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Pi Off-Grid-Project : powered by power bank and solar-panel: some calculations
dear friends hello and good evening, 
A new little winter project is just around the corner: I still have a few Rpi3 lying around here - and I've had a few projects in mind for a while - some of which are "off-grid" - because a Pi is particularly suitable for that. It requires comparatively little energy - can be operated with a battery or, if necessary, a small solar panel. But to ensure that the energy is always sufficient here, so that the operation is "stable", a lot of preliminary considerations are necessary - and that's why this thread:
My question is: Can I power my Rasphberry 24/7 with a solar panel and a 5000mAh power bank? Or to put it another way - how big does the panel have to be? So a small one Off-grid project in the planning phase:
At the beginning I think about the stable power supply: What if I want to recharge the power bank with a PV solar module: How big does this panel have to be!? The thing is, if I can't charge the battery, the RPi won't run for more than 5 or 6 hours. So for the remaining 18 hours of the day we rely on the panel, whose characteristics are rather low, to both power the RPi and have enough left to charge the battery for the next 6 hours...
Do you have any experience here - have you ever done something like this?
Approach: So I think at the beginning I have to determine how much power the Pi actually needs to run for a whole day,
So at the beginning I need a rough estimate of power consumption, charging requirements and solar output; So I can then determine (or estimate) how big the PV solar panel should be?
Do you have any experience with power supplies that include a PV panel in such a way that operation is guaranteed over the long term? At the moment I'm particularly interested in how big the PV panel should be.
I'm looking forward to hearing from you!
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How much current depends on CPU load and if you are using USB, wifi or even bluetooth. You can turn off unused peripherals. Worst case for a Pi 3 would be somewhere around 1.3 amps but you might want to measure the actual load with it up and running everything like it would in actual use. Basically the current load in amps x estimated run hours x 1000 would be the size of the battery in mAH. You might want to oversize it by some factor. Worst case a 5000 mAH battery would last around 4 hours.
How much daylight depends on where you are located in latitude... The solar panels needs to provide enough current to power the Pi plus charge the batteries. Basically how laptops do it.
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dear michaelk 
first of all: sorry for the delay. Had so much Things to work on in the mean time.
Thanks a lot for your detailed and very engaged answer! That really helps break things down into measurable components. 😊
You're absolutely right that power consumption will vary depending on peripherals, CPU load, and usage patterns. I'll do some real-world testing with a power meter to measure the actual current draw under typical conditions.
I suspect that running WiFi and logging data will push consumption towards the higher end of the scale.
Regarding the battery calculation: So, if my Pi 3 at full load needs around 1.3A, and I want it to run 24 hours, that would be:
Code:
1.3A x 24h = 31.2Ah (31,200mAh)
Which means my 5000mAh power bank wouldn?t last long at all! 😅 Definitely need a larger battery.
Now, for the solar panel sizing: Since the panel needs to both power the Pi and charge the battery, I?d need to produce at least 1.3A continuously while also charging. If I assume around 5 hours of peak sunlight per day, that means the panel should generate:
Code:
(31.2Ah / 5h) = ~6.2A at 5V (~31W panel at 5V, or higher at a different voltage)
So in theory, a 40-50W panel might be safer to account for inefficiencies and cloudy days. Does that calculation seem reasonable to you?
Also, do you (or anyone here) have experience with charge controllers that can efficiently handle both powering the Pi and charging a battery simultaneously? I want to make sure the setup is stable and reliable over time.
well to sume up: thanks dear Michaelk - its awesome to see such support: thank you for the insights?this is turning into a really fun design challenge!
and i am Pretty sure that this will help not only me - this might help others too - that are here in the fourm.
have a great day
ps. I am so glad to be here in this great forum. Thank you for all!!
....Love Linux - and the Digital Innovation Hubs (network) supporting digital future: towards digital transformation, commitment to sustainability,
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Dhubs..
Spring is approaching.. Hit the garage sales at 7:30 AM Saturdays in search of cheap solar panels.. Last year I acquired 3 precious solar panels for $2 each from the silly garage sales.. One large one, and two 32-volt battery charger panels...
The key to getting a garage sale item cheap is to say: "How much for this wrecked thing?.. Will you take a couple bucks for it?.. Maybe I can use it for parts".. Always when you approach a garage sale, say "WoW! Nice garage sale".. (they worked long a hard setting it up, respect that).. Make friends with them.. Friends get lower better prices... On a ten dollar item, say "I'm a bit of a garage sale dicker'er. Will you take 2 dollars for this busted old thing?.. You'll probably get if for $3... Smile is crucial.. NEVER ever gaze into a seller's eyes while dickering.. Christians believe they are gods.. Their cult dictates that 'everything is made of god', therefore in their delusion they being part of everything are essentially god'... You never ever gaze into a god's eyes, like peasants never ever gaze into the king's eyes, nor into a huge wild carnivore's eyes... Gaze into their eyes, you'll get their price.. Look down, you'll get your price.. They'll make noises trying to acquire eye contact.. Hold your ground gazing down... Sometimes 'sharpydoo horse-trader' buyers offers get rejected, when my lesser offers get accepted simply because I prevented eye contact...
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You are probably not going to build a charger and you do not want the battery level to normally drop below 20%.
It also depends on what type of li-ion battery you buy as to what the charger may use as its max charge rate but plan on it being the same as the battrry capacity. That plus the Pi 1.3 amps is what is needed output from the panels.
I have not really played much with solar panel hardware lately.
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