Ideas on small weekend place

Re: Ideas on small weekend place

Add this: Do you have your batteries on charge? If you do not keep them charged (or recharge them for ~1 day per month), they will quickly die (sulfate) over the next few months without charging.

Since you have the batteries--Lets work from there. Using our typical rule of thumb for an "optimum" battery bank (2 days of storag eand 50% maximum discharge):

• 952 AH * 24 volts * 1/2 days of storage * 0.50 maximum discharge * 0.85 AC inverter eff = 4,855 WH per day of 120 VAC power

That would easily power everything excluding the A/C units (basically leave around 1,500 to 2,500 WH "left over" for A/C).

Using the 5% to 13% rate of charge for the battery:

• 952 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,793 Watt array minimum
• 952 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 3,585 Watt array nominal
• 952 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 4,661 Watt array "cost effective" maximum

Looking at a fixed array tilted to latitude via PV Watts for Houston, we get:

Month    Solar Radiation (kWh/m 2/day)
1      3.68     
2      4.12     
3      4.82     
4      4.98     
5      5.24     
6      5.53     
7      5.43     
8      5.44     
9      5.40     
10      5.19     
11      4.33     
12      3.34     
Year      4.79      

Looks like you have a significant marine layer/thunderstorms in the summer that really kills your summer sun???

Using 4 hours of sun per day minimum (use a generator for winter--when needed):

• 4,855 Watt*Hour load * 1/0.52 end to end AC system eff * 1/4 hours of sun = 2,334 Array for ~9 month of the year no genset

That tells us that a system from 2,334 to 4,661 Watt array (using all the digits so you can see where I get the results) would be a good fit, with 3,585 Watt or larger array being a "nice" healthy array (perhaps too much for a weekend cabin, but really nice for a full time 9+ month residence.

Now--The above is all based on power usage where you basically charge during the day and use power at night (or during 2 days of cloudy weather).

Since much of your power usage with A/C will be during the day--you can use "excess power" from your array to run the A/C during sunny/hot weather (no A/C during mostly cloudy weather? Although, humidity may be an issue requiring more A/C even under clouds).

Say you have 5 hours of sun and use the nominal to large array:

2,334 Watt Array * 0.52 system eff * 5 hours of sun = 6,068 Watt*Hours on a sunny day minimum array
3,585 Watt Array * 0.52 system eff * 5 hours of sun = 9,321 Watt*Hours on a sunny day nominal array
4,661 Watt Array * 0.52 system eff * 5 hours of sun = 12,119 Watt*Hours on a sunny day maximum "cost effective" array

So--If you limit your night time usage to 4,855 WH (or really bad weather day)--Power from the Battery Bank... During sunny weather you can use the above to power your daytime summer sunny day loads:

• 6,068 Watt*Hours - 4,855 WH night loads = 1,213 WH daytime load
• 9,321 Watt*Hours - 4,855 WH night loads = 4,466 WH daytime load
• 12,119 Watt*Hours- 4,855 WH night loads = 7,264 WH daytime load

The "maximum cost effective array" on an 80 Amp Outback controller would be:

• 80 amps * 29 volts charging * 1/0.77 panel+controller derating = 3,013 Watt array per single controller

If you got a small mini-split A/C (or heat pump if you need heating during winter) and wanted ~10 hours of cooling (400 Watt average load):

• 4,466 WH from "nominal array" 1/400 Watt load for A/C on load = 11 hours of operation

Anyway--A lot of information and guess work here--A starting point.

Your thoughts?

-Bill

Re: Ideas on small weekend place

Regarding the battery bank--Assuming the batteries are near full charged, you would need ~1-2% rate of charge to keep them "happy" at the minimum (you should have a hydrometer and check all the cells to ensure that all cells are fully charged).

The minimum solar array size recommended to float your battery bank would be (6 cells/12 volts at a time):

952 AH * 17.5 Vmp panel * 0.01 rate of charge = 167 Watt panel/array
952 AH * 17.5 Vmp panel * 0.02 rate of charge = 333 Watt panel/array

Or 9.5 to 19 amps minimum battery charger (solar or AC mains).

167 to 333 Watt array would be the absolute minimum I would suggest (you can monitor your cells' specific gravity to ensure they are remaining near fully charged).

I would hate to tell you that 3,000 Watt array is the minimum for what you need... Energy usage is a highly personal set of choices--The math I went through made lots of assumptions about your energy usage and power needs. Many which may be wrong.

If you do not want to spend a lot of money (budget, remote site with security issues), you may be very happy with a 1,793 Watt array and light A/C usage just on weekends.

For heavier A/C usage (and the occasional 1-2 week stay), you may wish to make up for the extra loads by running the genset more (at least for now).

Regarding generator sizing... A typical system may look like:

952 AH * 10% rate of charge ~ 95.2 Amp nominal AC battery charger @ 24 volts
952 AH * 29 volts charging * 0.10 rate of charge * 1/0.9 charger eff * 1/0.9 PF * 1/0.8 genset derating = 4,260 VA rated genset @ 10% rate of charge
952 AH * 20% rate of charge ~ 190 Amp nominal AC battery charger @ 24 volts
952 AH * 29 volts charging * 0.20 rate of charge * 1/0.9 charger eff * 1/0.9 PF * 1/0.8 genset derating = 8,521 VA rated genset @ 20% rate of charge

So, your 6.5 kWatt genset seems to be a good fit in general.

The Conext has an 90 amp DC current charger output--so you could go with a smaller 4kWatt genset--But the 6.5 kWatt would be OK too (gensets should operate at ~50% to 80% of rated power for most efficient operation (fuel efficiency).

Too large of genset can waste fuel (a standard gasoline generator running at 25% of load will use about the same amount of gallons per hour as a generator running at 50% of rated load).

The most efficient way to charge a battery bank is typically from 50% to ~80-85% state of charge, then shut down the genset and let the solar array finish the charging.

Of course, if you are charging and running the A/C or other heavy loads at the same time--You can continue to share charging+AC loads on the genset.

Inverter/Chargers can be very nice. Usually efficient charger, with the ability to program the AC input and DC charging current based on the capabilities of the Generator (many very nice options).

-Bill