solar power for camping

see also:

• camping off-grid - power, batteries, solar, fridges
• solar regulator / controllers for solar panels for camping
• 12V batteries, battery boxes and power stations

• current solar panels are based upon silicon cell technologies which combine the output of a number of cells
• these are usually rated at around 80-350W when tested at Standard Test Conditions of 1000W/sqm incident sun, 25degC and AM1.5 air mass spectral conditions.
  • in real usage, incident light is likely to be 800W/sq.m or less and thus one often uses nominal operating testing conditions of 800W/sq.m, 20degC with a 3.5kph breeze to keep it cool
• the larger the power rating, the larger the surface area needs to be to create that power output for a given panel efficiency
• sellers (especially those on Ebay) may over-estimate the output of panels, test actual outputs or do the maths:
  • The standard test of efficiency and output is done with sunlight falling on the panel with an intensity of 1000 watts per square metre and 25degC. The efficiency of the panel will be in the specs, and should be around 17%. So the panel should be producing 1000 x 17 / 100 = 170 watts per square metre of its area. So to get the actual output of the panel just use the dimensions of the panel, calculate its area in square metres, and multiply that by 170.
• traditional panels have glass fronted panels mounted in an aluminium frame making them bulky and heavy
• more modern “soft fold” panels are much lighter, more compact and more convenient but considerably more expensive than the older aluminium framed ones
• for max. efficiency place at correct angle to the sun (at midday, this angle should be your latitude, whereas in the morning or afternoon, the angle will need to be more than this), this may increase output of a 200W panel from 7A laying flat to 9A placed at a more optimum angle (11A/18V in perfect conditions), plus this allows better ventilation to keep them cooler and they wont kill the lawn as may occur if you lay them flat
• older or cheaper panels may not have blocking diodes in each section and the output may become zero if even one section falls under shade of a tree
• a portable panel is generally more useful when camping instead of a fixed roof panel
  • it allows you to park your vehicle in the shade and place your panels in the sun (you may need an Anderson plug extension cord for this)
  • it allows you to optimise the angle of the panel to the sun throughout the day
  • you can combine the output with a fixed roof panel output for added versatility
  • however, they are at higher risk of theft and damage and obviously you do need to set them up
• you can combine two panels into one controller which should give the combined current
  • just use an Anderson plug splitter (2 plugs going into one plug)
• you also need a solar regulator to charge a battery and ability to choose the correct charge mode for certain batteries such as LiFePO4
  • see solar regulator / controllers for solar panels for camping

• see also https://www.redarcelectronics.com/au/resources/solar-faqs/testing-solar-panels-and-blankets/
• testing the solar panels in full sun with no shadows on the panel disconnected from any controller or battery
  • solar panels are primarily current generators not voltage generators and as such they are not damaged by short circuiting as are batteries but they may fail due to damage to the solar cells or the wires
  • ONLY test ONE panel at a time
  • DO NOT TEST panels with ratings more than 10A ISC or more than 90V VOC without expert help
  • check the open circuit voltage (VOC)
    • set multimeter to DC voltage setting up to 100V and connect its red wires with the panels +ve output and the black wire with the panel's -ve output
    • most panels for 12V systems should have a reading of 20-22V (check the specs for your panel)
    • if the voltage is much lower than this despite being in full sun, then there is something wrong with the panel - if it is 0V then it may be a wire fault
  • check the short circuit current (ISC)
    • set multimeter to DC current (~10A setting) and connect its red wires with the panels +ve output and the black wire with the panel's -ve output
    • most panels for 12V systems in full sun should have a reading of 3-12A (check the specs for your panel FIRST, if rated ISC > 10A ensure multimeter can cope with this!)
    • if the VOC was correct but the current is under 1A despite being in full sun, this would suggest a faulty solar cell which is now acting as a resistor in series
    • current = voltage/resistance
      • a 100ohm resistor in series would result in a VOC reading of 22V but an ISC of only 0.2A current
    • a blown bypass diode inside the junction box may be the cause (eg. from connecting a battery with reverse polarity)
    • lower than rated ISC in full sun may be due to faulty panel section which could be checked by:
      • have panels in full sun and cover each section one at a time to see if it impacts the ISC current (if the ISC current drops that is normal, if it doesn't then that section is not working)
• if the VOC and ISC are as per specs, then load testing may reveal hidden faults
  • a simple visual test can be done using 2 x 12V 21W globes in series or 1 x 24V 55W incandescent globe which should shine brightly when the panels are in full sun
  • if this fails (and the globes are not faulty), then the panel may be faulty or the wire connections may be faulty

