# Is there a sizing sticky someplace?

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Solar Expert Posts: 2,397 ✭✭✭✭
I am getting hit with lots of questions on the gm-volt forum on using a battery based solar system to charge a Volt.

A full charge is about 13.5 kWh and a peak load of 3300 - 3500 watts X 4ish hours using a 240V charger. Also the Thermal Management System does a 800 watt draw for about 15 minutes every 4 hours when temps are high (1-1.5 kWh a day). Also the car has the capability to do a remote start and cool the cabin with a limit of 2 times between actual starts, drawing the full 3300 watts for 10 minutes.

Is there a sticky someplace with all the calculations in it for a battery based system? I am just trying to convince most users that grid tie is way more cost effective but I want the math to be correct. Plug in the loads, size the battery, then size the solar accordingly.

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Re: Is there a sizing sticky someplace?

The guy who was supposed to put up the sticky post with all the formuli in it hasn't got around to it yet.

If you're talking about he amount of panels to obtain the charge rate it's basically:

Rate * Voltage / Efficiency Factor (typical 0.77) = Array Wattage

If you're talking about the amount of panels to obtain a daily Watt hour capacity it's basically:

Watt hours / Efficiency Factor (typical 0.52) / Hours of Good Sun = Array Wattage

And you're absolutely right that the grid will be cheaper almost all the time, as opposed to putting up 7kW of solar array and being able to charge the car only during the day (night charging would be preferable in most cases I'd think).
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Re: Is there a sizing sticky someplace?

Assuming that is the only loads (pretty high for off grid) and like 3 days autonomy with no sun and a resonably small SOC impact. How much battery would that work out to? I think from that I could determine the solar requirements, 10% charge rate ....
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Re: Is there a sizing sticky someplace?

Most people would probably use a GT system which ups the efficiency rate considerably. But if you were going to have a dedicated off-grid system for recharging your Chevy Volt (the mind boggles) you'd need more autonomous battery capacity than the car itself has by a factor of about 2X for one recharge. If you add extra "no sun" days it gets absurd.

13.5 kW hours on 48 VDC is 282 Amp hours, or a minimum 564 Amp hour battery bank. Just to recharge the car? That's more power than most off-gridders use for the whole house!

If there were a way to connect the panels directly to the car's charging system that would improve efficiency considerably.

Looked at in dollar terms (based on my locale) it would cost \$10 to recharge "off grid" (after a big initial investment) or \$1.35 using BC Hydro. Which would you choose?
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Re: Is there a sizing sticky someplace?
Most people would probably use a GT system which ups the efficiency rate considerably. But if you were going to have a dedicated off-grid system for recharging your Chevy Volt (the mind boggles) you'd need more autonomous battery capacity than the car itself has by a factor of about 2X for one recharge. If you add extra "no sun" days it gets absurd.

13.5 kW hours on 48 VDC is 282 Amp hours, or a minimum 564 Amp hour battery bank. Just to recharge the car? That's more power than most off-gridders use for the whole house!

If there were a way to connect the panels directly to the car's charging system that would improve efficiency considerably.

Looked at in dollar terms (based on my locale) it would cost \$10 to recharge "off grid" (after a big initial investment) or \$1.35 using BC Hydro. Which would you choose?

That is my point with them exactly. That is a good jumping off point I guess to add to a thread over there. It is the typical not understanding the issues with off grid capacity and cost. Lots of these guys have TOU plans to boot and can get the charging costs for the Volt down well below \$1, but of course they like the green aspects of Solar and think that battery backed vs Grid TIe would be great. I suppose if cost is no issue.
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Re: Is there a sizing sticky someplace?
The guy who was supposed to put up the sticky post with all the formuli in it hasn't got around to it yet.

If you're talking about he amount of panels to obtain the charge rate it's basically:

Rate * Voltage / Efficiency Factor (typical 0.77) = Array Wattage

If you're talking about the amount of panels to obtain a daily Watt hour capacity it's basically:

Watt hours / Efficiency Factor (typical 0.52) / Hours of Good Sun = Array Wattage

And you're absolutely right that the grid will be cheaper almost all the time, as opposed to putting up 7kW of solar array and being able to charge the car only during the day (night charging would be preferable in most cases I'd think).

So my 870 array would be 669 (at 77%) (which I did get the 112w per panel in testing).. so that * .52 = 347.88 / hour * 5 hours = 1739.4 watts for 110-120 VAC usage.. (granted for my location down in TX it says 6-7 hours in the summer)..

