One of my intentions is to make the greenhouse an autonomous entity, connected wirelessly to a gateway (either RF or WLAN based). The greenhouse should be powered by a 10 or 20W solar panel that charges the batteries.

Initial thoughts

Initially I wanted a full autonomous system. The greenhouse should have both the electricity and computing power to be self sufficient and serve webpages. I looked into two candidate systems for automation:

• Raspberry Pi Model A
  
• Beaglebone Black
  

The latter being more powerful and efficient. The Raspberry Pi has a couple advantages compared to the BBB:

• A better community with a lot of shared experience.
  
• The possibility to add a relatively high quality camera for timelapse.
  

The Beaglebone black however has more memory, more processing power and a wider array of I/O ports. 

Both systems have pretty much the same power consumption ranging from 0.5 to 0.8 amps depending on the workload. Based on a 5V power supply, this equals 2-4 watts of power. 

The solar balance

The system will be powered by a battery that is charged by a solar panel. After building some nice panels myself, I decided to go for an assembled version sold by your favorite Chinese broker (Aliexpress). I selected a 10 and 20Watt panel costing approx. 40 dollars together. 

I've been investigating the electricity generated by these panels and was struck by the impact of a cloudy/rainy day. The power plummets and is only a percent of its power with full sunlight. These panels will not deliver enough power to create a sustained power source for the greenhouse.

The compromise

Although I had my high hopes for a full autonomous greenhouse, I've learned that the Netherlands doesn't have the sunlight to power a Beaglebone or Raspberry Pi without resorting to large panels not fit for a small garden.

Therefore I've chosen to go for a low power platform in the greenhouse using cheap NRF24L01+ dongles which connect to a Raspberry Pi as internal gateway.

The low power platform

Thanks to the wonders of Arduino and Energia, I've been able to use the same source code on vastly different platforms (albeit with some minor modifications). The current systems I am evaluating are:

• Arduino Leonardo
  
• MSP430 Launchpad - MSP430G2553 
  
• Stellaris/Tiva C Launchpad
  
• Fraunchpad
  
• F5529 Launchpad
  

Each offering has its own strong and weak points. The jury is still out on the definitive platform. I suspect the Tiva C platform could be a serious contender.