Simple math for newbies..?? Watts to Amps similarities..

ywhic

I know my system will put out say 8 amps per panel.. (with MPPT kicking I get like 9.2-9.4..)

I have 8 panels.. so like 72+ amps (for example @ 9 amps with the MPPT)

When charging batteries I know the C/ rule..based on % of charge rate.

So for 72 amps I would want like a 720 amp battery bank.. C/10.. this would if I run it to 50% DOD would be back up and full to my 100% in my (let say) 5 hours of sun (360 amps back in)..

I also know I will be at about 52% efficiency on the VDC to VAC conversion (on avg.) (I am leaning toward 62% in my head for my system)

So 1160 watts @ 62% eff. = 719.2 watts per hour on the VAC side.. (I noticed a slight co relation on the numbers 719.2 watts and 72 amps..)

Even when running lower numbers like 870 @ 62% = 539.4 watts per hour.. which equals 54 amps for the same 6 panels @ 9 amps each.. << and again the numbers seem to be at almost *10 factor..

Without new people going nuts with lots of numbers couldn't they stick with amps produced per panel array * 10 and get their minimum battery bank size?? and then they would also know how many watts they will get on the AC side by mulitplying the panel array amps by say 10 as well.. like 72 amps of panels will give you 720 watts per hour..

I know these numbers are not PURE per-se' , but they have to be closer then most ball-parking people do from the start..

Cariboocoot

Re: Simple math for newbies..?? Watts to Amps similarities..

Nope. No chance.
The simple fact is, it isn't simple.

I don't know where you got 62% efficiency from.
The 52% rating is the difference between the panel name plate rating in Watts * hours of good sun to the total AC Watt hours available from the inverter for a given day.
So if you have a 1000 Watt array and four hours of good sun you can expect (1000 * 4 * 0.52) 2080 AC Watt hours per day.
This assumes things like an MPPT controller, good inverter conversion efficiency, and sufficient battery bank to handle the load.
This is the daily Watt hour harvest, and is not the same thing as the peak charge current available.

That one, using an MPPT controller, is array Watts * 0.77 panel+controller efficiency / nominal system Voltage.
This does not mean you will see that much current at any time, or even ever, and especially not always. It is a rule-of-thumb for sizing the array to be sure it has enough "oomph" to charge the battery (factoring in the % charge rate). You need to check both to make sure there's enough power available from the array to A). provide a fast enough charge to accomplish charging within the amount of good light available and against load draws, and . provide enough Watt hours to 'replace' those used by loads through the whole day (charge period to charge period).

Unfortunately these things can't be absolutes because every system is different. Many things will affect the performance, especially temperature. The use of a PWM type controller changes things. The desire for a higher or lower charge rate changes things.

My panels are rated at 5 Amps. Doesn't fit well with a 232 Amp hour battery bank, does it? The four panels are configured for 48 Volts nominal, feed an MPPT controller, and charge a 24 Volt battery bank. Usually the charge rate peaks in the low 20's, but is sometimes less than that because the batteries don't draw more. It has been as high as 25 Amps under certain conditions. The 3200' of elevation improves the panel efficiency.

If only it were simple, eh?

BB.

Re: Simple math for newbies..?? Watts to Amps similarities..

The 52% is the "end to end" system efficiency (estimated, from solar panels thru charge controller to battery bank to inverter to load). Roughly:

• 0.81 hot panel derating * 0.95 mppt charger losses * 0.80 flooded cell battery discharge/charge eff * 0.85 inverter efficiency = 0.52 end to end eff

Note that major variable losses include:

• Solar panels--If in near/sub freezing weather can meet or exceed rated Vmp-array--for MPPT charge controllers, more output energy. Also includes wiring and some dirt on the panels too.
• Batteries--A "worst case" old flooded cell battery operated inefficiently (lots of charging to >90% SOC--bubbling/equalization losses). If you cycle them a lot below 80-90% SOC, they will be more efficient (closer to 90%?). AGMs I use 0.90 eff, but they have been reported upwards of 98% efficiency. And if you are running your loads in the daytime--Then the batteries are not being cycled at all.
• 0.85 inverter efficiency--If you operate near zero watts, lots of losses. If you operate near 100%, a bit more losses. In between, probably closer to 90% or better.

