Wednesday, November 16, 2011

Sizing Solar to Routerstation Power Specs

As I mentioned in the last post, we've been having a little trouble solar powering our nodes (in the woods) through a New England winter.  Turns out we get crap for sunlight.  Who knew? :P.  As a result I'm trying to get the most out of limited resources and redesign the nodes so they fail gracefully when they run out of juice.

I did a little experimenting with the Ubiquiti RouterStation earlier today to figure out exactly what its characteristics were.  Here's a few things I learned:
  • Minimum reliable operating voltage 9V? (It was difficult for me to determine this number exactly due to the limits of my power supply, but it will DEFINITELY not operate reliably below 8.5V and at 9V it seems to boot reliably and send data over one radio.  Using multiple radios hit the current limit of my power supply at 9V, so we just have to assume that the RouterStation itself is not current limited in the same way)
  • Peak current draw with three radios transmitting at capacity @12v 1.5A(est)
  • Average power consumption with three radios transmitting full time: 17.5W(est)
  • Average power consumption with three radios powered, but idle: 7.6W
The first element to consider when working with 12V batteries is that the margin between the minimum expected battery voltage and the minimum operating voltage of the device is quite small (~2V).  Even in a perfect world, the voltage drop when sending the peak current over 24AWG cat5 wire (assuming worst-case single conductor, 11V battery voltage), limits the cable length to just over 20 feet.  This number gets smaller if you have questionable cabling.  This short analysis suggests the use of 24V solar systems instead of 12V.

An additional problem that may arise, in the event that I'm right on the edge with the amount of battery storage I'm using, is that the devices will brown-out and hang.  This has already happened a couple of times during some cloudy weather, and requires someone to go out and hard-reboot the devices.  An easy solution to this problem is to use a power supply with a Low-Voltage Dropout (LVD) circuit instead of powering devices direct from the batteries.  This has the dual feature of protecting the batteries from over-discharge and cleanly cutting off your device before the voltage gets too low; in this manner enabling devices to turn off in the wee hours when there is insufficient light and come back in the morning without fanfare.

I've been eying these gadgets for quite some time now:

They come in 12 and 24V inputs and with direct power out or DC-DC conversion.  They take input from a solar panel and passive POE, charge your batteries, and provide both passive POE and wired output with LVD.  At $60 in quantity 1, they're a reasonable one-stop solution to building a single setup that will work almost anywhere. 

Since I'm stuck with some big 12V panels that I can't easily re-wire, I bought the 12-24V up-converting version.  According the company, this conversion is 80-85% efficient, which is why in the final spec we want to run native 24V.

Now some solar math (doing all this math for the intended system, not the one at Farm School)...

First find yourself a solar energy calculator.  For the US, this one is pretty cool.  For locations outside the US, try the older version.  Using the map, you'll be able to find the amount of energy per m^2 at your location, and then import into the PVWATTS calculator.  The PVWATTS tool calculates total available solar energy by month, as well as the total energy output of your system after derating (since we're using direct DC, you can omit the derating for any of the AC components). 

Now you might be asking "what does kWh/m^2/day have to do with the 60W rating on my panel?".  To make sense of all this, you must read the fine print
PV module power ratings are for standard test conditions (STC) of 1,000 W/m2 solar irradiance and 25°C PV module temperature. Caution: these are different than PVUSA (PTC) test conditions
 The important take-home from above is that "standard" conditions for the rated wattage of most solar panels are based on a solar energy of  1kW/m^2.  As a result the, value of kWh/m^2/day, is equivalent to the number of effective hours that the solar panel will be able to operate at its rated output (assuming the panel operates at the same efficiency over a broad range of input energy, which is generally true).  for example, a 60W panel during a day with 3kWh/m^2/day of available solar energy will generate 180Wh of energy before derating.

Looking at our device, we draw 17.5W at full throttle and 7.6W at idle.  If we're only using a little under 50% of the radio capacity on average, we get about 12W worst-case power draw.  Over one day that amounts to 288Wh of energy.  In Athol, MA, the worst months provide about 3kWh/m^2/day of energy..  Our DC power system, based on the PVWATTS numbers, probably derates to about 0.86 of the nameplate value, so we need:

288Wh / 3h / 0.86 = 111 W of solar to operate 24hours during the dead of winter.
Since mounting etc, are bound to be imperfect, you'll probably want to round this number up a bit too. 

Though a 120W panel might be enough to power our device on average, any given day might have more or less solar energy.  In order to size panels to the average, you need enough battery capacity to cover cloudy days.  Batteries are generally rated in Amp hours (Ah), which is the number of hours they can provide the equivalent of 1 Amp at 12V, which is 12W.  Conveniently, our device draws 12W, so the number of Ah in a battery is the same as the number of hours it will operate the device, except for one twist.  The Ah value usually describes the capacity of the battery to full discharge.  Fully discharging the battery over and over can damage it, so we want to plan for a less than full discharge.  For a deep cycle battery (deep-cycle = old-fashioned lead-acid battery with thick, unsophisticated electrode plates), 80% is a drop-dead limit.  For easy math, we'll say 75% is our discharge limit, and that we want to operate for 36h sun-free.  In the worst case, that means a 48Ah battery, which is on the order of the battery in your midsize passenger car. 

At Farm School, I'm operating two radios on the solar-powered devices at near-idle, so estimating 8W and derating a little extra for the DC-DC conversion gives me a need for about 85W of solar and 36Ah of batteries.  We'll see how this goes...


  1. Where can I get the reflectors files for download?

    Any antenna recommendations?


  2. Go here to learn more about reflectors:

    The best antenna to use depends on the application. As a general statement, parabolics are the best for long-range applications. Devices like the Ubiquiti PowerBridge have integrated parabolics. For short-medium range applications we have been using simple high-gain omnis (9dB) for 2.4Ghz and the Ubiquiti AirMax Omni for 5Ghz.

  3. Have a look at MPPT solar chargers instead of the PWM ones - at maximum sun, you'll capture a much larger percentage of the power coming from the panels.

  4. Solar power will really help us in stopping global warming, I hope that many people follow this. Thanks.

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  8. Informative post! I love it. As an expert, I'd like to share a little bit.

    Panel voltage output
    Panel quality
    Wiring quality
    Switch gear durability
    Inverter quality
    Operating temperature ranges
    String arrangements
    Strength of mounting solution
    Resistance to weather
    Optimum positioning to eliminate panels getting dirty
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    All the best-
    Devorah Cherry

  9. This is a good way on sizing on your solar router station's power with this you can definitely cut down on your electricity bill.

  10. The first element to consider when working with 12V batteries is that the margin between the minimum expected battery voltage and the minimum operating voltage of the device is quite small (~2V). solar panels home

  11. An additional problem that may arise, in the event that I'm right on the edge with the amount of battery storage I'm using, is that the devices will brown-out and hang. This has already happened a couple of times during some cloudy weather, and requires someone to go out and hard-reboot the devices. Portola Valley solar

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