System set up for a weekend cabin.

[Blantons Bluff]

I know I’m working a little backwards here but I acquired a few parts to build a solar system and hoping I could get advise on the missing pieces that I need. I have 2- 245 amp AGM intimidator batteries that are 2 years old each both in good shape and a 1500 watt inverter. i am only looking to start with using under 5000 watt hours one day a week. Could I make out ok with 2-200 watt mono panels and a 40amp Mppt charge controller? I will have an average of 4 hour of good sun a day. Again this cabin is only being used max 1 day a week.

 

[OzSolar]

Welcome!

I have 2- 245 amp AGM intimidator batteries

Let's do some rough math. 12 volt system, right? 2 x 245 AH x 12V = ~5.5kWh (5500 watt hours) of storage but do to a lot factors you'll likely only be able to regularly get ~3kWH out of them.

Could I make out ok with 2-200 watt mono panels and a 40amp Mppt charge controller

400 watts/12v = 33 amp so your CC is big enough

400 watts x 4 hrs per day x 6 days that you're not there = 9.5 kWH of generation

If you can remember to turn that inverter off when you're not there it looks like you can make it.

 

[svetz]

Welcome to the forums!

DoD With Lead acid batteries the first question is what's an acceptable depth of discharge (DoD)? The usual answer for prolonged life is 50%, but... since you're only using them for one day a week let's say you opt for 100% discharge and expect 200 cycles out of them, that's 200 cycles or about 4 years at one cycle per week. At 100% DoD, that's 245 Ah x 12 volts x 2 batteries x 90% inverter efficiency = 5 kWh. So, just enough for your needs if your batteries are still in pristine condition! Cold weather will reduce the capacity of lead acid batteries, so it also depends on when you plan to use it.

...Could I make out ok with 2-200 watt mono panels ... I will have an average of 4 hour of good sun a day.

The 4 hr of good sun per day is always questionable. It's crazy high for Winter and probably about right for summer. You can look up your seasonal insolation from charts like this one, or use a tool like PVWatts.

But, let's say you're only using it from late spring to early fall and the insolation really is 4. You get to charge for 6 days. First, you need to charge up the 5 kWh you consumed plus efficiency losses in both the MPPT and the battery, let's say that's 80%; then 5000 Wh / .8 = 6250 Wh needed.

You have 6 days, so 6250 / 6 = 1050 wh/d is needed and with an insolation of 4, then 1050 / 4 = 260 Watts of solar panels.
So yes, two 200 watt panels should accommodate you.

Hope that helps! You can find more in the FAQ section, some topics are:

• How much power does my device consume?
• Energy Audit
• How much Solar Energy should I expect where I live?
• What does it mean to have solar panels in parallel and series?
• How to figure out how much Solar and inverter wattage is needed to charge a battery
• Figuring out how many panels in series and parallel based on your MPPT
• What wire gauge should I use? (video, calculator)
• Comparing Solar Panels

 

[Checkthisout]

No. You need more battery and panel. 5000 wh in a day is a lot.

Like 2 more of those batteries and at least 4 200 watt panels.

I would just get something you can expand. Start out with what you have. Just make sure you get a charge controller and panel types that make it easy to add more.

 

[Blantons Bluff]

Thank you for all the comments. it Sounds like I need to reconsider how much power I plan to use and get it down to 2500 watts, shut the inverter off when I leave and consider a 500 watt panel system . will a 40 amp charger still work.
2nd question... is battery life effected when it takes more than 1 day to fully charge back up. Should this be considered in my panel purchase.

 

[Substrate]

2nd question... is battery life effected when it takes more than 1 day to fully charge back up. Should this be considered in my panel purchase.

As a weekend warrior, taking more than 1 day to *fully* charge up is not a big deal. Daily-cyclic is different. You're good.

Even though you say your batteries are good, I'd still recommend you use that 40A charger to charge each one up individually before doing anything with them in your permanent setup.

 

[Checkthisout]

Thank you for all the comments. it Sounds like I need to reconsider how much power I plan to use and get it down to 2500 watts, shut the inverter off when I leave and consider a 500 watt panel system . will a 40 amp charger still work.
2nd question... is battery life effected when it takes more than 1 day to fully charge back up. Should this be considered in my panel purchase.

With the temperatures you'll be operating at and the fact that managed/monitored operation will be intermittent, just prepare yourself that your batteries will probably last 3 years tops.

Anything "nice" you put I there might get stolen or vandalized.

Intermittent use cabins aren't cheap to own, even if connected to the grid.

