Off grid Autonomy: Why not increase Solar Panels?

[raisnhell]

Hello everyone

I am a novice in off-grid and have just started learning about system sizing. So please excuse me if the question below seems too foolish.

While designing an off-grid system with autonomy, why do we only increase the battery size and not the number of panels? For a 2 day autonomy, for example, we double the batteries, but leave the number of panels the same. It doesn't make sense to me.

Please help me understand this.

Thanks!

[Naptown]

Actually both are increased.
If you want 2 day autonomy while only discharging your panels 20% you would multiply the daily load by 10.
For the panels you would need to increase their size also. By how much depends on where the system is located and how much sun it will get at the worst time of year.

[billvon]

While designing an off-grid system with autonomy, why do we only increase the battery size and not the number of panels? For a 2 day autonomy, for example, we double the batteries, but leave the number of panels the same.

Generally you increase both.

In an ideal world you could increase only the batteries, and as long as your array met your energy needs, that's all you would need to do. But you also need to have enough solar capacity to charge your bank within a day or two so that the bank does not spend a lot of time at low charge levels; that degrades batteries. So generally you have to increase solar capacity as well. This, of course, is a waste of money if you don't need the extra energy or storage, which is why planning for your energy needs is so important.

[Sunking]

Minimum autonomy is 5 days or 20% discharge on the batteries per day.

When we design an off-grid battery system we size the panels for the worse case. Most times that works out to winter months. So for Example in KCY we design for 1 Kwh of usable electricity. In KCY during December/January receives3 Sun Hours. So panel wattage if you use MPPT controller is (1 Kwh x 1.5) / 3 Sun Hours = 500 watts

OK a 500 watt panel into a MPPT controller on a 12 volt battery generates roughly 40 amps of charging current. For 5 day reserve at 12 colts takes a 400 Amp Hour battery. With me so far?

For flooded lead acid (FLA) batteries the charging current has to be between C/12 and C/8. So for a 400 AH battery 400/12 = 33 amps, and 400/8 = 50 amps. So with 40 amps of charge current we are perfect at a C/10 charge rate on a 400 Amp Hour battery. Understand so far?

No try your 2 day reserve idea and see where it falls apart. At 2 days requires a 12 volt 160 AH battery. What happens to a 160 AH FLA battery with 40 Amp or a C/4 charge rate? You get a fried battery. Solution. Real simple use a AGM battery as they can take a C/4 charge rate with no problem. But there is a catch to AGM batteries. They cost more than twice what a FLA does and only last half as long. You are better off with the recomended 5 day FLA battery. Cost the same or less and last twice as long.

No let's flip it around to a 10 day reserve and you need a 12 volt 800 AH battery. Well you got a problem now because 40 amps on a 800 amp hour battery is a C/20 charge rate and is not going to work. So not only do you double the battery cost, you now have to double down on panels and charge controllers because you now need a 1000 watt panel and 80 amp charge controller.

OK final point. Th epanels are sized to deliver a minimum of your daily requirement in th eshortest month of December. For our example was Kansas City with 3 Sun Hours. In summer those 500 watt panels with 6 hours are way overkil and sit there most of the day collecting dust.

It is all about balance of economics to meet the requirements. Best balance is 5 day reserve with a generator and AC charger for those times you are cloudy for several days or over use in winter.

Darn I get tired of explaining this 1000 times.

[inetdog]

Darn I get tired of explaining this 1000 times.

Set up a form reply into which you can fill the OP's particular numbers?
Effectively a client-side sticky for those who do not read the actual sticky threads first?

[green]

Darn I get tired of explaining this 1000 times.

What is your motivation? Just out of curiosity. You don't have to answer. It's just....I've only been here a short time and even I am growing tired of the same questions and same unrealistic expectations people have when they come here. Maybe a PM would be better, I dont mean to put you on the spot.

BTW Thank you for all you have taught me.
Green

[raisnhell]

Hi everyone, firstly I'd like to apologize for posting this tread in the wrong section of the forum. I am going to try to move the post into the correct section, but if I am unable to, then I'll ask the moderators to do that for me.

And secondly, thanks Naptown and BillVon! Here I was thinking I've lost my mind while disagreeing at the training academy. I am taking a course to understand off-grid technology. Some of the explanations made no sense. Now I have something to fight the incorrect information with.

Thanks again!



Hi Sunking

I appreciate your reply and honestly, I've tried to find a logical explanation to what was being taught to us at the training academy. It made no sense what-so-ever to just push up the storage capacity without working on the modules. As for the battery calculations, I've understood those from your previous posts, and was not confused about the battery sizing methodology. The 2 day autonomy was just an example, but I really appreciate that you've corrected some of my other assumptions. Thanks for doing that! However, I'd like to point out something. Putting in a minimum 5 day autonomy is fine for areas with low sunlight conditions during the worst part of the year. But when you are in areas like KSA, Dubai, Sri Lanka, Singapore, South India, etc., the worst time of the year still has a average useable sunlight for 4.5-5 hours. At most, there are 1 or 2 days without sunlight. In most of the areas such as these (it is where I will be doing most of my work after my training) even a 3 day autonomy is overkill.

Regardless, I would like to thank you for the time you took to explain me the concepts. Much appreciated!

[Sunking]

I'd like to point out something. Putting in a minimum 5 day autonomy is fine for areas with low sunlight conditions during the worst part of the year. But when you are in areas like KSA, Dubai, Sri Lanka, Singapore, South India, etc., the worst time of the year still has a average useable sunlight for 4.5-5 hours. At most, there are 1 or 2 days without sunlight. In most of the areas such as these (it is where I will be doing most of my work after my training) even a 3 day autonomy is overkill.

No 3-days is not enough IMHO, even for those areas. Whoever told you that is not taking economics and recharge into consideration.

To maximize battery cycle life you do not want to discharge more than 20% per day, and never ever discharge more than 50%. So with 5 day reserve capacity only gives you 2.5 cloudy days of usable capacity. Here is a good realistic example of cycle life of a good quality RE Battery.

10% DOD @ 5000 Cycles
20% DOD @ 4000 Cycles
30% DOD @ 2700 Cycles
40% DOD @ 2000 Cycles
50% DOD @ 1600 Cycles
100% DOD @ 700 Cycles

[russ]

Many of the areas listed have one or two annual monsoons when very little useful sun is available for days on end.