Note the variations for PWM charge controllers are different. The "current" through the controller is based on the amount of sunlight (Imp very slightly goes up with increased temperature--but not enough to worry about). And PWM controllers are "more efficient" (use less power internally)--But do not "match" Vmp-array to Vbatt-charging like a MPPT controller can--So, we still have about 0.77 efficiency from panel thru charge controller "efficiency" for PWM (i.e., ~16-18 Vmp panel output to ~14.5 volts battery charger output thru PWM which gives around ~0.78 to 0.83 "energy transfer" from solar array to battery bank).

There are many places where efficiency can be worse or better... The above numbers seem to get people "in the ball park" for sizing a system to support their loads...

Even then, never plan on using 100% of available power every day--Weather/temperature/system performance varies. Some days you will use less, and sometimes you will use more--And recharge the bank back to 100% on those lighter use days.

I used to go into excruciating details when sizing systems. I probably did not improve accuracy by much--And, frankly, if I guess system output to within 10% of actual numbers (and if you measured your loads to within 10%)--You are doing really well. Weather itself will vary system output by 10% (sometimes +/-20% for coastal areas) over time/year to year.

And I spent more time explaining/fudging numbers to get "more accurate" answers--Which really just served to confuse more.

So--I tend to spend more time on conservation and the idea that you either have to manage your loads+system+battery storage and/or make up power with genset during bad weather (or larger system) instead.

In the winter--for many folks, their amount of sunlight is so small, and weeks of bad weather, a back up genset it going to be needed at times anyway (making a huge array just is not cost effective).

-Bill

Photowhit

Re: Simple math for newbies..?? Watts to Amps similarities..

BB. wrote: »

...Even then, never plan on using 100% of available power every day--Weather/temperature/system performance varies. Some days you will use less, and sometimes you will use more--And recharge the bank back to 100% on those lighter use days....

I think this is the most important thing Bill said, indeed, to be responsable to your battery bank you should strive to use less than what is available...

ywich wrote:

...So for 72 amps I would want like a 720 amp battery bank.. C/10.. this would if I run it to 50% DOD would be back up and full to my 100% in my (let say) 5 hours of sun (360 amps back in)...

Do you understand that this is NOT true? more likely 90-95% as you reach @85-90% the charge controller tapers back the charging as the battery accepts less current, Read Battery FAQ's about absorption charge Here.

For max battery life you should strive to fully charge yor batteries as often as possible...

ywhic

Re: Simple math for newbies..?? Watts to Amps similarities..

Cariboocoot..

I am basing my numbers on the actual numbers I had gotten from my temporary setup in testing in 93-99'F heat..

I have 8 amp rated panels getting 9.2+ amps with full 15' of wire run.

I've got 8 panels.. and 8 * 9.2 = 72 amps for DC.. I figured I should get about 44 amps on the AC side.. so if I run my Air during peak hour of sun I should still have some juice getting to the battery bank. With no Air running like in the winter I would gain a faster charge rate. Midnite Calc in perfect world says 80 amps for the panels. but I know the heat kills it..

BB.. based on 0.90 inverter eff (using your formula) I should be at 642 watts on the AC side then when starting with 1160w of PV.

So your saying worst case I get 52% and 603.2w * 5 hours = 3016 watts per day.. (or 3618w on a 6 hour summer day)..


Also, so whats more important as the day goes on?? Amps to the battery correct??

ggunn

Re: Simple math for newbies..?? Watts to Amps similarities..

ywhic wrote: »

Cariboocoot..

I am basing my numbers on the actual numbers I had gotten from my temporary setup in testing in 93-99'F heat..

I have 8 amp rated panels getting 9.2+ amps with full 15' of wire run.

I've got 8 panels.. and 8 * 9.2 = 72 amps for DC..

You have all 8 modules wired in parallel?

ywhic

Re: Simple math for newbies..?? Watts to Amps similarities..

ggunn wrote: »

You have all 8 modules wired in parallel?

The Midnite Classic 150 can go to 8 of the 145 panels in parallel per their Calc..

I did testing locally with 1 and then 2 panels to get some numbers when it was really hot to simulate the temps for TX install..

I'll be doing a 24v panel setup.. 4 parallel strings of 2 panels each into a 12v bank. (12v bank subject to change without notice.. LoL).

ggunn

Re: Simple math for newbies..?? Watts to Amps similarities..

ywhic wrote: »

The Midnite Classic 150 can go to 8 of the 145 panels in parallel per their Calc..

