Mesh/Power

Power usage

Ubiquiti routers all use 8 watts. That comes to 5856 watt-hours per month. As of May 2013, the average price for electricity in the bay area was 22.8 cents per kilowatt-hours. $0.228 * 5.856 kilowatt-hours ~= $1.335. So less than $1.5 per month.

Voltage

The Ubiquiti gear takes between 10.5 volts and 25 volts. I believe most of their official numbers state something like 12 to 24 volts, but one of their engineers posted a spreadsheet on their forum which listed the actual limits.

Solar

The shortest day of the year in Oakland is about December 2013 and has about 572 minutes of sun. December also has 14 days of average overcast days (coming in second after January that has 15).

It's difficult to find info on the performance of solar cells on overcast days, which is likely because there is no such thing as a standard overcast day. Let's assume that the performance of a solar panel in Oakland is falls to 20% during overcast daylight hours. On the shortest day of the year, that means we have an average effectiveness over a 24-hour period of:

 (572 minutes / (24 * 60) minutes) * 0.2 = 0.0794

So approximately 8% efficiency in the worst-case scenario. If the overcast efficiency is 10% then that's a 4% efficiency.

Given that the Ubiquiti gear needs 8 watts, we'd then need:

 8 watts / 0.08 = 100 watts

Or 200 watts if the efficiency is 4%.

A 50 watt panel can be bought for a bit under $100. A 100 watt panel can be bought for about $150.

On top of that some electronics are needed for battery charge control. I'm not sure what the price for that is, but probably around $20. And then there's the battery.

Battery

Shortest day of the year in Oakland has (24 * 60) - 572 = 868 minutes without sun. At 8 watts that's:

 8 watts * (868 * 60) seconds = 416640 joules = 115.73 watt-hours

You can get a small 10 ampere-hour 12 volt led-acid scooter battery for about $23. That's

 10 ampere-hours * 12 volts = 120 watt-hours.

That should be just enough, but since the battery will degrade over time, it might be prudent to go with something with a bit more capacity. I also don't know if these types of batteries will quickly degrade if they're routinely discharged to near depletion (I think they will).

Total cost

A very approximate answer to "how much will it cost to make a node off-grid" is:

• About $200 + the price of the router.

So maybe $300 to $350 including the router.