Designing (and Sourcing) a DC-only Solar System for a Single Device

[SoonerLater]

NEED: permanent, 24/7 power for a Google WiFi wireless access point (WAP) where conventional power is not available.

SOLUTION: solar panel to charge 12v battery to power a 12v female "car cigarette lighter" receptacle to a male USB-C car adapter to a Google WiFi WAP

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A Google WiFi device (or the newer generation Google Nest Wifi) is a "mesh wifi" device. It works in concert with other devices of the same family. The original Google WiFi device is rated by Google to draw 9 watts, which is supplied via a USB-C charger (which is DC). Real world measurements of the device show it really only draws about 3 watts.

If I want a system which can supply 15 watts (just to be safe) at 12v DC continuously 24/7:
(1) what size solar panel is required?
(2) what size battery is required?
(3) since a DC-to-AC Inverter is *not* required, is there any need for any sort of "solar controller" even required?

 

[gnubie]

15 watts is pretty high consumption. Is that just the label rating? In my experience a lot of this stuff is usually closer to half that. The extra power is only used if you plug in USB storage etc. If you can hook a multimeter in line with the DC and check the current. It'll move around quite a bit during startup but then it'll settle down. Throw a lot of traffic across the wifi link and take a note of the average current.

The actual long term consumption will tell you what you need to terms of panel / battery / etc. Given that its all low power you'd use a PWM charge controller board (not a recommendation to buy this particular one) rather than anything more serious. You can get those generic ebay style PWM chargers but I'm not sure of their own power use, never checked.

 

[darrenbeau1]

Will you be able to reach it with Ethernet or will it be a wireless bridge? If you can get Ethernet to it I would suggest power over ethernet to save you a lot of headache. My understanding is that Google WiFi points can do both.

 

[PHoganDive]

Some general figures based on supplied numbers:
15 watts x 24 hours is 360 watt/hours per day of usage.
A 12 volt, 100 amp hour battery has 1200 watt/hours total capacity.
If you use lead-acid and a 50% discharge, that gives you a bit under 2 days of autonomy, a lithium battery with deeper discharge could get you around 3 days.
A 100 watt panel with 4 hours of sun a day will give 400 watt/hours of charge per day, with system losses it will barely break even...200 watts of panel would give 800 watt/hours per day, which should give enough extra for system losses and less than fully sunny days.
200 watts @ 12 volts is 16.66 amps, so the charger needs to be able to handle that much current...20 or more would be fine.

 

[SolarRat]

@PHoganDive has you covered with battery and panel size, 12v, 100ah with a 100-200w panel and 20 amp charge controller would work great.

Cig sockets are common fail points, so I'd just use something like this (but for USB C!!):

Amazon.com: DC-DC 12V to 5V 3A Micro USB Converter Voltage Step Down Regulator Waterproof Power Converters for Car Smartphone: Electronics

Buy DC-DC 12V to 5V 3A Micro USB Converter Voltage Step Down Regulator Waterproof Power Converters for Car Smartphone: Power Converters - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

Real world measurements of the device show it really only draws about 3 watts.

If this is the case you could go much smaller on battery and solar if needed....obviously

 

[AaronBrand]

NEED: permanent, 24/7 power for a Google WiFi wireless access point (WAP) where conventional power is not available.

SOLUTION: solar panel to charge 12v battery to power a 12v female "car cigarette lighter" receptacle to a male USB-C car adapter to a Google WiFi WAP

------------

A Google WiFi device (or the newer generation Google Nest Wifi) is a "mesh wifi" device. It works in concert with other devices of the same family. The original Google WiFi device is rated by Google to draw 9 watts, which is supplied via a USB-C charger (which is DC). Real world measurements of the device show it really only draws about 3 watts.

If I want a system which can supply 15 watts (just to be safe) at 12v DC continuously 24/7:
(1) what size solar panel is required?
(2) what size battery is required?
(3) since a DC-to-AC Inverter is *not* required, is there any need for any sort of "solar controller" even required?

I’m doing something similar. What did you end up using? I’m thinking about hard-wiring the power supply to a fuse bar rather than having a 12v plug. The ac power supply with the unit (MeshForce) is rated for 1 amp and 12 volts, so it is similar to what you have. I’m wondering what you’ve used. I don’t think that it needs to be bigger than the 1 amp rating, which would be 12 watts max. Like you said, it uses about half this power. I’m working through the math for my location and I think it will be smaller than the 200w solar panels mentioned by one person here. I have that setup and it runs a light and water pump and small fridge on most days so far (though I had a few days of dark clouds and the fridge was preventing the battery from charging all the way. I’m looking at the shortest day of the year and trying to go from that number of hours of daylight.

 

[12VoltInstalls]

I’m working through the math for my location and I think it will be smaller than the 200w solar panels

In my opinion 200W panels is ‘just barely’ and one battery at that rating? I’d want more. Several more.

More panels charge the battery(ies) better AND the cloudy days are less of an issue. Particularly with lead acid you don’t want to deep discharge or stay discharged over days at a time - not discharging deep: that will tend to help the batteries last a long time.

 

[JohnAntonioCollins]

I stumbled on this old thread searching for something unrelated. But I happen to have set up a simple solar power kit for an Arlo wifi security camera. If the appliance has it's own battery, you certainly can simply connect a CLA (cigarette lighter accessory) usb charger directly to a nominal 12v solar panel. With the Arlo camera I encountered a problem. The camera requires the QuickCharge usb protocol. So I had to get a CLA QC charger. I encountered another problem. The CLA QC charger required some minimum wattage to operate. I started with a 15w panel and that was not enough. I changed to a 25w panel (amorphous from Harbor Freight) and it's been working well for years now.

For reference, the Arlo's internal battery gives it a few days to a few weeks of standby depending on how much activity it does by motion-sense or manual remote viewing. This camera is located outdoors and temperatures regularly go below freezing here in the winter. I assume that it has some kind of bms and a lithium cobalt oxide battery?

That said,
I imagine there exists a usb battery that can simultaneously charge and discharge. That could be used between the CLA-usb and a usb appliance. But beware of low temperature behavior if it is relevant.