I did testing locally with 1 and then 2 panels to get some numbers when it was really hot to simulate the temps for TX install..

I'll be doing a 24v panel setup.. 4 parallel strings of 2 panels each into a 12v bank. (12v bank subject to change without notice.. LoL).

If you are connecting them that way your array current is your module current multiplied by four, not eight. The available power (Watts) is the same, of course.

ywhic

Re: Simple math for newbies..?? Watts to Amps similarities..

ggunn wrote: »

If you are connecting them that way your array current is your module current multiplied by four, not eight. The available power (Watts) is the same, of course.

I know 8 amps (rated) * 4 parallel of 2.. upto 32 amps in @ 24v.. upto 80 amps out @ 12v.. should be more like 64 (low side)- 73(high end) based on numbers I got..

I got 9.2 amps for a single panel and about 15.3-16.3 amps (out to 12v bank) with 2 panels hooked in series...

Won't know real numbers till install date next Spring or Summer.

ggunn

Re: Simple math for newbies..?? Watts to Amps similarities..

ywhic wrote: »

I know 8 amps (rated) * 4 parallel of 2.. upto 32 amps in @ 24v.. upto 80 amps out @ 12v.. should be more like 64 (low side)- 73(high end) based on numbers I got..

I got 9.2 amps for a single panel and about 15.3-16.3 amps (out to 12v bank) with 2 panels hooked in series...

Won't know real numbers till install date next Spring or Summer.

I must not be getting what you are saying. It appears to me that you are saying that two modules in series gives you about twice the current (Amps) as a single module. This cannot be; two modules in series will give you twice the power (Watts) of a single module, but at the same current as a single module. The voltage will be doubled. Power is current multiplied by voltage, so the same current at twice the voltage is twice the power.

Connecting two modules in parallel will indeed give you twice the current, but the voltage will be the same as a single module. Again, you get twice the power but it comes from doubling the current instead of the voltage.

My apologies if I appear to be talking down to you, but from the title of the thread it looks like you are looking for information on the fundamentals.

Cariboocoot

Re: Simple math for newbies..?? Watts to Amps similarities..

On a 12 Volt system the eight panels should be wired as a nominal 24 Volt array on an MPPT controller: four parallel sets of two. This means the max current going in to the controller would be 8 Amps * 4 as ggunn said: 32 Amps.

However what goes in to an MPPT controller isn't as important as what comes out of it. Your MidNite Classic will pick what it thinks is the best Voltage and current for the panels to output appropriate charging power to the batteries. That's why we use this formula for MPPT output: array size * efficiency / minimum charge Voltage. Or 1160 Watts * 0.77 (typical) / 12 = 74 Amps max current. That would fit with a battery bank around 720 Amp hours as you desire.

Emphasis: lots of things effect the actual efficiency of a system. Even if you do the "long math" beforehand and it all works you can't count on the weather. The formuli I explained before work best with a 25% maximum DOD. Trying to plan for precise amounts of power in/out is asking for trouble. Always give yourself plenty of margin for error. And have a generator on standby.

ywhic

Re: Simple math for newbies..?? Watts to Amps similarities..

No problem been around since January..

With my MPPT controller and 2 panels in series I got the VOLTS converting to more AMPS for me.. Pumping 36-37 volts in to a 12v battery bank with Midnite Classic Controller.. hence the higher AMPS coming from the controller.. sorry if I confused you.

BB.

Re: Simple math for newbies..?? Watts to Amps similarities..

MPPT controllers are "constant power" or down converters -- Basically they follow Vmp*Imp*0.95=Vbatt*Ibat

If Vbatt is less than Vmp/Varray, then Ibatt will be higher than Imp/Iarray (which the charge controller is in "bulk" or MPPT mode. This is not necessarily true in absorb or float modes (depends on DC loading and battery state of charge).

-Bill

ggunn

Re: Simple math for newbies..?? Watts to Amps similarities..

ywhic wrote: »

No problem been around since January..

With my MPPT controller and 2 panels in series I got the VOLTS converting to more AMPS for me.. Pumping 36-37 volts in to a 12v battery bank with Midnite Classic Controller.. hence the higher AMPS coming from the controller.. sorry if I confused you.

Ah, yes, I see; that was amps out of the controller, not the PV source circuit. That is very different, of course. As you were.