tabbycat wrote: »
Is it fair to say that using a charging voltage of 14.4 volts in determining the size of your solar array will compensate for battery losses? If you used 12 volts the array calculation would be smaller. Thanks.
lkruper wrote: »
If I understand you, you are referring to two different things. If you refer, first, to calculating the amps for an MPPT controller by dividing the voltage of the panel by 14.4 and multiplying by the amps of the panel, then you will probably get a more accurate idea of the real amps from your system. However that difference will likely be well within any fudge factor that might be used (typically 1.5 for MPPT and 2 for PWM).
Secondly, if you refer to the fact that you must put in more % into your battery than you get out, then there is nothing wrong with taking that into consideration. Don't recall off the top of my head, but you might need to put in 110% of what was used to get a full charge but only get 100% out.
tabbycat wrote: »
The calculation from BB is bank AH's * Charging voltage / 0.77 (panel & controller derate) * charging rate = solar array req'd in watts. There is no allowance for bank charging efficiency. Thanks
inetdog wrote: »
A much more common number for an MPPT CC is 95% efficiency (1.052 fudge factor). The panel efficiency relative to Standard Temperature and Conditions (STC) rating is handled separately.
It is true that the panel voltage loss with temperature will affect the output of an MPPT CC but will not (up to a point) affect the output of a PWM CC.
lkruper wrote: »
So, would you say that SK's 1.5 fudge factor is overly conservative and recommend using 1.052 instead?
BB. wrote: »
I generally try to answer specific questions about folk's systems rather than a post on how to size your system... The actual sizing of a system is really a "turn the crank" set of calculations--It is the questions about loads, needs, conservation, alternatives to electric power, etc...
I try to be careful and run the same calculations (fudge factors) in specific order and based on specific system needs (DC vs AC inverter, flooded cell vs AGM, etc., where the system will be installed, local weather/solar insolation, availability of others fuels, etc.).
But, I can run a quick set of calculations to show the basics. The first pass calculations work for almost any size system (small or large). The second pass equipment selection, etc. is when the differences between hardware/choices become important. Note, I am using the "standard" deratings (flooded cell battery, AC inverter, etc.). You can modify to your needs, or ask for suggestion on tweaking for you application.
A 3.3 kWH per day system: Refrigerator, well pump, clothes washer, Laptop+Internet, LED lighting, cell phone charger, TV... As close to "normal" electrical living with off grid solar (and lots of conservation--Using wood/propane/solar thermal/etc. for heating/hot water/etc.)--Without spending a huge amount of money. Near Atlanta Georgia.
First, sizing the battery bank... 1-3 days of storage, 50% maximum discharge. 2 days of storage (no sun), and 50% max discharge (for longer lead acid battery life) seems to be a good system for most people.
3,300 WH per day * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 maximum discharge = 647 AH @ 24 volt battery bank
Why 24 volts? I have done this calculation a bunch and know that 24 volts is the minimum bank voltage I would suggest. The detailed answer is that when a battery bank is over ~800 AH in capacity, for various reasons, it is better to got to the next voltage level.
For example, the above for 12 volts would give a 1,297 AH @ 12 volt battery bank--Usually too large of AH capacity for our needs. Very heavy copper wiring, several 80 Amp Solar charge controllers, problems with high current flow (inverters, charge controllers) and voltage drop... So, 24 volt battery bank (or possibly even 48 volt battery bank) is a better fit.
Next, sizing the solar array... Two sets of calculations. First set is based on Battery Bank capacity. A large lead acid battery bank needs a large charging source to properly recharge the batteries (for long life). Typically we use 5% to 13% rate of charge (based on battery's 20 Hour capacity numbers). 5% can work for a weekend/seasonal cabin. 10% or more for full time off grid cabin/home (9 months or more per year) suggested.
Solar panels are as cheap, and batteries are expensive. Installing more solar panels is usually a good idea (healthy batteries, don't have to watch the system as closely, less generator run time/fuel costs).
647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,218 Watt array minimum
647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,437 Watt array nominal
647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,168 Watt array "cost effective" maximum
More "fudge" factors. Notice that I use the battery charging voltage of 29 volts. And a panel+controller derating of 0.77 (77% efficiency). AGM batteries charge at 27.8 volts or so.
And notice that I do not differentiate between PWM and MPPT type charge controllers--For our discussion here, there is not enough difference between the two types of controllers (over all system efficiency) to worry about... They have quite different calculations for efficiency and such. But in the end, for a properly designed system in a moderate to warm climate--Not really any difference in overall harvest (for our needs, they are within ~10% of each other's overall harvest).
There are reasons to pick MPPT over PWM in many cases (larger systems, cost issues, design decisions, etc.)--But leave that for another post.
Next, need to size the array based on the amount of sun. PV Watts 1.0 (old version) was just taken down in the last few days, so will use another data source for now (I like simple/accurate data that does not through in more "fudge factors"--Which can make things more confusing).
For a fixed array, tilted to best year round performance, in Atlanta Georgia (note, the SolarElectricHandbook seems to have mixed up Georgia USA with the Republic of Georgia--Hopefully the data for Atlanta is OK--Would need to verify with another source for "real" calculation): [h=3]Atlanta
Average Solar Insolation figures[/h] Measured in kWh/m2/day onto a solar panel set at a 56° angle (from vertical):
(For best year-round performance)
Typically, I toss the bottom three months (winter) and assume that folks will run a genset for bad weather--Of course, you can make other decisions. Here, February (and November) with 3.76 hours of sun would be the "break even" month for genset usage (some years you need the genset, others you may not).
3,300 WH per day * 1/0.52 end to end off grid system eff for AC inverter+Flooded cell battery * 1/3.76 Hours of sun = 1,688 Watt array "minimum" based on loads+sun
So between the 1,688 Watt array needed based on loads and hours of sun, and the numbers based on battery bank capacity of 1,218/2,437/3,168 Watt array--For a full time off grid system I would suggest ~2,473 Watt array... And such an array would generate on an average February/November day:
2,437 Watt array * 0.52 off grid system eff. * 3.76 hours of sun = 4,765 Watt*Hours per day
So--That gives you some extra power for optional loads during sunny weather--Plus some extra power for cloudy days/when guests come by, etc.
We now have the basics--Battery Bank sizing and Array sizing... And now can look at specific hardware to support those power levels.