# Helping with a project

Solar Expert Posts: 1,973 ✭✭✭
I am helping some guys with a project involving solar lighting, and I am a little out of my element. I remember you offgridders talking about a rule of thumb for the ratio of PV in STC Watts to battery capacity in Watt-hours. What is that?

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Re: Helping with a project
ggunn wrote: »
I am helping some guys with a project involving solar lighting, and I am a little out of my element. I remember you offgridders talking about a rule of thumb for the ratio of PV in STC Watts to battery capacity in Watt-hours. What is that?

It is a fallacy. Don't fall for the "one Watt of panel per Amp hour of battery" nonsense.

Take a standard 12 Volt system:
220 Amp hours @ 12 Volts = 2640 Watt hours
Usual panel recommendation on a PWM controller;
22 Amps * 17.5 (typical panel Vmp) = 385 Watts
Ratio: 2640/385 = 6.8:1

Now, change to an MPPT controller and expect better array performance, needing only 343 Watts and get a ratio of 7.7:1

Alter the peak charge current, get different results. Change the insolation, get different results. Temperature coefficient? Different results.

Best bet:
Calculate the battery capacity according to the loads, and round that size up to the nearest available.
If the lights are only at night you can use a 5% of the battery size for the peak charge current, otherwise go with 10% (up to 20% if possible). Calculate the array size based on this. Again, round up to the nearest available size.
If the array is then >400 Watts look into the economics of an MPPT type controller.

Sometimes you just can't shortcut to the end.
• Solar Expert Posts: 122 ✭✭✭
Re: Helping with a project

Short answer: 8 watt-hours of battery storage to each watt of solar array.
• Solar Expert Posts: 1,925 ✭✭
Re: Helping with a project
Desert Rat wrote: »
Short answer: 8 watt-hours of battery storage to each watt of solar array.

I have 5. Would have 3 if my financial independence project wouldn't lag far behind my energy independence project.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Helping with a project

Here's a bit more information on the project:

It's an art project for the Burning Man Festival. It only needs to be functional for about 10 days. They want the display to run from dusk to dawn. They calculate their steady state load at 12VDC to be about 12A and they are figuring on 9 hrs/day, for 108A-h/day. I have advised them to get 200A-h @12VDC worth of battery.

They have acquired a couple of mismatched PV modules (240W and 220W) which have almost identical (29.4V and 29.5V) Vmp and they want to know if it is enough to charge the batteries during the day. It's the high desert, so cloud cover other than the occasional scattered T-storm is unlikely.

I figure they can get some sort of Morningstar lighting controller and run the PV modules in parallel.
• Banned Posts: 17,615 ✭✭
Re: Helping with a project
ggunn wrote: »
Here's a bit more information on the project:

It's an art project for the Burning Man Festival. It only needs to be functional for about 10 days. They want the display to run from dusk to dawn. They calculate their steady state load at 12VDC to be about 12A and they are figuring on 9 hrs/day, for 108A-h/day. I have advised them to get 200A-h @12VDC worth of battery.

They have acquired a couple of mismatched PV modules (240W and 220W) which have almost identical (29.4V and 29.5V) Vmp and they want to know if it is enough to charge the batteries during the day. It's the high desert, so cloud cover other than the occasional scattered T-storm is unlikely.

I figure they can get some sort of Morningstar lighting controller and run the PV modules in parallel.

Okay, good on the load calcs. Couple of GC2's is the cheapest bet here and drawing it down to 50% won't be an issue because it's a short duration project.

The panels are an issue because they're the "wrong" Voltage without an MPPT controller. Too much Wattage for the little Morningstar unit, so either an expensive controller (\$400+) or throw away half the power potential. With an MPPT controller and good daylight hours the 460 Watts might make enough recharge. Without it ... you're looking at 15 Amps peak current instead of 29 and a need for more than 7 hours equivalent good sun to recharge.

There's the problem in a nutshell.
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Helping with a project

I'll play 'Coot for a minute.... tell them to get an Eu1000i generator & car charger (To Bulk) and finish with the panels

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Banned Posts: 17,615 ✭✭
Re: Helping with a project
westbranch wrote: »
I'll play 'Coot for a minute.... tell them to get an Eu1000i generator & car charger (To Bulk) and finish with the panels

But the gen is even more expensive than an MPPT controller!
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Helping with a project

And they will have it for next year and other applications

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Solar Expert Posts: 1,571 ✭✭
Re: Helping with a project
ggunn wrote: »
They have acquired a couple of mismatched PV modules (240W and 220W) which have almost identical (29.4V and 29.5V) Vmp and they want to know if it is enough to charge the batteries during the day. It's the high desert, so cloud cover other than the occasional scattered T-storm is unlikely.

I figure they can get some sort of Morningstar lighting controller and run the PV modules in parallel.

Yeah, I'm with you, if it only needs to work for 10 days then panels in parallel to a simple PWM controller and 2 x 12V batteries in series. Given high temperatures they may not reach absorb voltages so the controller should be configureable for longer absorb times. Given the loads, they could even configure it to avoid float all together, just stay in absorb all day. They could also orientate the panels differently to gain a longer day: one east the other west.

Then when the party's over, charge the battery on a proper charger.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Helping with a project
westbranch wrote: »
I'll play 'Coot for a minute.... tell them to get an Eu1000i generator & car charger (To Bulk) and finish with the panels
They want it to be solar and they don't want to run a generator.