Sun Apr 02, 2023 7:06 pm
Specified is input voltage range. So anything inside that range is fine, including 12V. If you use 12V, then device will draw 7W/12V=0.6A (actually slightly less). If you'd power same device using 28V, then current would be 0.25A.
Following an exercise in calculations (take results with lots of grains of salt):
So if you target autonomy of 5 days, then needed battery capacity (12V setup) of 120hx0.6A= 70Ah (I've used exact value for current, not the 0.6A aproximation). It might be possible to get to target autonomy with slightly smaller battery if solar pannel set is decent - even during cloudy periods solar cells produce some small amount of power. Be sure to use "deep discharge" batteries (usual car batteries don't like being deeply discharged) to make them last longer (more deep discharge cycles).
Dimensioning of solar cells is next thing: you want something that will be able to power your gear and charge battery at the same time (because it has to top up what was consumed during previous night). Consider winter time with night duration of, say, 16 hours (depending on latitude, winter nights are longer in more Northern latitudes) and not so sunny during day light. Which means deficit of 9.5Ah or 112Wh.
Decent solar panels have efficiency of around 27% (check the specs of those you're considering) and typical winter cloudy winter day might provide 300W per square meter (if panels are tilted to point directly towards sun ... tilt them for winter, summer has longer days and panel efficiency is not as important). During 8 hours of day light, solar panels have to produce 56Wh (for direct gear power consumption) plus 112Wh (to recharge battery ... actually it will be more because charge/discharge cycle of battery has losses as well), so around 200Wh. Divide it by 8 hours and consider actual solar power (300W/m2 versus theoretical 1000W/m2 which is usually used by panel manufacturers), which gives you needed nominal power of installed solar panels of about 85 Watts. Which should be around 0.35 square meters of solar panels (net effective area).
Now the above calculation only covers night time consumption. Mentioned battery size should be enough to power your gear for 3 days of gray, but recharging to full capacity will be long and painful ... perhaps too long to be rady for next 3 days of gray. So you should consider going for a slightly larger solar panel, something with nominal power of 100W-120W would probably suffice.
Probably it's needless to say that during summer this setup will be pretty overdimensioned, but a decent solar power regulator / battery charger should take care of it.