String Problem

Re: String Problem

Just to be clear, that is the range of a single model of panel that you can pick. But you should not mix three or more models in the 5.4 to 6.6 amp range-Because the high/low panels are more than 10% apart.

Of course, you have to do the series/parallel configuration of your array too and make sure that they meet the input limits of the charge controller (i.e., for a 150 VDC input charge controller, Vmp-array>~100 VDC gets dicey in climates with cold winters (Voc-cold of the array could exceed 150 VDC).

And if you parallel panels or strings of panels, the same 10% range rule would apply too (some folks believe that 5% range is better--But the question "is it worth limiting your choices of panels" or not?).

The closer you match, the less miss-match you have and the array will be slightly more efficient. But if you can get 10% match panels for 1/2 the price of 5% panels (to match your existing system), then limiting yourself to 5% match may not be worth it.

Also, it goes to the question of even keeping your existing panel(s)... If you can match 140 Watt panels at $280 per panel (~$2/Watt) or you can get other panels (that do not match at all, and you have throw away or sell your older panels) at $280 per 240 Watt panel (i.e., ~$1 per Watt for GT designed panels)--Then it may be more cost effective to simply start over with a new array--Especially if you already have a good MPPT charge controller of the right capacity.

Do a couple costed paper designs and see which works out best for you. Remember to include shipping to your front door (or pick up location) for your solar panels. Shipping is not cheap, especially if you are getting only a few panels at a time (vs an entire pallet load).

Check with local solar installers and see if you can get large format GT panels from them at a good price (as an example).

-Bill

Re: String Problem

You look at the same thing for all panels. I.e., mix/match Vmp/Imp for STC of all panels (not some panel PTC ratings).

However, when you look at actual power output, the PTC (or similar) ratings are usually more accurate for a typical installation (in a warm climate) for most people.

We use lots of rules of thumbs around here to give "real" numbers (not marketing numbers) for system performance. That is why we use 0.77 derating for solar panels+charge controllers and 0.52 derating for end to end system performance (as well as ~4 hours of sun as a beginning estimate--it is close enough for 80% of the people asking their initial questions).

For example:

• 1,400 Watt solar array * 0.77 panel+controller derating = 1,078 Watts average peak over the year (will only exceed a few times a year, or in sub freezing weather)

And for an off grid system:

• 1,400 Watt array * 0.52 system derating * 4 hours min sun for ~9 months of the year = 2,912 Watt*Hours typical minimum energy for ~9 months of the year (not winter)

Use a generator for poor weather/winter operations.

These rules of thumb will get you to within ~10% of an "optimum system"... and, frankly, any performance predictions are within 10% or so anyway. Between variablity in weather (year over year) and errors in measurements (voltage, current, losses), etc... It is just easier to "round" up by ~10% (if you have a choice, pick the slightly larger battery, solar panel, array, etc.).

You can "play with the numbers" if needed. AGM batteries are ~10% more efficient than flooded cell. If you use power during the day and not much at night, you do have (almost) no battery losses, etc.

At least, you will meet your minimum requirements.

I/we also do this because as the system ages, panels lose a bit of output, dust on panels, batteries become slightly less efficient, you add a few more loads to the system, etc... We want the system to still meet your requirements 8 years down the road when the battery bank is on its last legs, etc.

-Bill

Re: String Problem

Our rule of thumb is 5% to 13% of the battery bank's 20 Hour rated capacity:

10% of 135 AH = 13.5 Amp charging "nominal".

5% is the minimum we ever recommend for a deep cycle battery system. And 13% is about the maximum "cost effective" solar array for most folks. You can go higher, it just usually does not make economic sense.

Deep cycle flooded cell lead acid batteries are the cheapest. AGM are ~2x more expensive, very clean, high surge current, efficient, possibly easier to damage by over charging, and may last 1-2 years less than similar flooded cell.

GEL--Don't use these for solar. In general, they have a 5% maximum charging current. If you are outside the US (Europe), you may have access to GEL batteries that do not have the 5% maximum charging current--Those might work very well for solar.

Picking 12/24/48 volt battery bus... Start with your loads. The more kWH per day you use (and or higher wattage loads/charging), typically the higher battery voltage you need.

As a starting point, 1,200-2,000 watt AC inverter, about the maximum a 12 volt system will support. Around 2,400 to 4,000 Watt AC inverter, about the maximum a 24 volt system will support. Over ~2,400 Watts, you should really consider a 48 volt battery bank. And similar questions about battery AH capacity and size of charge controllers (i.e., 60 amp charge controller will work for 12-48 volt battery bank, but will supply 2x or 4x the 12 volt wattage on higher voltage battery banks--save money on charge controllers for larger systems).

Sometimes, you have to do a paper design of several systems and see which works best for you and the over all costs. It is not really a black/white type of answer.

The is the "short answer".

-Bill

Re: String Problem

Can you tell us a bit more about your loads? How many hours per day at what average current (i.e., 10 hours * 20 amps = 200 AH @ 24 volts).

Summer/winter, hours of sun per day?

And what is the AH capacity of your battery bank? Battery type is Flooded Cell, AGM, ?

And how big of solar array do you plan/want to install?

-Bill