Thats not too bad for the \$\$ I spent.. IMHO.. I'm at like \$1350 without the battery bank, inverter, or mounts.. which I figure is another \$1400 (\$600 batteries (220ah) and \$800 24V inverter).. so \$2750.. is about \$1.58/watt for initial outlay..

Still bearable.. if not for this site I'd be a mess I'm sure..
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Re: Is there a sizing sticky someplace?

Not quite, Al; that example assumes a lot including an MPPT type charge controller (necessary for the size array needed to charge the car) and "average" temperatures, no long wire runs, et cetera.

It's like if you wanted to go psycho on efficiency and use monocrystaline panels on a dual axis tracker, and build the charging station at 6000 feet above sea level ...

That would change the 77% factor and the 52% factor considerably.

It's just amazing the difference in system performance with what seems to us mere humans as a 'minor' fluctuation in the operating conditions.

Sort of a continuing problem on the forum: having to answer questions in generalized terms initially and then having people assume those answers are across-the-board absolutes.

Heck, my panels run over 80% all the time and the Voc goes 30% higher in Winter. You won't get that in Texas!
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Re: Is there a sizing sticky someplace?
ywhic wrote: »
So my 870 array would be 669 (at 77%) (which I did get the 112w per panel in testing).. so that * .52 = 347.88 / hour * 5 hours = 1739.4 watts for 110-120 VAC usage.. (granted for my location down in TX it says 6-7 hours in the summer)..

Just to fix a double derating (0.77 * 0.52 --- The 0.52 already includes the 0.77 for the panels and charge controller).

Also, Watts is a rate--So Watts * Hours = Watt*Hours

870 Watt array * 0.52 end to end system derating * 5 hours of sun per day = 2,262 Watt*Hours (for "average" 120 VAC inverted power)

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Is there a sizing sticky someplace?
BB. wrote: »
Just to fix a double derating (0.77 * 0.52 --- The 0.52 already includes the 0.77 for the panels and charge controller).

Also, Watts is a rate--So Watts * Hours = Watt*Hours

870 Watt array * 0.52 end to end system derating * 5 hours of sun per day = 2,262 Watt*Hours (for "average" 120 VAC inverted power)

-Bill

The 870 * 0.52 = 452 Watts/hour.. which I think is also about 45 amps correct??

I don't think it matters that I'm only at 3500' elevation..
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Re: Is there a sizing sticky someplace?
ywhic wrote: »
The 870 * 0.52 = 452 Watts/hour.. which I think is also about 45 amps correct??

I don't think it matters that I'm only at 3500' elevation..

The 0.52 factor is an over-all efficiency rating for off grid systems; from the panel "nameplate" rating to the AC Watts. So your 870 Watts of panel could produce 452 Watts AC out of the inverter. This is usually tempered with the time factor, as in over five hours of equivalent good sun you'd get 2262 Watt hours per day.

Since it's on the output side it would be "452 Watts / 120 VAC = 3.75 Amps" but that's not the purpose of that formula.

Your 3500 feet of elevation should help: thinner air = more insolation. Not a lot, but maybe a couple of percent on the efficiency scale. The only way to know for sure is to try it and see.
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Re: Is there a sizing sticky someplace?

i'm with everybody else on this point that if you have the grid that grid tie is the best and greenest way to go about it because of the higher efficiency and it would free up the time one could use the car too instead of waiting for the car to charge while the sun is shining. a big enough battery bank could allow night charging, but that's a big system.

to me the greenest way is where i get to keep more green, but saving on the need for charge controllers and batteries is definitely greener as many green enthusiasts will cite that the manufacture of these products uses energy that often is generated by coal, oil, and lng. also fewer pvs needed would also mean fewer pvs manufactured too
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Re: Is there a sizing sticky someplace?

I was also wondering about a thread with all the formulas. Could somebody explain this one to me? I get the other one is the formula to calculate the watt*h on the ac output side. But what does this one do and what are all the factors?
If you're talking about he amount of panels to obtain the charge rate it's basically:

Rate * Voltage / Efficiency Factor (typical 0.77) = Array Wattage

I also understand that the 0.52 derating is for end to end. What about the 0.77?

