I use a smart solar PV diverter for our hot water resistive element storage tank.
Australian device:
Catch Power Green
For single phase 230/240 V supplies. Grid tied systems only.
There are various such diverters available but Ive little idea what is available in the USA.
The unit monitors our power flow with a CT clamp and knows when we are exporting energy to the grid. It then adjusts up how much power is sent to the HW heating element, in the case of my unit it uses a burst fire method of power control, although some diverters use PWM.
Depending on how much power solar PV my system is generating and what other loads are operating at the time, it might be sending 100W to the heating element or it might be sending 2 kW or more to the heating element, it instantly adjusts the power delivered to the heating element based on the available excess PV generation. It can divert anything up to the element's rated 3.6 kW. If other household loads exceed available solar PV generation, then it will cease diverting energy to the hot water system.
It tends to keep the balance such that there is still some small amount of export to the grid so that grid imports are avoided.
Here's a chart showing the power flow for one 24 hour period for the supply phase this system operates on.
Positive = importing from the grid.
Negative = exporting the the grid.
From midnight to 5:30 am just a small base load of ~80 W imported from the grid, mostly electronics.
At 5:30 am a reverse cycle aircon system is turned on for heating in the second dwelling. Grid imports rise to 600-650 W.
From ~7:00 am the solar PV system begins to generate, and production gradually ramps up as the sun rises higher. As a result grid imports begin to decline.
At about 8:00 am there is enough solar PV generation such that we are no longer importing energy from the grid.
From ~8:30 am the diverter begins to send power to the hot water heating element. As more solar PV output becomes available it diverts more power to the hot water, and exported energy is kept to a low level.
Just before 11 am the hot water thermostat switch opens and so no more power is being diverted to hot water. The excess energy is now being exported to the grid.
In the period during which diversion was occurring, the second dwelling heating would have been operating, and eventually turned off. That increased the power being diverted to heat water, but you can't see it because the diverter automatically and quickly adjusts how much power is diverted based on changes in production (e.g. clouds pass over) and consumption (e.g. household loads are turned on and off).
Can also see later in the afternoon the system diverted a little more energy as the thermostat likely closed again, but these were short top up heating cycles of 5-10 minutes.
If by the end of the solar day the diverter has not been able to complete a full water heating cycle (e.g. it was a lousy solar day), which it will know because the water heater's thermostat switch didn't open at all during the day, then it will use overnight grid off-peak power to complete the heating cycle. I can tell it to never use grid power and instead wait until the next day to heat, but for safety I prefer to ensure the tank never has the opportunity to linger at lower temperatures for too long (legionella risk).
I did some readings over the last few weeks. It's mid-Winter here, so PV production is limited. It's operating with what is effectively a 3.6 kW PV array (one phase of my 3-phase system) and a max inverter supply of 3.3 kW. In a three-week long period we imported an average of 0.7 kWh/day from the grid. Normally at this time of year we would import 5.5 - 6 kWh/day for hot water. As we head out of Winter there will be much less requirement for the system to top up from the grid. June is the "worst" solar production month here and we imported about 2 kWh/day on average.
Overall I am expecting to reduce our import of energy for water heating to about 10% of what it was, with 90% being supplied by the solar PV.
I'll have a much better handle on system's annual performance at end of Q1 2023. So far it's had the wettest and most prolonged crappy solar PV Autumn period on record and Winter to deal with. Even so our grid imports for hot water have been reduced by 2/3rds.
