help confirm new PV system specs

codlinjr · January 2015

I'm about to pull the trigger on my first PV system and after hours of research I think I finally have the components lined up. If some of the experts here can do a "sanity check" for me and let me know if you see anything out of line, included what I plan to pay, I would appreciate it.

System will be used on a rustic 16x20 off-grid cabin in north Florida that will get used on average 2-3 days/month. I have done some online calculators and guess I will need approx. 1500 watt/hours per day running LED lights, radio, a fan or 2, small chest freezer, coffee maker, small microwave (no more than a few minutes/day max), laptop charger (rarely), maybe an LCD flat screen/DVD player (rarely for 2-3 hours max). I also have a 5500 watt generator for power tools, an AC unit in the future, or emergency situations if needed. The solar will be hardwired into the cabin breaker box with a transfer switch in the emergency event I need to run everything off generator power instead at some point. I only need one days worth of battery storage I figure, and I plan to shut the inverter down during the times I am not at the cabin (is that wise?) as I wont be leaving any appliances or lights on.

I think I have a system on the high end of what I will need most of the time but wanted it to be big enough to run the items mentioned above on rare occasions. Specs:

2x Sonali 300W 24V poly panels wired in parallel (local pick-up for $231 each plus tax)
100' run of 6 ga copper wire from panels to controller (suggestions on best place to buy??)
Prostar 30amp 24volt PWM controller ($177 online)
4x 6volt 230 AH Deka deep cycle flooded batteries wired in series (local pick up for $89 each plus tax)
Samlex PST2000-24 2000/3500W 24V pure sine inverter hardwired into cabin ($642 online)

Any help or information you may have that I am missing will be appreciated. Thanks in advance

BB. · January 2015

Re: help confirm new PV system specs

Welcome to the forum Codlinjr.

This is a solar forum--But we also try to be practical.

Would a Honda eu2000i genset (~$1,000) and two gallons of gasoline per weekend work for you?

The issue with solar is the batteries will last ~3-5 years. And for running large loads (coffee maker, microwave, and even a freezer--starting surge), needs a relatively large battery bank (to supply surge current, 5-10 minutes of high power for the microwave, etc.).

And only using power for 3 days a month--That is ~27 days a month that the energy falls on the floor unused. So, $/kWH wise, solar is not the best investment.

If you can replace the coffee maker and freezer with propane (or ice box)... Just run the microwave from the genset (again, the smaller Honda or similar inverter-generator would be nice).... You could get away with a smaller battery bank and a ~300 Watt 12 VDC TSW inverter. Less costs, less overall maintenance costs, and less to be stolen (if there is a security concern).

If you still want solar--Not a problem--But wanted to confirm your needs first.

-Bill

codlinjr · January 2015

Re: help confirm new PV system specs

Thanks for the info Bill. I have considered the generator only option but am leaning toward solar for various reasons. I haven't eliminated the option but wanted to see if what I had in mind solar wise was on the mark.

westbranch · January 2015

Re: help confirm new PV system specs

Alternately you could have a few 'heavy duty circuits' for the fridge microwave coffee maker and deep freeze that you run off the gen in the daytime and a small battery bank for evening lights, laptop. Also consider battery powered hand tools.
hth

BB. · January 2015

Re: help confirm new PV system specs

OK, 1,500 WH per day, ~1,500 AC inverter, 24 volt battery bank, north Florida.

You are right on the edge of a 24 vs 12 VDC battery bank... Either will work OK for you (pending panel/charge controller/AC inverter selections).

If you want to run ~1,500 watts for a period of time (fridge, microwave, coffee maker, mix and match). Your battery bank will need to be around (assuming a maximum discharge rate of C/5 for flooded cell batteries):

1,500 Watts * 1/0.85 AC inverter * 5/C discharge rate * 1/24 volts = 368 AH @ 24 volt battery bank

We generally recommend a 5% to 13% rate of charge--For a weekend cabin, 5% is probably OK. For a full time 9+ month residence, 10-13%+ is recommended. Note also that larger battery banks also require a larger solar array to make minimum rates of charge:

368 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 693 Watt array minimum
368 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,386 Watt array nominal
368 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,802 Watt array "cost effective" maximum

Note, we also size the battery bank based on power draw for (typically) 2 days of "no sun" and 50% maximum discharge:

1,500 Watt*Hours per day * 1/0.85 inverter eff * 1/24 volts * 2 days storage * 1/0.50 max discharge = 294 AH @ 24 volt battery bank

In your case, the maximum current/power draw is setting the minimum size of the battery bank.

Using PV Watts for a fixed array, tilted 31 degrees from horizontal, using Jacksonville FL:

Month
Solar Radiation
(kWh/m²/day)

1
4.29

2
4.31

3
5.41

4
6.10

5
5.82

6
5.50

7
5.44

8
5.37

9
5.13

10
4.81

11
4.51

12
3.77

Year

5.04

Toss the bottom 3 months (use genset during bad weather), gives us 4.51 hours of sun for November:

1,500 WH * 1/0.52 end to end system eff * 1/4.5 hours of sun per day = 641 Watt array minimum

So--I would suggest your battery bank is too small for your power needs (C/5 is good for a maximum of minutes a bit more of maximum power from flooded cell batteries). If your peak power needs were less (just fridge and smaller appliances, you could do with less).

And your AC inverter is way to big to be useful with such a small battery bank (roughly, for 24 volt battery bank, you should have 200 AH for every 1,000 Watts of maximum load--A 2kWatt inverter should have ~400 AH @ 24 volt battery bank minimum if you expect 2,000 Watts from the inverter for more than a handful of seconds. If you plan on drawing less maximum current--You can get away with a smaller battery bank. Note that larger inverters usually draw more power when running (just turned on)... So, for larger inverters, their "tare" loads can be significant... Say 20 Watts:

20 watts * 24 hours per day = 480 Watt*Hours

Or almost a 1/3rd more power than your actual AC loads (larger battery bank, larger solar array).

If I have the correct panels, they are "true" 24 volt panels (Vmp~35 to 38 volts or so) for charging a 24 volt battery bank:

Watts 300
Cells 72
Panel Vmp 37.58
Panel Imp 7.98
Panel Voc 44.64
Panel Isc 8.45

And two of those panels in parallel should be OK with a 20 amp PWM controller (check the controller specifications to be sure).

Quick back of the envelope check of your system.

-Bill

codlinjr · January 2015

Re: help confirm new PV system specs

Thanks again Bill, that is helpful.

help confirm new PV system specs

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