I also had gotten some help from BB with my calculations for my system. And there was a few formulas I would like to get explained. What I would like to know is where all the factors come from:

1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 942 Watt Array "minimum"
1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,883 Watt Array "nominal"
1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,448 Watt Array "cost effective maximum"

And also what "rate of charge" really means.

Thank you all for the help,

Aleman
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Re: Is there a sizing sticky someplace?
aleman83 wrote: »
I was also wondering about a thread with all the formulas. Could somebody explain this one to me? I get the other one is the formula to calculate the watt*h on the ac output side. But what does this one do and what are all the factors?

I also understand that the 0.52 derating is for end to end. What about the 0.77?

This is average panel output + MPPT charge controller loss on a typical system. In other words a 2000 Watt array can reasonably be expect to produce 1540 Watts average over the hours of "equivalent good sun".
I also had gotten some help from BB with my calculations for my system. And there was a few formulas I would like to get explained. What I would like to know is where all the factors come from:

1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 942 Watt Array "minimum"
1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,883 Watt Array "nominal"
1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,448 Watt Array "cost effective maximum"

This is calculating array size based on battery bank capacity for various rates of charge. So for a 1000 Amp hour 12 Volt battery bank you could have between 942 and 2448 Watts of array.
And also what "rate of charge" really means.

Rate of charge is the peak expected charge current for a battery. A 100 Amp hour battery (any Voltage) would charge at 5 Amps peak for a 5% charge rate, 10 Amps for 10%, et cetera. This is sometimes expressed by the fractional equivalent "C/#" where "C" is the battery capacity in Amp hours (at the 20 hour rate) and "#" is a number which relates to the rate of charge (as is C/10 is a 10% rate).

Any clearer?
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Re: Is there a sizing sticky someplace?

Batteries are rated for Amp*Hour capacities at different discharge rates... C/5 would be battery is 100% discharged in 5 hours... C/10 is 10 hour discharge rate... C/20 is 20 Hour discharge rate... C/100 is 100 hour discharge to dead.

For example a Trojan battery rating:

L16RE-A
6 volt

325 AH 20 hour rate

360 AH @ 100 hour rate

We (I) like to use the C/20 hour discharge rate... This is a pretty close match to discharging a battery to 75% state of charge over 5 hours of discharge at night and 5 hours of charging the next day... (4 nights at 5 hours each, equals 20 hours to 0% state of charge).

The faster you discharge a battery bank, the less "apparent capacity" the bank has (and the more energy turned into heat).

The 0.77 derating I take from PV Watts:

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/derate.cgi

Basically, around 81% derating for solar panel output (hot panels, drop Vmp, wire drop, dirty panels, etc.) and 95% for controller losses.

Note the whole derating for MPPT type charge controllers is entirely different vs PWM type charge controllers... But for our first cut estimates (warm climate, generically/correctly configured array/battery bank/etc.) the 0.77 derating is "good enough" to give a conservative derating (from solar panel "name plate" STC standard test conditions in lab/factory).

The "rate of charge"... We use the 20 Hour rate (more or less typical industry number)--0.05*(20 Hour Rate)=charging current and then use:
• 5% minimum rate of charge... Lead Acid batteries need a somewhat minimum current to obtain optimum performance on the lead plates (opening pores, chemical conversions, possibly mixing of acid in tall batteries, etc.). Also, we what batteries quickly recharged... The longer they sit at less than 90%+ state of charge, the more they sulfate. And there are self discharge losses, charging losses, etc... When you go below 5% minimum rate of charge, it is difficult to fully recharge the battery bank every few days (ideally).
• 10% rate of charge... Sort of the "optimum" (or even minimum from Trojan) rate of charge. Batteries recharge quickly... If you add a bit more loads to your system, the "extra panels" still will quickly recharge the bank and avoid long term "deficit" charging.
• 13% rate of charge is about the maximum rate of charge for a standard flooded cell lead acid battery to take from 20% state of charge to 100% state of charge (i.e., 8 hour forklift shift). Battery bank may begin to overheat if charged (for long period of time) above 13% or C/8 rate of charge (although, you can approach 25% rate of charge--I would recommend a Remote Battery Temperature sensor for charge controller to reduce chances of thermal run-a-way).

Got to go...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Is there a sizing sticky someplace?

