Does this argument for higher efficiency make sense?

jcgee88
jcgee88 Solar Expert Posts: 154 ✭✭
A solar company in my area posted advice on their webpage
which argues for "higher efficiency" PV modules (he sells
SunPower) for certain situations. I sense that his argument
is flawed. Can some of the experts comment? I have
bolded what I consider his most dubious statement. Below
is the text from his website:


Example A
Installer A proposes to install eighteen 230 watt (W) panels from a leading solar panel manufacturer. The 230 W power rating is calculated using standard test conditions accepted in the industry. This means that that each panel is capable of generating 230 watts of power, and under ideal circumstances the panel could produce 230 kWh in an hour if operating under perfect conditions. The array of 18 panels forms a total of 4,140 watts, or 4.14 kW. You’re told that the system will require approximately 400 ft² of roof space, and can be expected to produce 5,206 kWh of power annually.

Example B
Installer B provides you with a proposal to install sixteen “high-efficiency” panels from a different manufacturer, which are also rated at 230 W. Because of the higher efficiency (18.1% compared to 14.2% for the competitive panel), this 3.68 kW system is expected to produce 3% more energy than the larger system – 5,363 kWh annually – for about the same price. It also requires only about 250 ft² of roof space to install.

What's the Difference?
On the surface, it might appear that the higher rated system would be the better value. However, if the southern exposure on your roof is limited, you might opt to go with the higher efficiency panel with the smaller footprint. The return on your investment, though, is best measured in the actual output, or the kWh produced. By looking at the total amount of energy produced over the life of the system, you can project a fixed cost of the electricity produced. Panels typically last 30 years. By multiplying the annual production by the 30 years, the larger system (A) produces 156,180 kWh, while the smaller, more efficient system (B) produces 160,890 kWh. By dividing the rebated cost by the total energy production, you come up with a fixed cost p/kWh. Given the available financial incentives, in most cases this will be only slightly more than current rates.

--

Comments

  • Jakachira
    Jakachira Solar Expert Posts: 47 ✭✭
    Re: Does this argument for higher efficiency make sense?

    Their explanation is confusing lot. Efficiency in terms of better solar cells or what? This has to be clear before you buy these :confused:
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Does this argument for higher efficiency make sense?

    I'll agree with the "confusing" assessment. This is almost always a sign of someone trying to pull a fast one.

    If the two selected panels are both rated at the same Wattage then the efficiency is irrelevant. The more efficient panels will take up less space, but they won't take up less space and produce more power.

    Then they switch gears and start talking about a roof without good insolation. Yes, if you can confine your total equivalent Watts to the area that gets good sun and not have a larger array that will be partially shaded at times the daily harvest will be better. But that is not comparing on an equal basis.

    Then they throw in terms like "rebated cost", which may or may not apply to your situation.

    My guess is that the "more efficient" panels have a higher profit margin.

    I also think the terminology would be more accurate if they said "less efficient" for the "14.2" panels, as 18.1% efficient isn't that unusual these days. Maybe they're comparing amorphous to crystaline?

    In any case, it to boils down to this: solar electric power systems are highly site-specific. There is no "this is better than that in all installations".

    I could be wrong about my assessment, but I think you're right to be suspicious. It is confusing. :confused:
  • jcgee88
    jcgee88 Solar Expert Posts: 154 ✭✭
    Re: Does this argument for higher efficiency make sense?
    I'll agree with the "confusing" assessment. This is almost always a sign of someone trying to pull a fast one.

    If the two selected panels are both rated at the same Wattage then the efficiency is irrelevant. The more efficient panels will take up less space, but they won't take up less space and produce more power.

    <snip>

    My guess is that the "more efficient" panels have a higher profit margin.

    <snip>

    I could be wrong about my assessment, but I think you're right to be suspicious. It is confusing. :confused:

    I already selected and installed, so this isn't for me.
    The reason I ask is because this company is using this
    argument with a friend of mine. The argument in a
    nutshell could be stated as: go with me because I
    offer the higher efficiency SunPower.

    When I first read this, I found it confusing, but that
    bolded part, i.e., you get smaller footprint AND 3%
    more power, that just leaped out to me as being
    false.

    Unfortunately, on the first go around, my friend seems
    to have swallowed this argument hook, line and sinker.
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Does this argument for higher efficiency make sense?

    A. 18 230W panels = 4.14 kW STC
    B. 16 230W panels = 3.68 kW STC

    System B will only produce about 89% of the power of System A - all else being equal.

    The only way that System B would do better is if their 230W panels are underrated and that System A panels are overrated. For standard PV panels, that is simply not the case - all panels perform basically the same within a couple percent.

    Looking at the more realistic California ratings, 230W STC panels seem to run between 198-211W in their testing. Or about 5% difference. The SunPower 230W module was one of the highest (if not the highest) at 212W.

