PV Array Sizing Basis

[andreb]

Hey everyone,
I'm seeking some guidance on correctly sizing a PV array.
Here's my challenge though, I'm trying to figure out the size of the Solar Array required to power a 3kwh system using a 3000 watt inverter and a single 48V/100 Ah rack battery. The maximum PV input for the inverter is 5000 w.

Here's how I went about calculating the PV array size:
1. Using an inverter efficiency of 94%, that requires my input power to the inverter to be 3191 w.

2. If I assume a battery efficiency of 90%, that means the input power to the battery should be 3546 w.

3. Let's ignore minor system losses for simplicity. If I assume 10 sun-hours (its very hot & sunny where I am in the Caribbean), then I need a daily PV power production of around 355 watts.

4. If I select a 315 w panel (VOC 39.9 V), then it would seem I require just two (2) panels to satisfy the battery.

5. The inverter, however, is rated for a maximum PV input of 5000 w and requires a PV array input voltage of 120 VDC to 450 VDC.

6. Considering these parameters, I can satisfy the PV array input voltage with just 3 - 4 panels in series but the input power will be only 1260 w.

Sooooo, I guess my question is, when sizing the PV array, should it be based on the battery charging power requirement OR on the inverter power requirement.
If I base it on the inverter power requirement I would need around 12 panels in series. This would give me a total PV array input voltage of 480 V and a total input power of 3780 w.

Your thoughts??????

 

[andreb]

Actually in the last line of my post, there's a small error. It should be 11 panels giving a total voltage of 440 V (<450 V) and a total power of 3465 w.

 

[Charleswgibbs]

I think you are seriously over estimating how much solar input you will get. Sunny is good but hot isn't, hot panels don't produce as much. It may be sunny for 10 hours but you aren't getting peak power for 10 hours, even in the Caribbean you are only going to get 5-6hrs solar hours.

Your total energy use determines how large of an array you'll need, do you have any figures for how many kWh you use on a daily/monthly basis?

 

[andreb]

I think you are seriously over estimating how much solar input you will get. Sunny is good but hot isn't, hot panels don't produce as much. It may be sunny for 10 hours but you aren't getting peak power for 10 hours, even in the Caribbean you are only going to get 5-6hrs solar hours.

Your total energy use determines how large of an array you'll need, do you have any figures for how many kWh you use on a daily/monthly basis?

Thanks for the point about sun-hours. Point taken. As far as energy use goes, I'm working with just about 3kwh daily.

 

[Charleswgibbs]

Thanks for the point about sun-hours. Point taken. As far as energy use goes, I'm working with just about 3kwh daily.

Oops! I saw the 3kwh figure in the op but thought it meant 3kw system, lots of people mixing up units so I just assumed, sorry! ?

 

[andreb]

No worries. Given the info though, what would you estimate as an appropriate PV size?

 

[andreb]

You know what, I think I finally found my answer. I was able to get the manual for the inverter (I only had the spec. sheet before) and in there, the manufacturer gives recommended PV array configurations. They recommend a minimum of 5 panels (330 w / 40.35 VOC) in series and a maximum of 10.
This works for me BUT I'd still like to confirm weather it is correct to base PV array sizing on Battery capacity, system efficiencies and sun-hours OR simply refer to inverter manufacturer's recommendations.

 

[Doggydogworld]

5. The inverter, however, is rated for a maximum PV input of 5000 w and requires a PV array input voltage of 120 VDC to 450 VDC.

Is this an all-in-one box that combines an inverter and a solar charge controller/battery charger? If so, you're correct that you'll need a few panels in series to achieve minimum voltage. Must the 120VDC minimum be maintained while operating? If so you should probably use the panel's NOCT Vmp as a guide instead of STC Vmp. The Voc rating is a max reading with cool panels, perfect sun/tilt and zero load.

6. Considering these parameters, I can satisfy the PV array input voltage with just 3 - 4 panels in series but the input power will be only 1260 w.

It sounds like 5000W is the max solar input. You can do less as long as you meet the minimum voltage requirement.

Four 315s will probably produce 4-5 kWh per day. Enter location and system info into the PVWatts calculator and it will estimate production for each month based on local weather records.

 

[Doggydogworld]

Forgot to mention you need to include your inverter's idle consumption. Some can consume over 1 kWh per day. And your inverter's 5 panel recommendation sounds like a rule of thumb. Five of those little 100W panels probably wouldn't work. So double-check the actual specs.

 

[OffGridInTheCity]

Sooooo, I guess my question is, when sizing the PV array, should it be based on the battery charging power requirement OR on the inverter power requirement.
If I base it on the inverter power requirement I would need around 12 panels in series. This would give me a total PV array input voltage of 480 V and a total input power of 3780 w.

Your thoughts??????

There are many ways to think about sizing a PV array such as
- How much power do I want to consume
- Is there a limited on space or $
- Will neighbors complain and/or will there be permit constraints?
- Can I do it myself or are there contractors available to help me?

But sizing 'to a battery bank' has not been my journey, it's been the other way around.

My journey was to size my array for a power consumption goal. Namely, I wanted to power my home 100% off-grid and it's still my goal. At first I thought I might get an ROI in my remaining lifetime but at 11c/kwh in my state, it's just not realistic. So it's clearly about off-grid / preparedness (+ FUN).

So far, I'm up to a 13kw PV array which produces about 18,000kwh / year with 15,000kwh / year of actual consumable power. This has gotten me about 1/2 way. The winter 3 months are just a HUGE problem. I did buy 12kw of panels stored under the house + wire and 'stuff' that I could deploy to the back yard in the event of a Puerto Rico type, long lasting emergency. So I have 25kw PV array capabilities you might say.

However, I'd put up a 50-60kw PV array if I could because that's what I need for 110% off-grid in really bad winter months. Crazy right? but it's your power consumption goals that determine ideal PV array size to my way of thinking

Tip: I've found PVwatts - https://pvwatts.nrel.gov/pvwatts.php - to be an excellent guide to predict array size to power. It's based on your location's weather / available sun but of course it doesn't know what the weather will be in future years.