How Many Watts Do You Leave On The Table? (Small Systems)

I had a heated discussion with a guy planning a solar system about how much, and WHEN he was going to use power,
Should he put more money in panels/batteries because he used the power at night, or panels/charge controllers because he planned to use power in the daytime when panels were producing...
Charge Controllers being considerably less expensive than batteries or panels.

The same subject came up in a thread here yesterday (MUCH more polite!) and today, so I want to know what others think and are doing...
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With REALLY SMALL 12 Volt systems, the types used in Vans, Trailers converted to campers, maybe a cabin,
Roof space seems to be a limiting factor,
4Ea. 100 Watt Panels, 30 Amp Charge Controller and 2 Batteries seems to be real common, so I'll run with that as an example.



Example, Before System Losses,

5 Peak Sun Hours,
4Ea. 100 Watt Panels, 400 Watts/Hour, 12 Volts, 33.33 Amps/Hour.
2 Ea. 12 Volt, 100 Amp Hour Batteries, 200 Amp Hours.

Batteries Discharged 50% or 1,000 Amp Hours.

1,000 Watts @ 12 Volts = 83.33 Amps
30 Amps/Hour x 12 Volts = 360 Watts/Hour From Charge Controller,
360 Watts/Hour x 2.78 Hours = 1,000 Watts/1kWh
2.78 Hours To Recharge Batteries

5 Peak Sun Hours (or more) Per Day, Minus 2.78 To Recharge Batteries,
Leaves 2.22 Sun Hours @ 360 Watts An Hour, 799 Watts Left.
799 Watts @ 110 Volts Left On The Table.

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Example, Before System Losses:

5 Peak Sun Hours,
200 Watt Panels, 800 Watts/Hour, 12 Volts, 66.66 Amps/Hour.
4 Ea. 12 Volt, 100 Amp Hour Batteries, 400 Amp Hours.

Batteries Discharged 50%, 2,000 Amp Hours (2kWh)

2,000 Watts @ 12 Volts = 166.66 Amps
60 Amps/Hour x 12 Volts = 720 Watts/Hour From Charge Controllers,
720 Watts/Hour x 2.78 Hours = 2001 Watts (200 Amp Hours/2kWh)
2.78 Hours To Recharge Batteries

5 Peak Sun Hours (or more) Per Day Minus 2.78 To Recharge Batteries
Leaves 2.22 Hours @ 720 Watts An Hour, 1,598 Watts Left,
1,598 Watts At 110 Volts From Inverter Left On The Table.

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Example, Before System Losses:

5 Peak Sun Hours,
200 Watt Panels, 800 Watts/Hour, 24 Volts, 33.33 Amps/Hour.
4 Ea. 12 Volt, 100 Amp Hour Batteries, 400 Amp Hours Total.

Batteries 50% Discharged, 200 Amp Hours (2kWh, 2,000 Watts)

2,000 Watts @ 24 Volts = 83.33 Amps
33.33 Amps/Hour x 24 Volts = 799.92 Watts/Hour (200 Amp Hours/2kWh)
2.50 Hours To Recharge Batteries

5 Peak Sun Hours (or more) Per Day, Minus 2.5 To Recharge Batteries,
Leaves 2.5 Hours @ 799 Watts An Hour, 2,000 Watts Left,
2,000 Watts (or more) @ 110 Volts From Inverter Left On The Table That Does NOT Draw From Batteries.

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2 Charge Controllers work best when they find the power point faster, and dump Amps into the Load End of the system (Batteries & Inverter) so the dirt cheap Charge Controllers aren't the best option.

With low internal resistance batteries, Like LiFePo4, the second charger will recharge batteries MUCH faster.
We all like seeing that 'Float', 'Charged' or ' ' Icon, and after that it's all excess Watts the panels are producing.

I set timers past the point where my batteries usually charge fully, Kicking on something I want done, using the excess production.
Everything from the crock pot to the washing machine.

@JeepHammer you are nailing my agenda for justifying the expense I put into solar that I am unlikely to ever get back in electricity savings. I want to develop Internet of Things (IoT) that can optimize our solar system based on goals.