    Assuming that System B is the SunPower system, it might get within 95% of the performance of System A - but that highly depends on the exact panel that System A is using.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Does this argument for higher efficiency make sense?

    go with the less efficient higher wattage systems unless you are cramped for space and then need to produce a higher amount of power per square foot of exposure area. the gain per square foot won't be as large as one might think either. understand that the gain in efficiency will also mean you pay a higher premium for it, making their smaller system even more expensive than the other systems that will produce more power.
    for an fyi, be also aware the thin film types will need about 2x the area normally used that many standard pvs need even though they may be cheaper per watt now. this is a big increase for mounting costs compared to standard pvs and not just an area increase for the thin films.
    remember, a watt is a watt is a watt. a more efficient watt is still just a watt and is likely to be a much more expensive watt.
  • BajaGringo
    BajaGringo Solar Expert Posts: 40 ✭✭
    Re: Does this argument for higher efficiency make sense?
    niel wrote: »
    ...remember, a watt is a watt is a watt. a more efficient watt is still just a watt and is likely to be a much more expensive watt.

    My thoughts exactly, but it sounds better when you say it!

    :p
  • blackswan555
    blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Does this argument for higher efficiency make sense?

    As Said , Watts is watts, All else being equal, A is bigger and will produce more kwh/PA,
    The other number you may want to look at / ask them about is
    Temperature coefficient of power -(0.5±0.05)%/ °C
    (Taken from a BP panel,)
    Panels lose efficiency the hotter they get, the above number is how much, ( 1/2 a % per deg C above 25 c in the above) so If you live in a warm climate, panel is say 75 deg C, That`s 25 % loss,
    A small difference in the above number can be a considerable difference in production.

    Have a good one
    Tim
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Does this argument for higher efficiency make sense?

    "This means that that each panel is capable of generating 230 watts of power, and under ideal circumstances the panel could produce 230 kWh in an hour if operating under perfect conditions."

    Correct me if I'm wrong, but a single 230w panel times 1 hour would put out 230 watt*hours (.23 kWh) and not 230 kWh - no?
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: Does this argument for higher efficiency make sense?

    dwh, I noticed the same thing, but it is JUST a detail.

    Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: Does this argument for higher efficiency make sense?

    I get this same thing from several Sunpower dealers around here. They really emphasize the "high-efficiency" angle then come out with inflated annual kwh numbers that are not believable. I go through the arithmetic with customers to prove that they are being lied to. Even though there is some variation in PTC ratings between modules (which should be used when figuring performance) with the same STC ratings, it isn't much. There is no way 16 Sunpower $2.50/watt 230w modules are going to out perform 18 $2/watt 230w modules. The only reason for using Sunpower is if you are very limited in roof space.
  • Gazz
    Gazz Solar Expert Posts: 38
    Re: Does this argument for higher efficiency make sense?
    drees wrote: »
    A. 18 230W panels = 4.14 kW STC
    B. 16 230W panels = 3.68 kW STC

    System B will only produce about 89% of the power of System A - all else being equal.

    The only way that System B would do better is if their 230W panels are underrated and that System A panels are overrated. For standard PV panels, that is simply not the case - all panels perform basically the same within a couple percent.

    I have just had a 2.88KW system installed and although the weather has been bad I have had a few good days of sun. I have Sanyo HIT 240 panels and was sold them as being "better" than the cheap Chinese units. I also had limited space and liked the finish so the supposed added output was not my main reason for getting these. But what I have noticed that during good weather I have had a total output 3.2KW and this has not been perfect weather conditions, is this normal?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Does this argument for higher efficiency make sense?

    If you are in the UK--I would guess that your readings are unusual.

    However, Sanyo seems to over-spec. their panels. (just a random spec.):
    Power Guarantee
    SANYO’s power ratings for HIT Power panels guarantee customers receive
    100% of the nameplate rated power (or more) at the time of purchase, en-
    abling owners to generate more kWh per rated watt, quicken investments
    returns, and help realize complete customer satisfaction.
    ...
    Warranted Tolerance (-/+) -0% / +10%

    Since panels are typically warranted for -20% over 20 years or so--It appears that your panels could produced 20-30% more than rated power when new.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Gazz
    Gazz Solar Expert Posts: 38
    Re: Does this argument for higher efficiency make sense?
    BB. wrote: »
    If you are in the UK--I would guess that your readings are unusual.

    However, Sanyo seems to over-spec. their panels. (just a random spec.):



    Since panels are typically warranted for -20% over 20 years or so--It appears that your panels could produced 20-30% more than rated power when new.

    -Bill

    Yes in the UK there is not much sun, but they have defiantly gone over 3kw on a number of occasions, not for very long though, just waiting for a full day of sun.
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Does this argument for higher efficiency make sense?
    Gazz wrote: »
    I have just had a 2.88KW system installed and although the weather has been bad I have had a few good days of sun. I have Sanyo HIT 240 panels and was sold them as being "better" than the cheap Chinese units. I also had limited space and liked the finish so the supposed added output was not my main reason for getting these. But what I have noticed that during good weather I have had a total output 3.2KW and this has not been perfect weather conditions, is this normal?
    On cool, partly cloudy days it is common for the power to jump up after the sun appears from behind a cloud.

