Solar system design questions

Welcome ot the forum Vdub411.

Ok--Understand you have lots of material--But until you have figured out how much load you have, we really cannot do much yet.

The only other option is to design the system around a major component (such as the solar panels you already have) and then estimate how much power would be available from such a system.

And, the other "warning", Off Grid Solar power is expensive. If you have Grid Power available, off grid solar power will never save you money. In some cases, such as for folks that have very unstable utility power (long outages after ice storms, afternoon power failures, etc.), off grid solar power can make sense as a backup.

Note that some items, like your RV refrigerator, are not really even good candidates for solar (or even grid tied power) for long term use. The RV fridge is probably ammonia/propane based unit. While running on propane, they are not bad (perhaps a 1/2 lb of propane per day). However, on 12 VDC or 120 VAC power, they are very inefficient--They just use the electricity to provide heat instead of a propane flame (using solar electric power to provide heat is usually inefficient).

At this point, I do not understand enough about your needs to even begin to design a system. If you want to power some LED lights, charge a cell phone, run a laptop, charge some battery powered tools... We could make a guess and estimate your needs at ~1 to 3.3 kWH per day.

A 1,000 Watt*Hour (1 kWH) per day system is pretty small and can be a good system to experiment with. A 3.3 kWH per day system, can be enough to run a full, very efficient, off grid cabin/home (lights, fridge, well pump, washing machine, laptop computer, etc.) and have a "near normal" electrical life. But a 3.3 kWH per day system is not small, and not cheap.

Getting batteries for "free" can be interesting--But that gets us into questions about exactly what kind of batteries you have. Telecom batteries are typically designed for "float" operation and may not sustain daily deep cycle operations. And "small batteries" (such as your 115 AH @ 12 volt AGM batteries), are not a great solution for larger/deep cycling operation (lots of parallel battery strings, and especially AGM types, can make for a "unstable' battery bank--Difficulties with equal current flow between strings, lots of electrical connections, finding the occasional cell that goes back--shorted or open, etc.).

And your solar panels are 60 Cell Vmp~30 volt panels... They are just fine, however, they do not "map well" to use a PWM (simple, inexpensive) charge controller to charge a 12 volt battery bank (really need Vmp~18 volts) or a 24 volt battery bank (Vmp~36 volts). So, you should use a MPPT type charge controller... These are very nice controllers, but they are not cheap (~$100 for PWM, ~$600 for MPPT soalr charge controller--Although, you can find less expensive import MPPT controllers of mixed quality).

Just to give you an idea of what your 4x 250 Watt solar panels can do for you. In general, I would suggest a 5% to 13% rate of charge for your system... Weekend/seasonal use, you can use 5% rate of charge. For full time off grid daily use (9+ months a year), I would suggest 10%+ rate of charge.

• 1,000 Watt array * 0.77 panel+controller derating * 1/0.05 rate of charge * 1/29.0 volts charging = 531 AH @ 24 volts maximum
  
• 1,000 Watt array * 0.77 panel+controller derating * 1/0.10 rate of charge * 1/29.0 volts charging = 266 AH @ 24 volts nominal
  
• 1,000 Watt array * 0.77 panel+controller derating * 1/0.13 rate of charge * 1/29.0 volts charging = 204 AH @ 24 volts minimum
  

Lets say you go for 2x12 volt batteries in series, and 3x parallel strings for 345 AH @ 24 volt battery bank (I would recommend avoiding lots of parallel battery strings--1x strings is "ideal", and 3x parallel strings maximum (unless you have no other options)).

A 345 AH battery bank, operated for 2 days of storage and 50% maximum discharge would give you around:

• 345 AH * 24 volts * 1/2 days storage * 0.50 maximum discharge * 0.85 AC inverter eff = 1,760 WH per day (or over night) use

Note, that a 2,000 Watt AC inverter would run for less than 1 hour per day if you followed our rules of thumbs.

And normally for a 345 AH @ 24 volt battery bank, I would be suggesting a typical maximum of wattage of Solar Array and AC inverter of (345 AH / 200 AH per 1kW=) ~1.73kW or 1,730 Watt array and AC inverter typical maximum. However, for AGM Batteries, they support much higher surge/continuous loads than flooded cell lead acid batteries, so, in theory, you could support your 2kWatt (or even larger) AC inverter).

However, you bank would still not support a 2kWatt inverter very long--Taking a battery dead (which ruins lead Acid/AGM batteries):

• 345 AH * 24 volts = 8,280 Watt*Hours of storage (until dead)
  
• 8,280 Watt*Hours * 0.85 inverter eff * 1/2,000 Watt inverter-load = 3.5 hours maximum operation (or less)

For various reasons, recommend that you use only 1/4 to 1/2 of your stored battery power per day... And avoid taking your battery bank below 50% state of charge very often, and never below 20% State of Charge (deep cycles will "use up" a lead acid battery pretty quickly).

And then, how much energy will a 1,000 Watt array produce... Lets guess you are near Olympia Washington, using a fixed tilt 1,000 Watt array:
http://solarelectricityhandbook.com/solar-irradiance.html

Olympia
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 43° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
1.68	2.79	3.52	4.15	4.55	4.76
Jul	Aug	Sep	Oct	Nov	Dec
5.21	5.18	4.68	3.13	1.95	1.51

Toss the bottom three months (use genset, cut back on energy usage):

• 1,000 Watt array * 0.52 system eff * 2.79 hours of sun (Feb) = 1,451 Watt*Hours per day of 120/240 VAC power (February "break even" month).
  

The above is what we call a "balanced" system design. And close to optimum for most people. If you have different needs (say use the system on weekends during summer days--We can do something different like a larger solar array). Or if you have "free" batteries, perhaps you want to discharge them to 50% state of charge every day--But then you would need a larger solar array to get them recharged by the end of the next day, etc...

However, for a garage where you plan on using power tools/larger amounts of power (air compressor, etc.), I would highly suggest you think about reconnecting to the grid for the property, or bring power from your own home next door.

If your power needs are random (a few weekends a month) a genset (plus gasoline powered air compressor, etc.) may be a more cost effective solution--And, if you want, use a 1,000 Watt*Hour per day system for "quiet power" and experimenting with solar (lights, laptop, radio, small tool charger).

Your thoughts?

-Bill