Project in progress
Solar UPS controller/automatic transfer switch 

Solar UPS controller/automatic transfer switch 

Get your essential appliances off the grid with just one solar panel

  • 80 respects

Components and supplies

About this project

[UPDATE] Now using MKR1000 to control the UNplug unit

I replaced the Particle Core with an Arduino MK1000 for several reasons:

  • The pricing policy for a production run with more than 25 units is locking the user into a monthly subscription to access to the cloud gateway.
  • Over-The-Air (OTA) software updates are awesome but having limited control over when this is actually happening was too scary. I had several incidents when the unit was doing an OTA that I did not trigger. Also debugging was very hard. It's a large library that does many things in the background and for this kind of devices it felt too 'big'

I have posted a picture of the current 'wiring' at the bottom: It looks chaotic but the MKR1000 is not pin compatible to the Nano and I had to wire the pins manually.

There is an ugly pitfall with the supply voltage of the MRK1000: The Vin pin is not filtered except a tiny cap to GND. It is impossible to find any power supply that is quiet enough to provide conditions where the ADC converter would create stable precise readings. I looked at the schematics and found a complex filter being applied to the USB +5V input. You can realistically only use that path to supply power. Also the on-board Vin voltage regulator only supports 5.5V and not 6V as mentioned on the aspects page.

[UPDATE] New rev. 3 PCB is now the heart of the UNplug toolbox 

Power plant in toolbox size: the new UNplug power generator can be carried around easily. This is the fully functional prototype - currently only the H4 solar connectors are still on order. I just got the green side panels but need to find the right screws first.

The new design includes:

1500W pure sine wave inverter, sealed lead acid or lithium type batteries in a sturdy metal case with a total weight of 77 pounds. In full configuration this power generator is able to charge an electric vehicle via a level 1 charger! (with enough solar panels attached)

Capacity of the batteries are 0.8 - 1.2 kWh depending on the type. This is enough energy to power your fridge during the night.

I am using a separate power supply now because creating you own stable power supply for 24VDC from 120-240VAC is not easy or cheap. I am also having my own PWM charger for the solar panels (2 rows of mosfets) now. The inverter in this version will need to be one with a remote power-on function. I am also down to 6 cable lugs to make things lighter. 

New and noteworthy items:

  • 30A, 120/240VAC heavy duty relay
  • external 40W switching power supply to charge the battery and power the unit at night
  • 50A, 32V solar PWM charge controller built-in

[UPDATE] Kickstarter campaign, please click here for the project page.

[Original text]

It all started during a stormy winter in 2011 when my power was out for 4 days. I was thinking: if I only had some independent power source available. Maybe I would not have lost all my food in the fridge…


I have developed a small 'box' that let you use solar energy in a very small scale. It powers just a few household items that run 24/7 like the refrigerator, internet modem, security system, laptop, etc.

It is modular, scalable, moveable and it connects any inverter, battery, charge controller, solar panel up to 1500W continuous power. It has wifi and hooks into Home Automation. It could also be labeled as Automatic Transfer Switch.

Project domain:

In the past 2 years I developed a small generator and assembled a shopping list so people can build DIY a solar micro grid. There is a separate page about all technical specs. All parts together costs less then $900 and you can build a solar powered uninterruptible power supply (UPS) that generates you daily 1kWh or more - and it will all fit in a shoe-box.

I think everyone can find one big item in the household that is "always on" like a refrigerator or computer/modem/etc. and pull it off the grid during day time. Just the refrigerator alone consumes between 40W - 160W on average. This adds up to 500kWh per year!

Hardware Design

I have gone from bread board proof of concept into actually manufacturing a small run of PCBs. Most of them are at friends houses who help beta-testing the system in combination with various inverters and charge controllers.

Here is the list of numbers explaining the components:

  • 24VDC supply for external hardware with a max draw of 20Watt
  • header pins to communicate with other hardware: Serial1 and I2C
  • Connector for a Spark Core or Arduino Nano
  • Buzzer
  • Temperature sensor
  • 120V to 24V transformer
  • Heavy duty 6 AWG terminals to connect to Solar panel, charge controller, battery and inverter
  • 120V DPDT relay to switch between 2 power sources which are connected at terminals 10
  • Power mosfet bank to switch up to 50A at 30VDC
  • Terminals for 10A at 120VAC

The current revision of the board is fitted to a standard enclosure from to make it easy to install.

You can find a more detailed description here.

Software Design

The basic functionality which was developed for Rev1 runs stable and has been ported to Rev2. This foundation code runs off-line and there is no interaction required. By switching to the Spark Core I entered the world of IoT which is very exciting but also poses some threats to the stability and reliability. I am currently working on a web page displaying live data coming from my unit.

I am not entirely sure how to make this unit immune to unwanted hacking. Since the beauty of Spark is the remote flashing of the firmware this is also on the other hand the biggest threat. If your power source is at risk you might feel direct impact on your quality of living.

Real-time solar power tracking

Here is a link to the website to access the live data. The graph shows the power generation (green) and consumption (red) for today and yesterday.

Next steps

I am using the above setup to run my fridge, TV, stereo, laptops and chargers from solar energy and its running stable for more than a year now. I am now working on setting up a crowd-funding campaign which is a completely different set of problems. I also need to look into UL certification because it hooks into the 120V wall outlet in your room and it seems to be a requirement for hardware projects.

I will update this page on a regular basis. If you are interested you can take a look at my project blog called 'One Fridge off the Grid'.


Custom parts and enclosures

Case cutout


Similar projects you might like

Integrated Solar ChargeController, Inverter, PowerBank, Lamp

Project tutorial by Shahariar

  • 57 respects

Solar Battery Powered Switch for Blinds, Lights, and Charger

Project in progress by Leon Chu

  • 15 respects

Home Smart Home

Project tutorial by Syed Sanoor

  • 253 respects

Arduino - PV MPPT Solar Charger

Project tutorial by Abhi Verma

  • 54 respects

IoT arduino ESP Garage Door opener (UD)

Project in progress by David Smerkous

  • 23 respects

Automatic Transfer Switch (ATS) with Web Server

Project showcase by Superfunk2000

  • 18 respects
Add projectSign up / Login