What are the ways to power 5V sensor using solar panels?
I have an outdoor sensor (which will have to work in the winter on Poland, however, we usually have temperature much over -10 Celsius degrees) which needs 5V.
The sensor operates 24 hours per day and consumes about 300 mA when working, for about 10 seconds every 5 minutes and less than 1 mA the rest of the time.

Energy required:
Operate:   10s in 5 mins = 10/300 = 1/30th duty cycle.
Sleep:     1 mA when not operating.
Can be seen as 1 mA continuous + 299 mA x 10/300 = 10.97 ~= 11 mA average
mAh/day = 24 hours x 11 mA = 264 mAh.
Say 250 mAh/day for calculations.
Warsaw in December  averages 0.67 sunshine hours per day - Wow! - that's low.
Due to losses from panel to sun alignment, panel to battery matching, battery store &amp; retrieve efficiency and more, storable solar energy is about 50% of the panels nominal maximumcapacity or less.
To get 250 mAh/day you want say 500 mAh + from PV panel.
If you use a 9V panel with linear regulator that's energy of
9V x 500 mAh = 4.5 Watthour per day.
As there is as little as 0.67 hours of average sun panel size needs to be
4.5 wh/0.67 SSH ~=  6.5 Watts.
A 10 Watt PV panel may be &quot;safe&quot;.
That is far larger than many would feel was necessary. It may not be.
I said 9V panel out above. I had in mind a 7.4V LiIon battery - 2 series cells. This will charge from 9v. It can use a linear regulator. This is wasteful of PV capacity but more easily implemented than a switching regulator type solution.
In sub zero temperature conditions (snowy) PV panel efficiency will increase slightly over nominal but all batteries will either have much degraged energy output capacity or &quot;just not work&quot;. Manufacturer's specification sheets should be consulted and a good idea of minimum temperatures is required.
Lithium based batteries are about as good as any at sub-zero temperatures. Claims vary by manufacturer and model but as a guide LiIon may be rated to -10C and LiFePO4 to -20C.

Battery capacity for 1 day is &gt;= 250 mAh as above.
Somewhat higher is &quot;most wise&quot;.
If you want operation over N days of no sun  you need N x 250 mAh of battery.
http://www.gaisma.com/en/location/warsaw.html


You're saying about 50% storable solar energy. What does it mean?

The apparent energy you can get from a PV panel of a given rating and the amount you can use after storing and recovering it vary by a factor of say Ks. In real world situations, if the panel does not track the sun (usually it doesn't), and if it is not perfectly clean &amp; snow free, and if the panel optimum operating voltahge Vmp is not exactly the battery optimum charge voltage AT ALL TIMES then available energy and mominally available energy will vary. I am saying that starting with Ks = 0.5 is in the order of right. eg a clean optimally pointed 10 Watt panel at 25C in full midday summer sun for one hour will deliver ABOUT 5 Watt hours of energy out of a typical battery system - and not the 10 Watt hours you might expect.  Battery to panel matching can make a useful difference - and this is what a MPPT (maximum power point tracking) controller does. MPPT is effectively an &quot;electronic gearbox&quot; that finds the optimum V &amp; I operating point for a panel in current situation.

BTW I can only find solar cells which have only over a dozen % of efficiency. Where should I put that efficiency in my calculations?

Panel output per solar input is what is relevant to you.
You are interested in how much power you can get at a given insolation (light power) level. While efficiency influences panel size  an eg 10 Watt panel has the same power output whether it is 10% or 20% efficient - the 10% efficint panel will be about twice as large - and this matters substantially in some applications but is not very important in others.
In this case efficiency is (probably) not too important in it's own right - unless perhaps size constraints apply.
eg if you have 1000 W/m^2 of insolation and a 10% efficient 10 Watt panel you will get 10 Watts from it. The 10 W depends on the efficiency and area but you do not need to know either to calculate output as the manufacturer has included those parameters when rating the panel. A 20% efficient 10 W panel could have half the area of active material - but it's still a 10 Watt panel. So, efficiency is not needed in your calculations if you use the manufacturers core ratings. - usually Wmp, Vmp, Imp.
Usually at 25C panel temperature and AM 1.5 (an optical path to panel spec).

Durability in outdoor use:
&quot;Crystalline Silicon&quot; PV panels are assumed. See comments at end.
Note that if the panel will be &quot;out in the weather&quot; continually and if you want years of service there are some panel types that are extremely good and some that are completely unsuitable.
Epoxy resin encapsulated = VERY poor lifetime. These have a rounded edge and no frame. Lifetime outdoors is a few  years for a &quot;good&quot; one and can be 6 months in some cases. Suitable (if then) for toys and products not constantly exposed to the sun.
&quot;PET&quot; encapsulated. Plastic outer layer similar to that used on softdrink bottles. Heat laminated using (usually) &quot;EVA&quot; adhesive/encapsulant. Will withstand &quot;years of exposure&quot;. Quality varies with manufacturer.
Glass front sheet + EVA encapsulant. The &quot;typical&quot; industry standard method of producing PV panels. Usually with an aluminum frame. 20+ year lifetime typically. 30+ years with care. 40+ years with luck and decreasing output (I have one). Heavier and more fragile. The best solution for outdoor long term use.
Other:  There are other &quot;frontsheets&quot; and panel types that are less liable to be encountered. Fluro-plastic (FEP etc) can have excellent results. Usually from specialised suppliers - not common on retail / hobby market.
Amorphous Silicon - smooth dark layer on glass with fine grooves between &quot;cells&quot;. Older technology. Works OK but has few advantages other than having low cost per Watt, sometimes. Low efficiency, fragile, output drops with age unless 'tempered' occasionally. Unlikely to be a good idea.
CIGS, CdTe, ...   Less likely on retail market. May be flexible or low cost. Best versions are competitive in efficiency with crystalline silicon.