 

[Checkthisout]

As a weekend warrior, taking more than 1 day to *fully* charge up is not a big deal. Daily-cyclic is different. You're good.

Even though you say your batteries are good, I'd still recommend you use that 40A charger to charge each one up individually before doing anything with them in your permanent setup.

Agreed. Charging them up as quickly as possible will help stir them up which they will crave with intermittent use.

 

[Substrate]

Intimidators are agm's that don't need to be stirred.

While charging them up as quickly as possible (up to 0.3C max charge current), is ideal in case insolation is weak, this is a "weekend warrior" application where you have 5 days of solar to catch up.

 

[Checkthisout]

Intimidators are agm's that don't need to be stirred.

While charging them up as quickly as possible (up to 0.3C max charge current), is ideal in case insolation is weak, this is a "weekend warrior" application where you have 5 days of solar to catch up.

Right. I think I mixed this up with another thread.

 

[Substrate]

That's cool - I do that all the time!

@Blantons Bluff - IMPORTANT! Those Deka/East Penn Intimidators should never be taken above 14.4v CV, unlike other agm ratings that go to 14.6v or higher. Deka's are a bit special. Unless your 245ah models allow for this, I'm pretty sure 14.4v should not be exceeded.

BZZT - I'm wrong again. I forgot about the Deka VRLA manual here: Page 12 shows that 14.4 - 14.6v ok.

https://www.dekabatteries.com/assets/base/0139.pdf

 

[Blantons Bluff]

Thanks for the info! i gather from all the comments that I’d be better off adjusting my wattage usage down to 2500 watts a day. Bumping the panels up to 600 watts and a 40 amp charger. I don’t want to bring bring the batteries below 50%.
2nd question- am I damaging the batteries by taking longer than a day to charge back up to 100%

 

[MichaelK]

Thanks for the info! i gather from all the comments that I’d be better off adjusting my wattage usage down to 2500 watts a day. Bumping the panels up to 600 watts and a 40 amp charger. I don’t want to bring bring the batteries below 50%.
2nd question- am I damaging the batteries by taking longer than a day to charge back up to 100%

Nobody has asked how you came up with the 5000Wh number? Can you itemize what it is you think that is? Air conditioning, refrigerator? At my own cabin, I'm finding I use ~3.5kWh on a daily basis, and that's with the refrigerator running, the lights bright, and the TV on. So, your 2500Wh number appears a bit more realistic.

I do the math a bit differently. Assuming you stick with the 40A controller, and you use a fudgefactor (85%) like I normally recommend, you could deploy (40A X 13V charging)/85% = 612W of panels, so I think the choice of going up to 600 is a good one. I would recommend going online to your local Craigslist venue and see what large grid-tie panels are on sale there. With cash and carry pickup, I'd expect myself to find panels in the 300W range for 80-90$ or so. Recently, I bought 260W REC panels for 65$ each.

I've never seen anyone here complain they've got too many watts of panels. Too many is not as bad as too few. You can really over-panel a lot if you orient the panels in different directions, say one SE, one South, and one SW. You'll get more watts per day without going over your amp limit.

 

[Blantons Bluff]

Great! Photon Sorcerer, much appreciate the advise. My 5000 watt # was the worst case senecio # with a couple more to spare. I’m sure I can getaway with less, it is a off the grid cabin and the whole idea is to get away from using electronics Anyway. I’ll have to haul water into my place so My biggest draw on power will be pumping 300 gallons of water From one container to another every once in a while. But if it’s an issue, I might go with gas pump instead of electric. I’ll look for larger watt panels. I did see a 600 watt panel, 40amp controller kit for 699 on amazon today but if search around, I’m sure I can do better. I’ll keep checking Craig’s list and market place.

 

[12VoltInstalls]

You can really over-panel a lot if you orient the panels in different directions, say one SE, one South, and one SW. You'll get more watts per day without going over your amp limit.

I think multi-directional is an ideal setup. Depending on region you may not harvest more watts daily. Nevertheless, the big benefit I experienced was maintaining full charge much later in the day, and usable production began about an hour and a half earlier.

This worked so well for me with too small of a system that it completely changed my direction in purchasing an upgrade. The focus became improving harvest for low-production days rather than buy more panels AND completely revamping my batteries. Saved me almost $2000 I’m guessing.

In the FWIW department: I was surprised to notice I can get as much as 200W on clear, sunny days from the ‘morning’ 400W array long after it no longer is getting any sun exposure- just atmospheric reflection.