Mini solar with AC backup & monitoring

A petite solar installation to achieve grid independence for a segment of your dwelling. Last log about a voltage hysteresis on Nov 7th '23.

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This project is pretty similar to, but I added a monitor to keep in touch with power, voltages and currents.


After designing, getting the parts and building this project on October '23, I realized the strong similarites with and, I guess, with many unpublished others.

However I published it here anyway because I think a few peculiarities deserve public interest.

Block diagram

Solar Panel 

i bought a nominal 170 W (111 x 77 cm) monocrystalline from Eco-Worthy. At the moment I cannot get more than 105W at noon with full sunshine in October at my 43° 11' Latitude. (I don't know why!).


Price: 85 Euros including shipping

Battery Charger

I bought an EPEVER MPTT 1210AN battery charger because I liked to monitor its working parameters by its RS485 output.

The Instructions say clearly to connect the Inverter to the battery (1) and not to the "user" dedicated output (2) without explaining why ☹️

I suppose to have found the reason. EPEVER asks to connect the Inverter to the battery because the "user" output is capable of supplying a maximum of 20A, while an Inverter like the one I adopted could draw 50A (600 W) and more. However, this solution has an issue, at least for me, because my Inverter starts working (generating 220V) when the input voltage rise to 12.2V and stops when  it falls below 10.8V. Such a range is too limited and well below the battery operating range, which is 11V to 13.5V. In fact the operating range of the EPEVER "user" output (i.e. when the "user" output is powered) is 11 - 13.5V.

Therefore I decided to add a 100A relay (like the ones for engine starters) powered by the  "user" output. In this way I solve also another issue: the inverter draws  around 1A when it has no load ☹️. This absorption leads the battery to discharge down to the limit when its BMS disconnects it and, as a consequence, during the night the battery is not operating and the monitor is not supplied.

The relay is driven by the Battery Voltage. See below the Voltage Hysteresis.


Epever RS485 ModBus Protocol

Price: 71 Euros including shipping

Voltage Hysteresis

This circuit connects the battery to the Inverter when the battery voltage rises above 13.5V and disconnects when it falls below 10.5V.

  1. Adjust R9 to obtain 3 Volts hysteresis (13.5 - 10.5 = 3V)
  2. Adjiust R3 to obtain max=13.5V min=10.5V
  3. (repeat if necessary)


I opted for a LiFePO4 12V 50Ah battery with internal BMS from TimeUSB.  

Price: 155 Euros including shipping


I found a Green Cell 500W pure sinusoidal inverter

The case remains pretty cool (<30° centigrade) while supplying 100-150 Watts, even if the noisy fan starts frequently. For this reason, I changed the original 12V 0.22A fan with a less noisy (and less powerful  😀) 12V 0.1A fan, scavenged from an old PC.


Price: 83 Euros including shipping

Automatic Transfer Switch

Until now, I didn't know the existence of such kind of device. The Jadeshay 2P 100A 220V I bought is nothing more than a very fast (less than 20 milliseconds) stepper relay but it is fast enough to switch almost seamlessly to the AC mains when the voltage from the inverter goes down and viceversa when the Inverter comes back to operate again. The supplied PC's I tested have no problem, while my 32" Samsung TV have a very short screen blanking (the screen becomes black and reappear in half a second). 

I see two issues: 

  • the relay sounds a very noisy "clack", which I attenuated inserting the ATS in a 1.5 cm thick polistirene box.
  • its electrical life is guaranteed for 2000 times (see Instructions) i.e. switching 6 times a day it should live less than an year ☹. I'll try to study a mosfet/TRIAC alternative or adopt a solution like this TRIAC based transfer switch.


Price: 35 Euros including shipping


I built this circuit with an ESP32, an RS485 board and a...

Read more »

  • A new battery supervisor

    Guido11/04/2023 at 21:31 0 comments

    Attaching the Inverter directly to the battery, It's up to the Inverter to decide when generating 220V ac and when not.

    My Green Cell 500W 12V pure sinewave decides to generate power when the battery voltage reaches around 12V and stops when the voltage goes below 10.5V. 

    These two parameters are fixed, while I wanted some more flexibility and extending the range to 13.5V,

    So I'm designing a voltage comparator, which you'll see soon in the Details section.

    Bye Bye!

  • Second solar panel

    Guido11/04/2023 at 21:15 0 comments

    After adding a second solar panel 😎 (170W, just like the first), I observed that when connected in parallel, the total power generated, as measured by the EPEVER 1210AN, was approximately 130W. This was the same as the power generated by a single panel under similar lighting conditions.

    Initially, I ruled out a malfunctioning panel and turned my attention to the MPPT battery charger, the EPEVER 1210AN. To my surprise, I discovered that its maximum power input from the panels for battery charging was limited to 130W 😂😂😂.

    To address this limitation, I purchased and installed a larger model, the EPEVER 3210AN, which can handle a maximum power of 390W 😀. I'm eagerly awaiting tomorrow morning (if not raining ⛈) to see the results!

  • Monitoring using Home Assistant and ESPHome

    Guido11/04/2023 at 20:59 0 comments

    As I explained in this chapter in the project details, I've decided to experiment:

    • Home Assistant dashboard instead of ThingSpeak
    • ESPHome fimware and configuration instead of my ESP32 custom software

    The funny thing is that the hardware remains exactly the same.

  • Power balancing

    Guido10/20/2023 at 18:04 0 comments

    In a sunny day, at noon my single solar panel can supply no more than 110W and 20-30W if cloudy.

    The appliances I connected (2 PCs, 1 32" monitor, 1 network switch, 1 LED lamp, two mobile phone chargers) need between 50 (1 PC in stand-by ad the monitor off) and 120 W.

    Therefore, even during the sunlight (120 > 110) the solar panel cannot charge the battery 🙁.

    So I decided to buy another solar panel to get 220W when sunny and 40-60W when cloudy. I'll let you know!


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Guido wrote 10/18/2023 at 17:53 point

Thank you Dan! That ATS is working very well. Anyway, beside the strong noise "clack" when it switches, I found another issue: its electrical life is guaranteed for 2000 times i.e. switching 6 times a day it should live less than an year ☹. I'll try to study a mosfet alternative!

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Dan Maloney wrote 10/18/2023 at 17:21 point

Cool build, I like that automatic transfer switch particularly. I'll have to check it out -- might help with my solar setup. Thanks!

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