Disaster Solar

Using what I learned from my previous 30W system, I decided to create an ‘ideally sized’ system for low powered devices. The device I had in mind is a low-cost router that is very popular with the OpenWRT community, the TP-Link 703N. This ~$25 router consumes about 1 Watt of energy (~200mAh @ 5V) and is powered via a standard micro USB port.

The solar setup for this system was sourced entirely from Amazon. It consists of four components, which you can see in the image above: A 10W solar panel, 8Ah 12V battery, charge controller, and a cheap 12-24V->5V power regulator which provides 2 USB ports, providing up to 2.1Amps @ 5V.

Total cost on the electrical side, sourcing entirely from Amazon:

$39.99 + $20.79 + $17.59 + $9.12 = $87.49 

The only cost left out is wire and connectors from my parts drawers, the most significant in this case being a salvaged cigarette lighter port which would run you $2-4 on ebay or Amazon.

Based on my calculations, this system could run 24/7/365, if it were positioned ideally (here in sunny California) and there were never more than 2 days in a row without sun. It would continue to run for 4 days in a row without sun, but this would damage the battery, so I use an Arduino with a TIP120 transistor to cut power to the 703N, preventing battery over-discharge.

My preferred battery type is an absorbed glass mat or gel battery. AGM batteries are sealed, do not requiring watering, and can be used in any orientation. They can even be carried in checked baggage on airplanes, so your system is truly portable! This is useful for rapid deployments to disasters or just for traveling to a conference with a demo sized solar system.

Make sure your battery is a deep cycle battery. If you keep the average cycle at about 50% discharge, you will have a long lifetime at a low cost. Over-discharging will reduce the battery lifespan, so my systems are designed to have 2 days of buffer– that is, run for 48 hours with no sun before discharging below 50%.

The panel in this case is sized to generate enough power from 1 day of winter sun to charge the battery from 50% to 100%, while simultaneously powering the TP-Link 703N. That means we generate 48Wh per day to charge the battery, and 24Wh per day to run the 703N.

The calculation I used to figure that out is below, and also explained a bit more fully in my presentation at the 2013 International Summit for Community Wireless Networks. Here’s a PDF download of the slides from my presentation: Solar Power Basics

The calculation below takes as input:

• The power consumed by the 703N
• The hours of sun on the ‘worst-case’ day, which you can find using a sun chart.
• The number of days in a row without sun you expect.

It then determines how large of a battery and solar panel you will need. You’ll notice the required battery capacity is four times the daily energy usage, providing ample buffer and preventing over-discharge. The 10W solar panel will be able to charge the battery from half to full with seven hours of ideal sun per day, while using the 703N as a Wi-Fi access point.

This is one of my first setups! It’s a 30W Solar Panel, 22 Amp Hour Battery, and a cheap charge controller.

$84.50+$45.15+$19.99= $149.64. 

Since these components are all low voltage DC, and the amperage put out by the solar panel is quite low, I suggest you use standard 2.1×5.5mm DC barrel plugs to connect everything. Grab a pack of 10 male  and 10 female from Amazon. If you keep developing solar power setups as a hobby, you’ll definitely use them all eventually, and a pack of 10 barrel plugs is roughly the cost of 1 ‘solar panel connector’ like this MC4. These also have the benefit of connecting directly to devices that use 2.1×5.5mm barrel plugs, though you should always take care to ensure you provide the voltage your device expects, and that your battery and voltage regulators are capable of providing enough current to power your device.

The Wi-Fi radio in this system is a Ubiquiti Nanostation Loco M2. It’s running the Commotion DR2 firmware, which allows it to broadcast an Access Point locally while simultaneously communicating with a distant radio.

In order to maintain portability, I went with a 22Ah battery, which weighs only 14 pounds (6.35kg). I wanted a system that could be put in a backpack or travel box, and still be comfortable to carry with 1 hand. This system is pretty powerful considering its portability, but is still a little undersized for Ubiquiti devices. I’ve used a 5W average power consumption for the calculations below.

Based on my power measurements, this is actually a slightly too small panel and battery to run this load 24/7. The M2 Loco draws between 4 and 8 Watts when being used. The Nanostation can drain a 22Ah 12V battery in 33 hours-53 hours, depending on the number of users. So this system works great for occasional or temporary use… i.e. 16 hours a day, or for up to 2 days in an emergency. I try to run it for 20 hours or less at a time, to ensure the battery never drops below 50% charge.  With a 50W solar panel and a 40Ah battery, you could run a Nanostation M2 Loco 24/7 even with 2 days in a row of no sun.

 if you want to run any device off solar power 24/7/365, step 1 is to determine the ‘minimum hours of sun per day in winter’ using a U. Oregon sun chart. You also need to determine the average and/or maximum power consumption of your device, and determine either how many days in a row without sun your area can get, or if the occasional failure is acceptable, the number of ‘reserve days’ you can be powered with no sun.

If you want to perform the calculation above for your own devices, I made a spreadsheet that does the math for you. You’ll also find power measurements from common devices, including the Nanostation M2, TP-Link 703N, Raspberry Pi Model B, and some newer devices, such as the Village Telco Catbert and Koruza Wireless Optical Device.