Solar Power - How many Watt is enough for photon?

Hi all,

Need help from smarter minds than mine :blush:

I need to source a photon with a solar cell, I think 150ma it’s a good average of what the photon will need to work with wifi on…

So… How many watts my solar cell needs to produce to source it?

So… Here’s my math which I think it’s wrong… I just don’t know where or what I am not looking at…

0.15amp x 3.3 = 0.450 watt

So half a watt? So say if I get a 5 Watt solar cell with 50% light, I should be able to power the photon. Since 2.5 watt will be enough to power it?

Yeah I know… I am an engineer wanna be :blush:


Do you only want the photon to work when it the solar panel is illuminated ?

If not, then you’ll want to use some form of battery shield (SparkFun have one, Particle have one and I believe will be selling it just as soon as they can get their ecommerce site to understand the shipping constraints on LiPo batteries…)

Well… Curious… My current setup involves a SparkFun battery shield, a WeatherShield and the Photon.

What I want is the system to have enough power to charge and be used during the day. I am charging a 2000mah lipo… When it goes dark, I will enter a battery conservation mode… Where the weather instead of being reported every 10 seconds, will be reported every 3 minutes and in between the wifi radio will be turned off to conserve energy.

Also a DEEP sleep mode will be used if the power for some reason goes bellow 30% charge. Then the deep sleep will be waken every 2 hours. After two hours of deep sleep the system will boot, report status and decide if it will keep in low power mode, deep sleep again or return to normal mode depending on the power.

What I want to determine is how big a solar panel I need to charge the battery, use the photon with WiFi and still contemplate a not so sunny day.

Now the take home value, is I want to learn how to calculate this things :blush:.

Any help welcomed!

PS @AndyW I tested the other project with the isolation transformer. Works great! I am still alive! On my first trip to US, Il send it back to you!


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I have been using 10W solar panel and 100Ah battery to power Core + Relay Shield + 4 solenoids since Apr 2015. It’s enough for 2 rainy days.

Thanks! @tdphan that is Great info!!

Do you have a part number for the 10W solar panel!

I bought a 5W from adafruit, if I can’t find a 10W il wire two in parallel.

Quick questions:

  1. Is the photon ON 24x7? Or do you use sleep mode or turn off the radios or shut down at night? Or at intervals?

  2. After the solar cell, I am guessing you have 12volt cell… Do you regulate the power to the VIN of the photon or how do you hook it up to the photon. I am guessing VIN input has to be regulated with no more than 5.5Volt?



@frlobo: I bought the panel from China and there is no part number unfortunately.

  1. My core (not Photon) is ON 24x7. Photon consumes less energy so should be OK for Photon too. I live in Ho Chi Minh city (Vietnam) and sun light is good for solar energy.
  2. As I wrote earlier I use Core with Relay Shield which has its own voltage regulator so 12V DC fed directly to Shiels’s input.

Hope this helps.

Hey @frlobo Ive been building a water monitor that uses parts salvaged from this really cheap and nasty solar light
so its a 2200mah battery and a tiny 6v 1.7W solar panel, with a sparkfun battery shield. so far it seems to be holding up when using deep sleep.

Its charges OKish, it gets to 85-90% during the day and doesn’t seem to get any higher… i guess its a voltage thing with the tiny solar panel. overnight it was dropping to about 50% and i estimate it was on for about 10min total.

last night i had the unit inside and had it in programming mode (keeps the unit on) and flashed the photon a few times and drained the battery down to 30%. It went back outside at 8am this morning, now at 10.30am its at 63%. The location of the panel is slightly obscured by trees too so its not direct sun the whole day.

Thank you for great information… I guess il use two 5 watt panels in parallel. I will leave the photon on while the power is above 50% when it drops below I will enter intermittent mode… (Radio off) for periods,… If it drops below 20% I will enter deep sleep mode for 4 hours to allow either day charge or to allow night to turn into morning… Il report my findings…

I wish I could calculate all this instead of doing it by heart.

I am still worries that I need to regulate the 6V to 5V… The weather shield has a regulator. I will try to configure it. And use it.


How often do you use deep sleep? How are you using the photon? Are you constantly have it on, and then turn it off to deep sleep or?

You can calculate with such a method: for simplicity assume your Photon draws 100mA at 5V. It means Photon consumes 0.1A x 5V = 0.5 W. If it runs 24x7 daily consumtion is 0.5W x 24h = 12 Wh. This amount of energy must supply your solar panel. Now look at this map: Solar Insolation Map and locate place where you live (this map shows data for USA. If you live in other country google to find out). Let’s take zone 5 for calculation: daily average (over the course of the year) amount of solar insolation is 4.2 hour. If your solar panel rated 5W means it gives 5W at STC (standard testing conditions). In practice it is only 70% of this value. So you have around 3.5W. Daily you get from solar panel 3.5W x 4.2h = 14.7 Wh. You can see it is enough to power your Photon. Use 10W panel or more if you want more power for some thing else or to secure when no sun light (rain for example).
Hope that it’s useful.


Thanks for the info! @tdphan!

I appreciate the knowledge!

You can have a look at the code in my water usage monitor thread

basically its coming on for 20 seconds each time a pulse is detected, then sleeps for 2hrs unless a pulse wakes it up prior. i also have a setting to make it stay on for programming

OK so i added some code to track how much awake time my photon has or you,
Awake 683sec 1.05% of total 65000seconds
and this dropped the 2200mAh battery from 85.1% to 75.2% while the solar panel was covered over 18hrs
I change the times slightly too, so that’s based on awake for 15sec, sleep for 30min, unless woken by pulse. And it calculates and publishes 4 strings during each awake time.

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