ac watts vs nameplate dc watts

Ethan Brush · January 2015

Ok lets here it. What is the most AC power you have seen relative to your array nameplate? OR for off-grid I'll take DC charging watts if known.

gww1 · January 2015

Re: ac watts vs nameplate dc watts

ethan
I have 5600watt aray. I have seen sperts of well over 6000 watts. Blips really. In the end my aray per the mate display likes to run at 4800 watts on good days.
gww

BB. · January 2015

Re: ac watts vs nameplate dc watts

4,800 Watts / 5,600 Watt array = 0.857 ~ 86% array output.

Good sunny day, cool weather, MPPT charge controller--Certainly possible.

As array ages (perhaps 2% reduction in output voltage in first year), get some dust/bird droppings, etc. and you will see less.

Also, at times, it has been reported that some MPPT charge controllers consistently report 3-5% more energy than is measured with lab equipment (very few reports of negative percentage reports).

Real lab grade/calibrated equipment can cost several $10,000 to do the measurements better (still not "perfect"). The solar charge / inverter equipment is going to be around 5% accuracy... 5% error on voltage, 5% error on current, etc... And you are looking at any numbers within 10% of expected values as being "dead on" from an engineering point of view.

I like to use 0.77 for panel and controller deratings--Remember there is ~5% MPPT controller drop and 1-3% typical wiring drop too (not clear if 4.8 kW includes those losses or not)... Being somewhat conservative means I don't have to say (at least very often); "I am sorry" your system is not generating 81% average power in the last month.

-Bill

gww1 · January 2015

Re: ac watts vs nameplate dc watts

bill

I like to use 0.77 for panel and controller deratings--Remember there is ~5% MPPT controller drop and 1-3% typical wiring drop too (not clear if 4.8 kW includes those losses or not)...

I'm not sure my numbers have those losses either. That is just the number on the mate where it totals charge rate. If I add whats on the two cc, it is usually higher and sometimes lots higher. Today was the best day I have had in a month and a half. I seen 5200 watts. I produced 29 kwh per the two cc. The 4800watts is usually a good day and not all day. One cc shows it is making about 200 watts more at all times for the same size aray. Every since I saw the mate say my loads was .4 kwh and my clamp meter said 800 watts I take readings with a grain of salt. What I don't take with a grain of salt is that my 2000 watt water heater element was on 100% for more then 3 hours today, sweet.

It is cold here today and I saw a 115 volts from the aray during one of the cc new voc period. I hate some things about the mx60 compared to the fm80 but it always shows more output.

Yea, I don't know about my numbers except that is what I see and I use it as a guide not, counting compleetly on its accuracy.

I did want to brag though, cause after the dismal month we just had, it feels good to crow.
gww

PS I just went through a whole month with probly 15 days not getting more then 3kwh per day.

gww1 · January 2015

Re: ac watts vs nameplate dc watts

I was watching the mate again today and at ten thirty I was getting 4600watts on the mate but the charge controllers said 4800 watts. The mate finaly reflected this but it is nothing for it to be 200 watts off from the cc, usually lower. I would like to point out that these are winter time numbers. I don't think it does as well in summer and though I don't remember exactly what it is. I do remember having a harder time in summer running my loads and carrying a 2000watt water heater element and holding absorb. I remember in summer thinking I might have to get a 1500 watt element, but in winter it carries all and still shaves production.
Just trying to be clear and lead no one astray.
gww

Ethan Brush · January 2015

Re: ac watts vs nameplate dc watts

Thanks for the info guys. So what about on nice cold crisp days (for those of you in such a climate)? Here in Central NY its been a high around 12 degrees the last few days, nice and clear and sunny. How much then? BB, just to clarify your figures were "most" and not typical/average? What were the temps when that happened (SSBA prob not so cold...)

BB. · January 2015

Re: ac watts vs nameplate dc watts

Just rough numbers... Assuming warm sunny days (9 months of the year). We are probably taking about a 77% to 85% range of typical maximum output at noon.

If the panel orientation to sun are off by 10 degrees, Cos 10 = 0.985 (98.5%), add a percent or 3% for wiring losses, 5% for charge controller losses (MPPT), 81% or so for slightly dirty panels at sea level, etc... 5,000ft with clear/cold sky, etc... you may get closer to 85% panel ratings. Tilt the panels vertical, throw a field of snow in front of the panels, sub freezing day, you may get >100% of rated output of the array.

We can go through the math of the panels and system if you wish... But for planning systems, quickly and with a minimum of fuss and confusion, using conservative numbers (not too conservative) gets us close enough for most installation (usually within 10% of actual output).