    What happens is that while sun is blocked, the panels cool off - then when the sun hits them it takes ~15 minutes for them to warm up - in the mean time, they'll produce a burst of energy.

    Now - if you really mean that they are pushing out 3.2 kW on a clear day - that's very good for a 2.88 kW DC system. How many kWh are you getting on those clear days and at what angle/direction are the panels mounted?
  • Gazz
    Gazz Solar Expert Posts: 38
    Re: Does this argument for higher efficiency make sense?
    drees wrote: »
    On cool, partly cloudy days it is common for the power to jump up after the sun appears from behind a cloud.

    What happens is that while sun is blocked, the panels cool off - then when the sun hits them it takes ~15 minutes for them to warm up - in the mean time, they'll produce a burst of energy.

    Now - if you really mean that they are pushing out 3.2 kW on a clear day - that's very good for a 2.88 kW DC system. How many kWh are you getting on those clear days and at what angle/direction are the panels mounted?

    The system is south facing at about 40 degrees, the problem I have is that since the installation 4 weeks ago the weather has been rubbish. I need some good sunny days to see what it can give, the best day to date is 14kw/h

    Gary
  • jcgee88
    jcgee88 Solar Expert Posts: 154 ✭✭
    Re: Does this argument for higher efficiency make sense?
    Gazz wrote: »
    I have just had a 2.88KW system installed ...what I have noticed that during good weather I have had a total output 3.2KW and this has not been perfect weather conditions, is this normal?

    I feel that peak is not really a good metric of
    whether your system is performing normally. This is
    in spite of the fact that you achieved a peak higher
    than your system's nominal max. In other words, if
    you hit 3.2kW, but it only lasted 5 minutes, all that
    really tells us is what your practical maximum is.

    Perhaps a better metric is to divide your best day's
    results by your rated capacity. For example, you
    mentioned that your to date best was 14 kwH. If
    you divide that by your 2.88 kw generating capacity,
    you get 48%. [I realize you haven't had a perfect
    sunny day yet, so we look forward to your reporting
    your results under better weather conditions.]

    For myself, I had a couple of perfect weather days
    at the beginning of July, and on my 3.055kW array,
    I got a best of 20kwH. That would put me at 66%
    actual production vs theoretical capacity. I'm satisfied
    with that number, but I am sure others do better
    than me, since my site isn't ideal.

    Be aware that with this metric, your site could be
    operating perfectly, yet still be well lower than others' -
    due to the fact that they have more solar hours per
    day than you. So, it's not perfect, but is certainly
    better than peak.

    John
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Does this argument for higher efficiency make sense?
    Gazz wrote: »
    The system is south facing at about 40 degrees, the problem I have is that since the installation 4 weeks ago the weather has been rubbish. I need some good sunny days to see what it can give, the best day to date is 14kw/h
    What is your latitude (hint, fill out your location in your profile) and do you have a graph of system output for that day?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Does this argument for higher efficiency make sense?

    gazz,
    i'm not sure what your point is and where you are going with all of this. the sanyo hits are a hybrid pv between standard and thin film and is nowhere near the concept of the sunpower pvs. if you are getting more power than rated now then that is good and i'm happy for you, but don't think that another pv might not have done the same for you as you don't know. thin films are known for their initial higher outputs when new btw.
  • jcgee88
    jcgee88 Solar Expert Posts: 154 ✭✭
    Re: Does this argument for higher efficiency make sense?
    niel wrote: »
    gazz,
    i'm not sure what your point is and where you are going with all of this. the sanyo hits are a hybrid pv between standard and thin film and is nowhere near the concept of the sunpower pvs.

    Oops, I guess he hijacked the thread! Happens.

    John
  • Gazz
    Gazz Solar Expert Posts: 38
    Re: Does this argument for higher efficiency make sense?
    jcgee88 wrote: »
    I feel that peak is not really a good metric of
    whether your system is performing normally. This is
    in spite of the fact that you achieved a peak higher
    than your system's nominal max. In other words, if
    you hit 3.2kW, but it only lasted 5 minutes, all that
    really tells us is what your practical maximum is.

    Perhaps a better metric is to divide your best day's
    results by your rated capacity. For example, you
    mentioned that your to date best was 14 kwH. If
    you divide that by your 2.88 kw generating capacity,
    you get 48%. [I realize you haven't had a perfect
    sunny day yet, so we look forward to your reporting
    your results under better weather conditions.]

    For myself, I had a couple of perfect weather days
    at the beginning of July, and on my 3.055kW array,
    I got a best of 20kwH. That would put me at 66%
    actual production vs theoretical capacity. I'm satisfied
    with that number, but I am sure others do better
    than me, since my site isn't ideal.

    Be aware that with this metric, your site could be
    operating perfectly, yet still be well lower than others' -
    due to the fact that they have more solar hours per
    day than you. So, it's not perfect, but is certainly
    better than peak.

    John

    Thanks, I totally agree but I was just wondering if beating peak is normal, it would be nice to get close to 3kw for a couple of hours around mid day that might get me closer to 17khH for the day which would be nice to see.

    Gary