5 solar facts you cant ignore

ByRV DailyNovember 24, 2016
5 solar facts you cant ignore

We all like something for nothing but there’s no point investing in a beaut solar set-up if you’re going to ignore or, worse, be misled on, the facts about solar energy and getting the real bang for your buck.
By Collyn Rivers

RV owners constantly flatten batteries – yet then add more. This is like enlarging a cash box without adding more cash. If energy (solar or otherwise) is not coming in it’s not there to store.

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A rough guide is that unless your batteries are fully charged (by noon much of the year with typical use) then there’s too much battery, too little solar or a fault in the system. For most systems you really need about 300 watts of solar for every 100 amp hour of battery capacity. This is affordable now that solar capacity costs a fraction of battery capacity. Another argument for ample solar is that there is typically 20-30% solar output on overcast days.

You need a good solar regulator that is set up correctly. Given that, there is no risk of battery overcharging, nor damaging the solar regulator: the latter self-limits solar output.

Fooling almost everyone, particularly those awake during school physics, is believing a solar module’s output is that claimed in the advert (it’s about 75% of that).

The industry has twin scales. One (Standard Operating Conditions – SOC) is used for development and selling. SOC output is achievable only atop a high tropical mountain around noon on a freezing cold day. The other scale (Nominal Operating Cell Temperature – NOCT) indicates typical usage.

What’s confusing is that the current output of a 12 volt, 100 watt solar module is not measured at 12 volts. It typically peaks at a (now) 17 volts or so – at which it produces about 5.9 amps (100 watts). At a more useful (say) 12.8 volts that’s about 75 watts. Most solar modules dislike heat and lose about 5% of that for every 10 degrees C increase. The most probable (NOCT) output is revealed but in the technical data and on a data panel on the rear of some modules. That for promotion and on the packing is the inflated SOC claim. The industry claims this is ‘long-term industry practice’. So is thieving in the burglary trade.

A Maximum Power Point Tracking (MPPT) solar regulator provides up to 30% more power An MPPT regulator does not ‘provide more power’ as such. It recovers some otherwise lost much as a car’s torque converter optimises engine power – but by juggling amps and volts to optimise watts. That ‘30% more power’ is mainly for an hour or so after sunrise and before sunset, when battery voltage is very low, and during light cloud, etc. That up to ‘30% more’ is of not that much.

MPPT works, but in reality is likely to be 10-15%. The function is built into most high-quality solar regulators costing $250 upwards and included with some portable solar modules. It is often falsely claimed for cheap ebay specials (as Many People Presumably Tricked).

While true of Australia’s south coast in mid-winter, output then is only 20% of the module’s maximum. Horizontal solar modules may as well not be there. Free-camping there in mid-winter requires a quiet inverter generator: using its 230-volt output to run a 30-50 amp battery charger during the day. This provides ample power and avoids infuriating neighbours at night.

For more typical use, locating solar modules horizontally will, for much of the year, provide close to that optimum (your latitude) tilt – e.g. 27.5 degrees (Brisbane) and 28.7 degrees (Geraldton) peaking around October and March.

True – but only in the winter.

A 100-watt solar module during tropical winter produces 375-400 watt hours/day. The tropical summer input for that same solar module is 550-650 watt hours/day. This is similar to that of a Sydney’s sunny summer day. This is less than many RV visitors expect. Worse, it stays hot all night as well, so the fridge works hard 24/7- not least as some drink more cold beer.

There are also long periods of extreme heat (40°C plus) and just prior to the rainy season, days of heavy cloud cover. Amorphous technology solar modules (e.g. Uni-Solar, Shott) are barely heat affected but, being larger per watt, are less suited for RV use.

Solar input is measured much as is rainfall via a ‘bucket’ that captures sunlight. It may take an hour to fill at midday during a Brisbane summer, but all day during Hobart’s winter. That sun ‘bucket’ is calibrated in Peak Sun Hours PSH – 1 PSH corresponds to solar irradiance of 1000 watts per square (horizontal) metre. A high quality solar module converts that to about 140 watts a square metre.

Input during a Brisbane summer is about 6.5 PSH/day, that during Melbourne’s mid-winter is about 1.5 PSH/day.