In looking at this thread – what are the calculations for the standard .52 derating for end-to-end? I understand the .77 for power to the batteries (~85% panel efficiency, 97% wiring efficiency, and 95% charge regulator efficiency).* What are the additional factors to get to .52 derating for the end-to-end system?* I would assume adding 90% for battery efficiency and 85% for inverter efficiency, but that would only get down to 60%.* What am I missing, or what numbers do I have wrong?
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Re: Is there a sizing sticky someplace?

The "0.90" or 90% efficiency is what I use for AGM battery based systems (reported efficiency from 90-98% efficiency for different users).

For flooded cell, I use 0.80 efficiency (reported efficiency is around 80-90% typical).

Using 0.80 for flooded cell will get you done to 0.52 overall system efficiency.

Note, how you "use" a battery will affect efficiency... If you deep cycle (80% or lower), the flooded cell will be more efficient.

If you shallow cycle (85% to 95%+), then it will be less efficient (battery is at higher voltage when charging, near 100% charge your battery is generating hydrogen/oxygen gases and heat, etc...

Note that this is "energy efficiency"... Take energy out of the battery at ~12.5 volts, return energy at ~14.5 volts. AGM batteries have a "lower" average voltage spread between discharging and charging.

Current/Coulomb efficiency is different (and pretty near 100% for lead acid batteries when not "gassing"). Take out 100 AH, return at ~100 AH.

And, I tend to be conservative when making estimates... So, you will see me "error" towards lower efficiency numbers for generic calculations.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Is there a sizing sticky someplace?
mjp24coho wrote: »
In looking at this thread – what are the calculations for the standard .52 derating for end-to-end? I understand the .77 for power to the batteries (~85% panel efficiency, 97% wiring efficiency, and 95% charge regulator efficiency).* What are the additional factors to get to .52 derating for the end-to-end system?* I would assume adding 90% for battery efficiency and 85% for inverter efficiency, but that would only get down to 60%.* What am I missing, or what numbers do I have wrong?

The 0.52 derating is over-all system efficiency for a battery-based system. It starts with the panel rating in Watts times the hours of sun and ends with the actual AC Watt hours you can expect. Part of it is the potential power wasted when batteries are full but panels could still be producing, only there's no place for the power to go.

So a 2kW array over 5 hours of good sun starts out with 10 kW hours DC, but ends up as 5.2 kW hours AC.
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Re: Is there a sizing sticky someplace?

You do know the Chev Volt has had over 30 fires and a few deaths.
The car is not green , as in the protected life .
It's costing 235 K per car so far , and they still can't give them away.

I wouldn't get involved with such a potential fire-cracker.

Different Trades see/hear , have inside info.

VT
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Re: Is there a sizing sticky someplace?
CDN_VT wrote: »
You do know the Chev Volt has had over 30 fires and a few deaths.
The car is not green , as in the protected life .
It's costing 235 K per car so far , and they still can't give them away.

I wouldn't get involved with such a potential fire-cracker.

Different Trades see/hear , have inside info.

VT

There is no evidence of any fires cause by the volt except the 3 tests done by the Government testing.
The car was well Into development before the GM bankruptcy.
I can find no evidence of any deaths in volts, they have a 5 star safety rating.
The Volt is on track to sell 20,000 units this year, more than any other plug in and meets GM's plan on the,car.

You are a bit gullible if you think the cost of the car is 235K and you just can't believe everything you read on the Internet. And taking bailout dollars and dividing by the number of Volts is voodoo math.
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Re: Is there a sizing sticky someplace?
Lots of these guys have TOU plans to boot and can get the charging costs for the Volt down well below \$1, but of course they like the green aspects of Solar and think that battery backed vs Grid TIe would be great. I suppose if cost is no issue.

Even if monetary cost is no issue, you might point out to them the environmental cost of manufacturing the battery bank needed for the off-grid charging system (vs. no batteries for the grid-tie system), and of course the efficiency gains of the grid-tie as mentioned above.

One thing though that might improve the math a bit for both types of system is that I would expect most EV users to not need a full charge every day. E.g., if you're daily commute is 25 miles, that would seem to require only about 8 KWH/day (based on numbers I've seen; maybe that's optimistic).

One other possibility: if these guys are already on-grid and go with an off-grid system for other reasons (such as there being no or only very unfavorable grid-tie terms, as with my utility), they could make the system 'grid supported' and thereby reduce the battery bank size while still getting good use of the solar panels when the sun is available.
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Re: Is there a sizing sticky someplace?

WARNING: the socio-political discussion on Chevrolet's Volt stops now. That is outside the venue of this forum.