Newbie: solar system sizing math

aronv

New Member
Joined
Oct 13, 2021
Messages
2
Hi folks,

I'm putting together a solar system as a new learner - and wanted to check out if I'm understanding everything correctly. Any help or pointers to more reading would be appreciated.

Here's what I'm looking at:

Panels: 365 watt (40.8 Voc, 11.33 Isc)
Inverter: MPP LV6548 (2 PV inputs, Max PV input 4000W (per side), 250V (max))
Battery: EG4 LifePO4 48V / 100 AH X 4 (parallel) = 48V / 400 AH of battery
In Austin Texas

PV Array Size/layout: p2S5 for each side.
To make sure the PV Array is the proper size for the invertor:
1 array(side) voltage == 204 V (40.8Voc *5 panels) <- this is < than 250V max for invertor
1 array(side) watts == Q1) I wasn't sure how to calculate, is it a or b?:
A) 365 watts * 10 (wattage of panels * number of panels) = 3650 Watts (which is < than 4000W per side)
B) 204 total volts * (2 strings * 11.33 Isc) = 204 V * (22.6 amps) = 4622 Watts (which is > than 4000W per side) - in this case I can only do P2S4

Q2) should I be using any values from PVWatts in the above?
Q3) any safety factors to add in? (I was confused around what the invetor can handle as UL listed vs. what to take off the top for cold weather )

PV Array -> Battery Size Check (1 PV array(side) only)
PV Battery Charging Amps (assuming P2S5 and 3650 Watts from above): 3650 PV Watts / 48V = 76.04 Charging Amps
Adding in energy loss = 76.04 Amps * (1 - 20% loss) * 95% LifeP0 efficiency = 76.04 Amps * (.8) * .95 = 57.70 Charging Amps

Battery Charging (1 PV array(side) only) = 400 AH * (1 /95% efficiency) / 57.70 Charging Amps = 7.29 hours to charge batter bank (given 2 sides would be half that and < than ~5 hours of sunlight per day).

Q4) any huge math/assumption problems?
Q5) any equipment tweaks you would do? (growwatt, etc..)
Q6) any other guidance/thoughts?

thank you so much,
Aron.
 

OffGridInTheCity

Solar Addict
Joined
May 23, 2021
Messages
551
Location
Southern Oregon
Hi folks,

I'm putting together a solar system as a new learner - and wanted to check out if I'm understanding everything correctly. Any help or pointers to more reading would be appreciated.

Here's what I'm looking at:

Panels: 365 watt (40.8 Voc, 11.33 Isc)
Inverter: MPP LV6548 (2 PV inputs, Max PV input 4000W (per side), 250V (max))
Battery: EG4 LifePO4 48V / 100 AH X 4 (parallel) = 48V / 400 AH of battery
In Austin Texas

PV Array Size/layout: p2S5 for each side.
To make sure the PV Array is the proper size for the invertor:
1 array(side) voltage == 204 V (40.8Voc *5 panels) <- this is < than 250V max for invertor
1 array(side) watts == Q1) I wasn't sure how to calculate, is it a or b?:
A) 365 watts * 10 (wattage of panels * number of panels) = 3650 Watts (which is < than 4000W per side)
Yes, 10 panels @ 365 watts each = 3,650watts max.

B) 204 total volts * (2 strings * 11.33 Isc) = 204 V * (22.6 amps) = 4622 Watts (which is > than 4000W per side) - in this case I can only do P2S4
VoC is not for power - its the max voltage that can occur and can even go higher based on co-efficient of temp. For example, if you're in Alaska at -50F it could go much higher. This is so you don't exceed the max voltage of the Charge Controller.

Vmp and Imp more aligned for power - e.g. volts * amps. You don't list these above but MPPT (max voltage -> power) will be much lower than VoC. My 150v VoC goes down to 95v under MPPT max power (as an example).

No if the panel is 365w you won't exceed that by much - no way will you get 4,622watts out of 3,650watts of panels :)

Q2) should I be using any values from PVWatts in the above?
Probably answered above.

Q3) any safety factors to add in? (I was confused around what the invetor can handle as UL listed vs. what to take off the top for cold weather
Really cold temps can add to VoC but Austin TX is probably not going to spike this much. I think you're OK for 250v max charge controller using 5s2p.


PV Array -> Battery Size Check (1 PV array(side) only)
PV Battery Charging Amps (assuming P2S5 and 3650 Watts from above): 3650 PV Watts / 48V = 76.04 Charging Amps
Adding in energy loss = 76.04 Amps * (1 - 20% loss) * 95% LifeP0 efficiency = 76.04 Amps * (.8) * .95 = 57.70 Charging Amps
I agree 3650watts / 48v = 76amp max charging. In practice it will be less than that 'most of the time' but sometimes in cool temps and cloud reflected sun I get full 100% PV and even a bit more.

Not sure what you man by 20% 'energy loss'? Whatever the MPPT of the Charge Controller puts out will go into the batteries (or load) for consumption - typically there's no significant loss. Maybe 1-2% between charge in vs charge out of the batteries - e.g. that's one of the reasons Lithium is so popular :)

Battery Charging (1 PV array(side) only) = 400 AH * (1 /95% efficiency) / 57.70 Charging Amps = 7.29 hours to charge batter bank (given 2 sides would be half that and < than ~5 hours of sunlight per day).
This varies greatly by time of year. I find that winter is about 25% of summer. Spring is the absolute best. On my 12.85kw PV array, I can generate ~ 80kwh/day at absolute max. By extrapolation I would guess 3,650kw PV would max out at 22kwh/day in spring/summer and 6kwh/day in winter. 400ah @ 48v = 19,200wh. At 80% DOD (15,360wh) you could do a full charge in 3/4 of a day in spring and 3 days in winter.

Here's another angle. The PVWatts website https://pvwatts.nrel.gov/pvwatts.php will give you good info for your specific location, angle of panels etc. Take the monthly number and divide by 28, 30, or 31 as appropriate to get an average kwh/day for your situation. This will give you good ballpark info. They used to have a download where you could see the daily info (based on last year's weather) for your location - which can give you insight into the worst and best days of a month if you really want to drill down into 'what if' scenarios :)

Q4) any huge math/assumption problems?
Q5) any equipment tweaks you would do? (growwatt, etc..)
Q6) any other guidance/thoughts?

thank you so much,
Aron.

I didn't answer everything but I hope the above is enough to advance the conversation. If you want to give Vmp (for example) and perhaps a bit more detail on the 'how long to charge' (e.g. winter, summer, average, what's the load situation, etc) we might be able to give you more direct info that would help.
 
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