What is the formula to caclulate or estimate the electricity generated in output of a solar panel system?
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Formula to Esimate Electricity output in a Solar Power
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Couple things to assume (starting with Grid tie batteryless systems and then to battery/offgrid systems)
1) PV panels produce (generally) 80% of their STC wattage
2) Grid Tie inverters are 95% efficient
3) Battery charge controllers are
MPPT 95% efficient when in BULK mode only, otherwise they are 
PWM efficiency varies with the the ratio of PV array output voltage and battery charge voltage
4) Flooded batteries start life at 90% efficient, decreasing to 70% at failure
5) Off Grid inverter efficiency varys from 7095% depending on loading and brand/model
Hope this quick guess works for you
Powerfab top of pole PV mount (2)  Listeroid 6/1 w/st5 gen head  XW6048 inverter/chgr  Iota 48V/15A charger  Morningstar 60A MPPT  48V, 800A NiFe Battery (in series) 15, Evergreen 205w "12V" PV array on pole  Midnight ePanel  Grundfos 10 SO59 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run  Runs off PV 
 Midnight Classic 200  10, Evergreen 200w in a 160VOC array 
 VEC1093 12V Charger  Maha C401 aa/aaa Charger  SureSine  Sunsaver MPPT 15A
solar: http://tinyurl.com/LMRSolar
gen: http://tinyurl.com/LMRListerComment

Hi Mike90250,
I want to make sure I understand your information correctly  you say PV panels generally produce about 80% of their STC (standard test conditions) wattage. So if I had a 300w panel (module) I should actually expect just 240w in real world conditions, (not really 300w as I might get under optimal conditions in a lab test).
Then, I should expect to convert only 95% of the panel (module) output from DC power to AC power. This is just because of the way electrical engineering and physics works (?).
So in summary, at any moment in time, one 300w panel would ordinarily be expected to generate 240w (as DC power?) ... and that 240w would then be converted to AC by the inverter (losing 5% in the process) resulting in something like 228w of AC power? So it is really just the 228w times hours of production that I am calculating to figure out my offset?
Thanks.Comment

Originally posted by entgegnen View PostHi Mike90250,
I want to make sure I understand your information correctly  you say PV panels generally produce about 80% of their STC (standard test conditions) wattage. So if I had a 300w panel (module) I should actually expect just 240w in real world conditions, (not really 300w as I might get under optimal conditions in a lab test).
Then, I should expect to convert only 95% of the panel (module) output from DC power to AC power. This is just because of the way electrical engineering and physics works (?).
So in summary, at any moment in time, one 300w panel would ordinarily be expected to generate 240w (as DC power?) ... and that 240w would then be converted to AC by the inverter (losing 5% in the process) resulting in something like 228w of AC power? So it is really just the 228w times hours of production that I am calculating to figure out my offset?
Thanks.
PVWatts is a great tool to estimate how much array is needed to achieve a targeted offset, and the hourly output option can be used for more granular analysis if needed.CS6P260P/SE3000  http://tiny.cc/ed5ozxComment

Well... except that the sun varies with time, too.
Take a look at http://pvwatts.nrel.gov, that implements the formula you're looking for, and takes the sun into account (assuming no shading).
(Watch out though  it gives the output for a typical year with both cloudy and sunny days, so don't take any one day's output too literally.)
And/or look at a nearby similar system, find out how many kWh it generated last year, divide that by the sum of the faceplate ratings of its panels, and you've got the infamous kWh/kW aka "sun hours" aka "yield" number (see eq 1 in http://www.nrel.gov/docs/fy05osti/37358.pdf ), also called 'efficiency' by pvwatts.org ( see my system's at http://pvoutput.org/aggregate.jsp?id...=44908&v=0&t=y )
This is roughly the "hours of production" you're talking about, but it already includes the inefficiencies of that system's inverter.
For my system over a whole year, it's about 4.5, but your mileage will vary.
Comment

Ok, so it sounds like a general rule of thumb is that any given panel will just give you up to 80% of the rated power at the peak moment in the day... And ordinarily a little less depending on conditions and time of day.
And that from that number (being generated at the panel) you will lose 35% at the inverter.
So you may build a 7.68Kw system... But don't be surprised if under optimal conditions,the readout from the system just says it is making 6200w AC.
?Comment

My system's nameplate output is 8950 watts DC. Today, a hot sunny day, the AC output was
8am 1000W
9am 2300W
10am 3800W
11am 4900W
noon 5700W
1pm 6100W
2pm 6100W
3pm 5700W
...
Peak today was about 68%, not 80%.
At no point has it ever gotten anywhere close to 8900W AC. Best I've ever seen it do is 7800W, but that's rare.
See graph at http://pvoutput.org/intraday.jsp?id=...08&dt=20160726Comment

Originally posted by entgegnen View PostOk, so it sounds like a general rule of thumb is that any given panel will just give you up to 80% of the rated power at the peak moment in the day... And ordinarily a little less depending on conditions and time of day.
And that from that number (being generated at the panel) you will lose 35% at the inverter.
So you may build a 7.68Kw system... But don't be surprised if under optimal conditions,the readout from the system just says it is making 6200w AC.
?Comment

Originally posted by DanKegel View Postfind out how many kWh it generated last year, divide that by the sum of the faceplate ratings of its panels, and you've got the infamous kWh/kW aka "sun hours" aka "yield" number
Sun hours" is generally considered to be the number of kWh/m^2 of solar radiation falling on a horizontal surface per time period (usually per day).
The term "sun hours" has nothing to do with system performance or system size. It is an anachronistic term that represents resource availability only. The currently accepted term is Global Horizontal Radiation or GHI.
Get it right.Comment

Originally posted by J.P.M. View PostGet it right.Comment

Originally posted by Mike90250 View PostCouple things to assume (starting with Grid tie batteryless systems and then to battery/offgrid systems)
1) PV panels produce (generally) 80% of their STC wattage
2) Grid Tie inverters are 95% efficient
3) Battery charge controllers are
MPPT 95% efficient when in BULK mode only, otherwise they are 
PWM efficiency varies with the the ratio of PV array output voltage and battery charge voltage
4) Flooded batteries start life at 90% efficient, decreasing to 70% at failure
5) Off Grid inverter efficiency varys from 7095% depending on loading and brand/model
Hope this quick guess works for youComment

Originally posted by Charlie W View PostIf I only spoke Acronym, I could understand that.
STC = Standard Test Condition
PWM = Pulse Width Modulation
MPPT = Max Power Point Tracking
the rest is english
I assume you can understand PV = Photo Voltaic since this is basically a solar power forum,
Powerfab top of pole PV mount (2)  Listeroid 6/1 w/st5 gen head  XW6048 inverter/chgr  Iota 48V/15A charger  Morningstar 60A MPPT  48V, 800A NiFe Battery (in series) 15, Evergreen 205w "12V" PV array on pole  Midnight ePanel  Grundfos 10 SO59 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run  Runs off PV 
 Midnight Classic 200  10, Evergreen 200w in a 160VOC array 
 VEC1093 12V Charger  Maha C401 aa/aaa Charger  SureSine  Sunsaver MPPT 15A
solar: http://tinyurl.com/LMRSolar
gen: http://tinyurl.com/LMRListerComment
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