Smart divert of excess PV generation

Long story short, I have a secondary, very weedy, solar PV system with a max output of 0.7 kWh installed on a barn that has its own mains meter. Background usage is only ~ 30w continual, so when the sun shines most of the generation goes out to the grid for no recompense.

I have very little that I can use this “wasted” power for. An obvious start would be to power two small fans that keep a classic car storage “bubble” inflated 24/7. AIUI they use 2 x 220 mA = 440 mA = 0.4A @ 12 vDC.

This is all very small amounts so I don’t want to spend £££ saving a few pennies, but it might be a fun project to divert the excess generation in to a 12v battery, and then run the fans from that.

What would be a good way to a) monitor when excess generation is happening and b) then turn on the battery charger?

I have an emonTx & an emonPi capturing generation and usage. They currently report to a local emonCMS.

Would this be a use for MQTT (something that I have yet to investigate) coupled to NodeRed or IFTTT and then a smart 13A plug socket for the battery charger? Or would there be something better? Ideally I’d like to include some form of buffer so that the charger doesn’t activate until there is specified threshold of excess production (perhaps 50-100W???).



if you want you can try my diverting software for emontx shield I could easily set it up for that that it turns on and off a relay for secondary devices as it has relay support built into it

I can easily adjust for that situation … but I update sometimes today as it currently has auto relay support on the third ssr I make that support 1,2 ,3 and 4th ssr depending on what amount of SSRs you choose to use

here you go if you would like to try–
you need a for a say the headless version
wemos r3 ( uno/wifi)
emontx shield ( or build your own)
and a Arduino relay block 1-4 ( your choice but i go with the 4 in case you would like to add more functions )
build as in the instructions
set sketch to
set uDIVS = 250 /// what step to turn on relay you want the step to be higher then the load
DIV = 0 // turn off relay if it goes positive
ios = 1 // number of SSRs to control
element to the size of your array

it will search for diversion but since you are not using a hotwater diversion the step will continue to climb until it reaches step 250 ( probably by about 3 seconds)
then it turn on the relays. then once it goes positive it turn it off…

after it all build and ready you need to send this web command

http://your_ip/control?cmd=pwm,04,355   //turns auto function on for switch1

if you set up the web espeasy web you need to enable task 4 with quad input and label switches 1 to 4 but then you will see that it sent command to switch 1 and set it to 3 which is auto mode

if you do not want to use auto function you could in the rules set it up
example – using onput from inverter

On Diverter#GTI>200 do  
  pwm,04, 295

the firmware will turn it off the relay once grid goes positive