• you should purchase Class A (or perhaps class B) rated solar cells, lower classes have too many defects and much less efficiency
• 3 main types:
  • polycrystalline - cheapest, individual cells have maximum efficiency of around 22%
  • monocrystalline - most efficient, individual cells have maximum efficiency of around 26.5% but most panels are rated around 20-23%
  • amorphous - expensive, thinner, more flexible and durable, slightly better in cloudy conditions but only 10% efficiency so need more area
• new shingle cell design allows for better efficiency as¹⁾:
  • there are no busbars (ribbons) required
  • can be joined together in overlapping shingle manner resulting in no gaps between the solar cells
  • they can be combined in parallel rather than series which allows partial shading to be not as problematic EXCEPT when shade is in vertical axis and this may actually result in less power output than conventional panels
  • produce more power per sq. metre as less inactive area
  • improved reliability as lower busbar failures and more resistant to external forces
  • more aesthetically pleasing as no visible circuitry

• the maximum wattage in ideal conditions = voltage at max. power x current at max. power
• the open-circuit voltage, V_OC, is the maximum voltage available from a solar cell, and this occurs at zero current and for most commercial silicon cells at 300degK is around 0.6V.²⁾ Most 12V solar panels have a total open circuit voltage of around 21-23V and generally is highest mid-morning when the panel has not become too hot. This can be measured with a multimeter across the open ends of the wires attached to the panel. The solar controller specifications for input voltage must be greater than this otherwise the controller will shut off.
• the short-circuit current I_SC is the largest current which may be drawn from the solar cell (this can be measured by passing the current through a multimeter configured to measure amps but care must be taken to avoid arcing) and this depends upon:
  • area of the solar cell
  • light intensity hitting the cell
  • wavelength spectrum of incident light (most silicon cells have sensitivity 0.4 to 1.1 micron wavelengths, glass removes most of the wavelengths shorter than 0.4microns)
  • optical properties of the cell - absorption, reflection (surface should be matte with minimal reflection)
  • collection probability of the cell (depends chiefly on the surface passivation and the minority carrier lifetime in the base)
• over-heating of panel reduces efficiency so keep them well ventilated
  • open-circuit voltage for a silicon solar cell (which is usually around 600mV) falls by about 2.2mV (or around 0.4%) for each 1degC rise in temperature (there is minimal change in short-circuit current though) ³⁾
  • the panel's Normal Operating Cell Temperature (NOCT) is an indicator of how well it dissipitates heat - the lower the better as it heats the panel up less during operation.

• what style best suits your needs - portable or fixed to a vehicle roof
• your budget
  • but beware, a cheap panel may break very easily (most are quite fragile) or under-perform and may represent false economy
• what power output you need - aim to get more than you need - most campers will want 100-200W
• does it have blocking diodes so that shade on, or damage to, one section doesn't stop the whole output?
• size and weight
• durability
• manufacturer reputation and warranty
• beware of scammers online who falsify output etc - do your research on them!
• how easy is it t set up on an angle to optimise efficiency
• how will you keep it well ventilated to keep it cool
• how will you prevent it from being stolen
• weather-resistance including hail damage but also resistance to being blown over in the wind - ability to peg down is useful
• how will you angle it to optimise the sun's direction - some have built in legs to assist with this

• panels are designed to work optimally when the sun is perpendicular to the surface
• in the middle of the day at the Summer solstice, the sun will be angled away from the perpendicular by the same angle as the location's latitude less 23deg and by the Spring and Autumn equinox's at midday the angle is just the latitude
  • thus for Victoria at around 38degS latitude, the panels need to be angled 15deg at midday in summer and more at other times of the day or in other seasons (38deg in Sept and March at midday)
  • ie. elevation angle of the sun = 90 - latitude + declination of sun
  • ie. angle of sun from zenith = latitude - declination of sun
• this has 2 main effects of solar panel efficiency:
  • reduction of intensity due to increasing Air Mass
    • Intensity perpendicularly direct onto panels = 1.353 x 0.7 (^{AM^0.678})
    • Air Mass = 1/cos(angle of sun)
    • Air mass at zenith (0deg) = 1; Air Mass at Summer Solstice midday at 38deg latitude = 1.09;
    • at latitude 38degS the intensity on a panel tilted perpendicular to direct sunlight at midday is:
      • Jan 1.03kW/sq.m
      • April/Sept 0.98kW/sq.m (by 7am it has already reached 0.55kW/sqm and by 9.30am 0.9kW/sq.m)
      • June 0.8 kW/sq.m
  • reduction of intensity due to incident angle on panels
    • intensity of sun onto a horizontal surface = intensity when perpendicular x sin(elevation of sun)
      • thus at 38degS at midday in Apr/Sept elevation = 90-38 = 62deg thus 88% of perpendicular intensity
    • intensity of sun onto a tilted surface = intensity when perpendicular x sin(elevation of sun + angle of tilted surface to horizontal)

