it from boiling), and if you try to switch DC with those, they'll likely weld shut and burn down your house.

author: 5U4GB
author_rank: Super Contributor
timestamp: February 09, 2025, 01:53:25 pm
content: There's a much easier way to do this which is to wire up a NO contactor that only closes when you're producing a certain amount of PV power, at which point the HWC will run as required. You won't optimally extract every watt of PV power from the system but it'll do reasonably well and is simple to set up.

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 09, 2025, 03:54:31 pm
content: Quote from: Siwastaja on February 09, 2025, 11:57:50 amYes I am on grid and my company has a fixed tariff. The point is to use as much solar energy as possible. In the summer that's easy, the panels produce more than 2.5kW peak, but in the winter it's less than that. So for example, if I don't need to heat the water in 30 minutes using 2500W (1000W from solar and 1500W from the grid), I can heat it in 1-2 hours using only 1000W solar.Good to know about the thermostat, maybe I could replace it with an immersion thermistor sensor?A triac should work well, and I could just pulse the power (I've done it before on a soldering station). Even one of those AC dimmers should do fine.QuoteThat's how I do it now, I have it automated to only run when peak solar production is expected.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 09, 2025, 04:20:50 pm
content: Really, the best way is to find out the netting period for your country / region / contract. I know how difficult it is to find with Google or from transmission company website. Can you download hourly data from your grid / energy company? In the best case, if you never see both import and export during the same hour, then you have hourly netting. You can also try an experiment of running the heater 5 minutes out of every 15 minutes and see if the export is recorded close to zero during that hour - then you would have 15-minute netting.If that is the case, then you can pretty much extend your current idea of turning it on with high production, to one where you PWM it with mid-level production, with PWM period synchronized to the netting period used for billing. For example, here with 15-minute netting, we could run the heater 10:10-10:20 and again 10:40-10:50, so total 20/60 duty cycle, without recording simultaneous export and import in the bill. In other words, utilize the grid as a virtual "capacitor".

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: February 10, 2025, 12:09:31 pm
content: You can buy an easy off-the-shelf unit to do this.It uses a clamp meter to monitor your import/export at your meter tails. Then PWMs the water heater to adjust export to zero.https://www.theecoexperts.co.uk/solar-panels/divertersThere are many different manufacturers e.g.:https://www.immersun.co.uk/products/power-diverterhttps://www.orionairsales.co.uk/solic200-solar-power-diverter-ce-solic200-solar-diverting-for-immersion-heater-16607-p.asp

author: 5U4GB
author_rank: Super Contributor
timestamp: February 10, 2025, 01:18:02 pm
content: Solar diverters work but are -ing expensive compared to the other solutions people have suggested.Also you just install them and they work, where's the fun in that?

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 11, 2025, 09:38:37 am
content: Quote from: 5U4GB on February 10, 2025, 01:18:02 pmThey look nice, but I almost paid that much on the inverter for my PV system.QuoteI think there was a misunderstanding, by on grid do you mean export? Sorry, I'm new to PV systemsI use a hybrid inverter that does not export back to the grid. It's set to use PV as the primary source and the rest from the grid.

author: benseno
author_rank: Regular Contributor
timestamp: February 11, 2025, 12:07:29 pm
content: Quote from: Siwastaja on February 09, 2025, 04:20:50 pmI like this idea of PWM, yet it can be extended to instant power utilization without accumulating it somewhere else.DC from solar can be connected to the negative rail and rectified AC from power supplier to the positive, with both having common ground. The ratio between negative and positive PWM pulses will be controlled to be proportional to the solar power generation and to be delivered via half mosfet/igbt bridge.

author: Zucca
author_rank: Unknown
timestamp: February 12, 2025, 03:03:04 pm
content: I have a Victron ESS system setup as no feed into the grid (when the 54KWh battery are full).As soon the solar charger (Victron SmartSolar MPPT RS) are not in bulk mode (=absorption or float) the panels are moved away from MPPT.In float the panels stay close to open circuit or move to MPPT to match what the home is demanding, automagically.It is more complicated than this, I just wanna point out the concept.Quote from: bogdan2014 on February 11, 2025, 09:38:37 amBefore start to invest time and money to mod the water hater, invest in batteries.Having a no feed into the grid system with no batteries is a sad story to tell.That said I support the PWM solution for the heater, I only fear the big DC capacitors in the inverters would not be too happy about it.

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 13, 2025, 06:21:28 am
content: QuoteBatteries are in plan as budget allows.The question is how do I actually tell when there's maximum solar production? Without loading the panels I can't really tell how much power they give.

author: benseno
author_rank: Regular Contributor
timestamp: February 13, 2025, 08:14:44 am
content: Quote from: bogdan2014 on February 13, 2025, 06:21:28 ambattery voltage? illumination/lux sensor?

author: Zucca
author_rank: Unknown
timestamp: February 13, 2025, 03:13:31 pm
content: Use a small cell as a sensor for solar power available.The current flowing in a cell in short circuit Isc (between + and - connected together) is proportional to the solar light intensity.Then you need a sensor for the solar panel temperature Tpanel (seeherewhy).Isc and Tpanel will give you a decent estimation of how much power is available from your solar panels.

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 19, 2025, 11:28:27 am
content: Quote from: Zucca on February 13, 2025, 03:13:31 pmThat sounds like a really cool project to tinker on

author: cmorgan
author_rank: Newbie
timestamp: March 01, 2025, 01:40:38 am
content: @bogdan2014 I've been working on a system like this for a bit, still haven't gotten it entirely off of the ground but I have a few installed that have been running for a few years and a new, more robust version in progress.https://op.solaris the site. Drop me a msg if you'd like to chat.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 08, 2025, 04:26:51 am
content: You might be able to get close enough with a heavy duty theater dimmer that accepts a 0-10V signal connected to a tiny 12V solar panel with a shunt variable resistor to set the effective gain.

author: Seekonk
author_rank: Super Contributor
timestamp: March 08, 2025, 10:26:37 pm
content: I heat water with PV and a power point controller that is quite efficient at transferring maximum power. These are easy to build. Blending power would be pretty easy if there was a constant need. Best option would be power element with a low voltage from grid with thermostat set to a very minimal voltage. Solar would blend in naturally. I pulse power to element from solar. When those pulses are present, no power comes from grid. Are you currently using something dumb like direct connect to panels to heat water?I have a washer that runs from solar array. When cloud comes over, inverter blends in up to 150W additional power to run washer. When enough power from solar the inverter turns totally off. Blending is fun.

author: Zucca
author_rank: Unknown
timestamp: March 15, 2025, 09:03:17 pm
content: Or use a VFD to control the heater power?