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The importance of data logging in any type of renewable energy system is often underplayed. This could be due to various things, but I believe that initial cost vs perceived importance is usually the deciding factor. Perceived importance is based upon a person’s understanding of an overall system and how each component factors in. Obviously, in a typical off-grid solar setup there are several essential components such as solar panels, batteries, charge controllers and possibly inverters without which a system just plain doesn't work. A data logger/monitoring system is not something that most people would even remotely place in the same category – after all, the system works great without it right? Right. Except when it doesn't work. What then?

So what does a data logger really do?

There are obviously different types of loggers that have various levels of sophistication and interactivity, the most basic being a small portable device that could for example just keep track of battery voltage over time and display it on a small LCD screen. This may or may not be useful for troubleshooting your system, it could certainly be tedious to try and discover what happened on a tiny screen. A more advanced data logger would store data on an SD card or USB flash drive and allow you to open up a log file on your PC, either as raw data or in a special software. That's already better, but still a bit of a headache – nobody wants to have to regularly unplug something to get data. So then we come to the more powerful types of data loggers – Internet connected devices with various degrees of intelligence and connectivity. This allows you to access your data through some form of web interface or specialised app on your smart phone. Now that's certainly better! Whenever, wherever you are you can see what's happening in near real time.

Cloud storage vs Local storage

Most internet-enabled data loggers have a small amount of local storage to buffer Internet outages, and will regularly upload their data to a cloud based proprietary website, through which you, the customer, needs to either pay for or use a specific brand of hardware to be able to access. Many inverters have compatible data loggers which export relevant data, but these can be somewhat pricey. The Internet of Things has brought several open source solutions for the DIY hobby space. So what are the benefits and down sides of these different types of devices?



Cloud based Easy to configure Requires a good Internet connection
  Historic data always available online Vendor-specific usually
Offline logger Low cost Limited customisation
  Low power consumption Not real time

Usually portable

Not user friendly
Wattmon Solution Accessible via LAN to customers on site Higher power consumption (2w)
  Can export data to a cloud Not portable, requires some installation
  Highly customisable  
  Real time viewing on any device  


Oh, you noticed the Wattmon solution, did you? Oops. I guess I did have a hidden agenda to introduce my product (doesn't everyone?). Now that the cat is out of the bag, let me tell you a bit about Wattmon and how it's data logging features can benefit you.

Wattmon is a monitoring and control platform designed to be highly flexible. It was initially conceived as a solar energy and battery monitor but it has evolved into much more thanks to customer feedback. I beta tested the product in 2013 in a tiny township in South India with die-hard solar enthusiasts and it grew from there - there's always a “oh, can you also monitor my…” And a “but I want to automate my ..”

The Hardware

The product consists of a master unit (I just release a new version called WattmonPRO) which is the brain of the system. It runs on anything from 5 to 60v DC and is ideally directly connected to a battery bank. It has Ethernet, a USB host port that supports several cellular dongle brands for low cost remote monitoring, and an SD card for storage. An embedded web server provides a convenient responsive interface for all devices from smart phones to large screens.

Several onboard connectors make basic data collection and control easy: 4 isolated digital inputs, 3 of them with pulse counting let you hook up flow meters or other counter devices. 4 open collector outputs let you hook up SSRs or small relays to control things. A built in isolated relay can handle 5 amperes of current, ideal for powering a small load. On the analog side, the power supply voltage is measured through one channel. A second channel can read voltages up to 330v DC, such as a large high voltage battery bank. A third input handles 0-5v dc, ideal for a sensor such as a pyranometer.

The real power comes from the interfaces: an RS-485 port with Modbus-RTU support makes it possible to communicate with thousands of third party modules such as meters, relays and sensors. We created a few devices that cover most commonly required function such as isolated current sensing, analog inputs, relays, frequency measurement and PWM output.

The Rs-232 serial port can be used to collect data from pretty much anything. We provide drivers to Victron and Studer inverters but could adapt them to any serial inverter protocol.

And finally a Onewire port makes it easy to add temperature sensors for monitoring and automated heating control.

The Software

Although the actual firmware is proprietary, the entire functionality of the front end application runs on what I call uPHP, a small (but powerful) subset of the hugely popular PHP language used in most websites today. The scripts are all open source, and a built in file manager and editor makes it easy to customise and program the device to your needs if you require it.

Everything is configurable through a browser and no custom software is required. As a user you can configure additional devices, decide which data to log, generate your own graphs and add automated actions based on anything ranging from battery state to time of day or external sensor values. All this is specifically designed for non-programmers and does not require you to enter a single line of code!

So what does this have to do with data logging?

Regardless of the platform or tool you choose, having the information you need in order to understand and resolve a problem is essential. Without real data you often resort to guesswork. As an example, I installed a Wattmon at a client who had a solar hybrid setup – a 48v battery bank with 1.5kw of panels, charge controller and an inverter-cum-charger. He was very happy to be on solar. After viewing the Wattmon graphs we suddenly noticed that he was not getting any energy from his panels. He even switched off the grid charger for a day. After a while, when the battery was nearly empty the charge controller kicked in. The installers were called and checked the system several times, each time ensuring the client everything was fine. It clearly was not working properly, and I informed the client he needed to get the set points on the charge controller adjusted. Finally after returning it to the company, it came back fixed and everything worked smoothly. This was not an obvious flaw and the system had been running for several months already, but thanks to detailed logging it was easy to identify visually.

Visual view of energy over 24h


Anyway, I could go on and on but if you are interested, please check out the website or get in touch with me. Thanks for taking the time to read this article.




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