• Atem aluminium frame 200W folding panel
  • $AU259 shingle celled, including lithium compatible MPPT controller; folds down to 120x45x5cm; 9.6kg;
• Atem 250W aluminium frame folding panel
  • 683x760x60mm folded in half with panels exposed; rear-attached PWM solar regulator compatible with LiFePO4; 11.4kg; 5m cable with Anderson plugs; durable design but no bypass diodes?; $AU209
    • 300W version is 100mm wider and 13.1kg; $AU259;
    • 300W lightweight version is 1005x685x50mm folded, 8.1kg; diode bypass; $AU319; PWM not for LiFePO4 though;
• Redarc
• iTechWorld
  • 150W single frame Shingle Solar Cells 1160x670x30mm designed for mounting on vans, etc

• Atem 200W blanket
  • $AU239 incl. MPPT Controller compatible with lithium; 7.4kg folds down to 475 x 420 x 60mm; All cables are terminated with Anderson plugs;
• HardKorr 200W solar panels with crocskin cell armour $579-$699 incl. HardKorr 15A 5-stage PWM Solar Controller for Lithium with Anderson plugs; 9kg; 4 panels 710x530x40mm folded; weatherproof; unlike cheaper panels, shade on one area does not impair all area outputs; 11A per hour on a clear sunny day (ie. 1A/hr fridge needs 3hours charge/day in the sun;
• Renogy 220W E.Flex panels
  • fold to 738 x 581 x 40mm ; 738 x 2135 x 4mm unfolded; 9.23kg; op temp -20°C to 65°C; 1yr wty; $AU591

• REDARC 112W Solar Blanket Amorphous Cells
  • flexible amorphous cell technology more able to withstand damage - can walk on it but large area (18 sections 1860×1185 unfolded) very expensive $AU2400 w/o controller!! 4.8kg;
• REDARC 190W Solar Blanket SunPower
  • 15 sections 1720×930 unfolded; 7.2kg; $AU2066 w/o controller
• HardKorr 200W Lifestyle solar blanket
  • $AU379; much more compact than their crocskin panels, similar output on testing, blocking diodes but less durable and controller is only a PWM which does not do lithium. 3×4 array of 12 panels folds to 350mm x 375mm x 70mm; 7kg;
• various brands such as Mobi on Ebay sellers such as Outbax (but they appear to have a poor service reputation and they seem to over-rate the output of their panels significantly ⁴⁾ ) who have various wattage “2021 tech” blankets with controllers for lithium at discounted prices - perhaps a nice size for output is a 300W Mobi for $AU219 but you may need to consider output may only be 200W max and these controllers apparently interfere with AM radio reception and also apparently get hot if used as output devices (use your battery to connect to devices to resolve this)

• the following Renogy panels are a little cheaper on the Renogy Amazon.com.au store!
• Renogy 50W flexible
  • 50W 683 x 508 x 4 mm; 1.3kg; MC4; op. temp -40ºC to +85ºC;flexes to 240deg. $A119 5yr wty
  • you could by 3 of them and join them in parallel via https://au.renogy.com/3-to-1-solar-branch-connectors-mmmf-fffm-pair/ to give 150W in a 12mm thick compact form factor and only 4kg at cost ~$A380
• Renogy 100W flexible
  • 100W 1093.0 x 582.0 x 20.5 mm; 2.4kg MC4; Operating Temperature: -40℃ to 80℃;flexes to 240deg. $AU219 5yr wty
• Renogy 200W flexible
  • 200W 160.5 x 74.8 x 0.3cm; MC4; 4.9kg; op temp -40ºC to +80ºC; flexes to 240deg. $AU389 5yr wty

• Renogy CIGS 150W Flexible
  • 150W 1658*646*1.5mm 2.1kg MC4; $AU459
  • “Unbreakability, unmatched durability against hurricanes and hailstorms” 360° flexibility can roll up to ~30cm diameter tube; 25yr wty;
  • mounting on roof: clean and dry roof, peel off the adhesive tape and attach the solar panel.