30 Watt PV Panel Review

[bdosborn]

Because of my illness (SPAS - Solar Panel Addiction Syndrome) I bought yet another solar panel for an up coming truck project. I got it from here:

La Vie Solar eBay Linky

This is a fairly small panel rated at 30 watts. One thing most people don't realize is that its a pretty rare circumstance when a PV panel actual produces its full rated power; the panel needs to be oriented to the sun just right, it needs to be a really sunny day and it needs to be a cool day. I've never seen more than 175 watts from the 220 watt panels on the Boxcar roof.

I plugged the La Vie panel into the Sunsaver MPPT charge controller in the Boxcar, propped it up on the tongue and it's outputting 31.6 watts! The output is slowly going down as it the panel heats up in the sun, its at 28.5 watts right now, but the panel performs well.



Since its so small you wouldn't need a a very big charge controller for it, the Morningstar Sungard would be a good choice. It isn't an MPPT controller so the harvest would be down 10%. That's a whopping 3 watts so MPPT would be a waste on such a small panel. You could probably get 10-12 amp-hrs per day from the La Vie panel/Sungard charge controller combination if you moved the panel during the day to track the sun.



This would be a nice, portable PV system for a teardrop that doesn't use a lot of battery. It costs less than the HF system, I bet it beats the HF output and its only a third the footprint.

Costs:

La Vie Panel ~ $95 to your door.
Sungard CC ~ $40 to your door.

I wish I had made the cover to the tongue box with this panel.

Bruce

[myoung]

Solar panel efficiency or lack of efficiency is dramatically affected by temperature, perhaps more than most people realize. When we installed a PV system on our roof, friends said we had the perfect place because the sun is so intense and the clouds are rare here around Phoenix, AZ.

Actually the best month for power generation is April followed by May as long as the temps don't get too high. Power generation drops off dramatically north of 80 degrees. In the summer months where 110 degree days are commonplace, we only get about 40-45 kwh a day compared to 55 or so in late April.

The other dirty little secret is that the efficiency of the panels themselves is only about 85% in the best of circumstances. For example, our 235 watt panels seldom exceed 185 watts of output. We have 190 watt microinverters on each one of the 34 panels and have real-time access over the internet to the performance/output of each and every panel separately. The graphs that are produced show clearly the effects of clouds. The slightest shadow like the one that is cast from a water heater vent over just a couple of cells in one of the panels in the early morning cuts the output from the entire 235 watt panel to as little as 10-15% output. Stunning. Glad we don't have panels ganged together in series like 99% of the installations out there.

[bdosborn]

The other dirty little secret is that the efficiency of the panels themselves is only about 85% in the best of circumstances.

Its not really a dirty little secret, its just the physics of how the panel works and a good design takes it into consideration. However, on teardrops, we tend to back into a PV system based on how big a system we can afford and how much roof area is available rather than designing the system. So the drop in output due to temperature really isn't something that we compensate for, its just a given.

For example, if I were to design a system based on nightly usage, the PV array size would be a lot bigger than 30 watts. Lets design around using 20 amp-hrs a night, which is what we use in a typical evening in the Boxcar watching a movie, lights, etc. (BTW, this is a lot, most tears probably don't use this much. Our DVD player is an energy hog and draws 45 watts.)

20 amp-hrs/5 hrs per day = 4 amp output from the PV array. Using only 5 hours a day of full sunlight takes into account cloud conditions and less than perfect orientation to the sun.

4 amps /0.8 = 5 amps. Here we've assumed a panel that only is putting out 80% of its rating. This takes into account the drop in output due to temperature as well as some voltage drop losses.

5 amps * 13.8v ~ 70 watt panel. I used 13.8V because that's a typical charging voltage for a PWM charge controller. But, that's not the peak power voltage for most polycrystalline panels. Peak power voltage is 18.7 for my panels so we need to add some more capacity to make up for the PWM losses (this is the loss that an MPPT controller helps recover).

(18.7v-13.8v)*5 amps ~25 watts

So we would need a minimum 100 watt panel to cover this usage. However, its probably a good idea to add capacity in case it isn't a perfect day. To make up for a rainy day or laying the panel flat, I doubled the capacity on the Boxcar. That's 200 watts of PV array. And here's the rub, a teardrop just doesn't really have enough roof for that much panel and most people don't want to spend the $6-700 dollars it would cost.

So, to make a long story short, temperature induced losses are very real but not that significant of a driver when designing a small PV system for a teardrop. First cost and roof area usually drives the system selection. Its a very different story when designing a grid-tied system like yours, the temperature induced losses can add up to some big numbers that need to be accounted for.

Bruce

P.S. Aren't you going to post a link to your Enphase website so we can look at your solar production? I'm very curious to see how the microinverters hold up in your heat.

[myoung]

Bruce,

I'll PM you the username and password to see our Enphase reporting. There are 5 other sites on the same link so I suppose it wouldn't be wise to broadcast the link too widely. I am happy to share it with those like yourself who are seriously interested.