-Bill

Photowhit · January 2015

Re: ac watts vs nameplate dc watts

Ethan Brush wrote: »

Ok lets here it. What is the most AC power you have seen relative to your array nameplate? OR for off-grid I'll take DC charging watts if known.

If you know what the panels are, make and model, you can often find the NOCT value, when panels are hot. I recently looked up for someone here and they were 87-88% of panel rating, but I have also seen as low as 72%. If you can't find a NOCT value for your panel you can find the temp for the panel rating and a correcting multiplier for higher temps.

This time of year in the northern hemisphere, arrays are producing close to their max. Some will produce at or above their panel rating.

Ethan Brush · January 2015

Re: ac watts vs nameplate dc watts

BB. wrote: »

Just rough numbers... Assuming warm sunny days (9 months of the year). We are probably taking about a 77% to 85% range of typical maximum output at noon.

If the panel orientation to sun are off by 10 degrees, Cos 10 = 0.985 (98.5%), add a percent or 3% for wiring losses, 5% for charge controller losses (MPPT), 81% or so for slightly dirty panels at sea level, etc... 5,000ft with clear/cold sky, etc... you may get closer to 85% panel ratings. Tilt the panels vertical, throw a field of snow in front of the panels, sub freezing day, you may get >100% of rated output of the array.

We can go through the math of the panels and system if you wish... But for planning systems, quickly and with a minimum of fuss and confusion, using conservative numbers (not too conservative) gets us close enough for most installation (usually within 10% of actual output).

-Bill

I am quite familiar with the average energy derating numbers, just curious about peak power and how often and what temps I would see >/= DC STC. My array now is not MPPT and has at least some branches in the way nearly all the time (work in progress) so of course Ive never been able to get high numbers. I have done grid tie systems, but Im never there on cold days to check output. I know peak power on the coldest days doesnt really mean much, just a curiosity.

BB. · January 2015

Re: ac watts vs nameplate dc watts

For PWM controllers and panels... They operate differently.

The Vmp-operating of the array is basically the voltage of the battery + voltage drop of controller + voltage drop of wiring... So as long as Vmp-array is > than Vbatt+stuff -- then that is all that matters.

Since the panels are (for the most part) current sources where the current is proportional to the amount of sun hitting the array (100% sun, then 1.00 * Imp, 50% sun then 0.5 * Imp, etc.--usually pretty linear to within 5% or so down to indirect sun on panels which gives virtual zero useful current do to internal leakage current), the output of the array is only limited by sun, haze, dirt on panels, etc. And Imp of the panel rises (slightly) with temperature (cold panels produce slightly less current)--In the big picture, the voltage temperature effects swamp the current temperature effects.

The only time Vmp-array "matters" is when you have Vmp~17.5 volts (Standard conditions), and a full/equalizing battery bank (perhaps a bit cool) that is running 15.0+ volts... A warm panel, with the 20-30 C cell temperature increase due to sun/poor cooling by air/wind, then Vmp-array-hot falls to or slightly below Vbatt+stuff--And current can fall. Usually not a problem--But with extreme conditions/heavy loads, a MPPT controller + Vmp-array >> 17.5 volts can be worth the extra costs for a MPPT charge controller.

-Bill

CDN_VT · January 2015

Re: ac watts vs nameplate dc watts

BB. wrote: »

For PWM controllers and panels... They operate differently.

The Vmp-operating of the array is basically the voltage of the battery + voltage drop of controller + voltage drop of wiring... So as long as Vmp-array is > than Vbatt+stuff -- then that is all that matters.

Since the panels are (for the most part) current sources where the current is proportional to the amount of sun hitting the array (100% sun, then 1.00 * Imp, 50% sun then 0.5 * Imp, etc.--usually pretty linear to within 5% or so down to indirect sun on panels which gives virtual zero useful current do to internal leakage current), the output of the array is only limited by sun, haze, dirt on panels, etc. And Imp of the panel rises (slightly) with temperature (cold panels produce slightly less current)--In the big picture, the voltage temperature effects swamp the current temperature effects.

The only time Vmp-array "matters" is when you have Vmp~17.5 volts (Standard conditions), and a full/equalizing battery bank (perhaps a bit cool) that is running 15.0+ volts... A warm panel, with the 20-30 C cell temperature increase due to sun/poor cooling by air/wind, then Vmp-array-hot falls to or slightly below Vbatt+stuff--And current can fall. Usually not a problem--But with extreme conditions/heavy loads, a MPPT controller + Vmp-array >> 17.5 volts can be worth the extra costs for a MPPT charge controller.

-Bill

Thanks , Nice explanation .
Lets just hope I can remember it as it was posted !

VT

ac watts vs nameplate dc watts

Comments

Categories