The biggest throttle on optimization is the SoC when PV input begins for the day, as well as how much you allocate for charging batteries. The one assumption will be that you do not want to waste any PV.

If you have utility power, you have more room to wiggle, and your goals may be different than 100% off-grid. For instance, someone on the grid will care about UPS, which is impacted by the lowest SoC in a normal daily cycle.

While I'll be focused on optimizing what one already has, I could use the same data with additional data like what you are looking at to recommend optimal ways to expand. Cost is a huge variable that differs for each person. It is not an automatic no brainer that adding panels is cheap when there are other costs to adding panels. Sadly, adding batteries has zero payback period unless you are literally throwing PV capacity away, so are the most painful type of cost. There are a lot of intangibles that will differ from each person as well, such as whether or not they are willing to vary their consumption schedule or how critical certain loads are.

That is why what I develop will be geared towards the goals of the individual, whatever those goals may be. It will simply present the data, and offer configuration to ensure 100% utilization of PV while also maximizing SoC of the batteries.

So, if the system anticipated 6 KWH of PV production over 5 hours, and load during that time would be 2 KWH, it would try to plan to use 4 KWH to charge the batteries.

Once this data has been collected for a user's setup for awhile, it could allow the user to do what if projections. What if they added more batteries to the bank? What if they added more panels? What if they added a new load? It could identify "watts left on the table" or an inability to fill batteries.

Approx 12kWh this time of year, but I don't leave it on the table because it'd cause it to catch fire.

My 2nd hand 1.9kWh array produces more far power than I can find a use for. That figure will drop as the days shorten but even then I suspect I'll be well into surplus land at winter solstice. About the only thing I could do with the excess is throw it at water heating but that's only going to soak up 2kWh in winter to replace use and thermal loss based on past observations. Anything that can be shifted to happen in the day has been.

I could burn more with an airconditioner but I prefer open windows and it is rarely so hot here that I want to turn it on during the day anyway. Overnight it uses about 0.6 kWh at present.

The test will be how things go when the sun doesn't make an appearance for over a week in the wet season but based on production during recent heavy cloud that's likely to not be a problem either.

Well, it's discounting ALL losses, which can be considerable...

I'm embarrassed to say how long it took me to figure out how much I was leaving on the table/wasting.
'Over Paneled' is fine when you have clouds, you just don't crank up the extras.

When people first crank up, I usually recommend getting as much panel, both quality & production as they can afford.
Extra batteries & charge controllers can come later.
I also have issues getting the higher voltage, 24 instead of 12, makes the system more efficient in the long run.

I GENERALLY don't recommend past 24 Volts for small systems, simply because of batteries and charge balance issues.
I'm still learning about LiFePo4s, but they seem to work really fine in 24 Volt strings so far, and I'm coming up on 2 years with them in a week.
The 48 Volt string is about 8 months old, and as far as I can see, it's doing well also.

For equipment, usually higher voltage is more efficient, but I'm sure I don't know everything I should about that either.

Since I have so few charge controllers, some homemade, I can't say what works best or what will work in tandem,
I know the big $$$ units will work in tandem/interconnect, but you pay for it, and they aren't the most efficient, designed to work with high volts and reasonably low amps.

I wouldn't tell people what they were supposed to do if I could, but I can demonstrate the differences on paper to them, and maybe they will become curious and ask questions, maybe have folks suggest things that will work better for them, their situation.

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Gnubie, if you call 12kWh 'Small', I'm thinking you are from Texas rather than Australia!

You could drop Texas into QLD and lose it

To be fair when I bought the panels I wasn't sure how they were going to go. They are 7 years old. I did the usual visual inspection and ran a high voltage test on them for leakage and they all came out OK but there are a few baked cells so I thought I might end up with a 6 being good, turns out they are all good.

*edit for more info*
The original intent was to add the working panels + another MPPT controller to provide additional charging to the existing battery.

JeepHammer said:

I had a heated discussion with a guy planning a solar system about how much, and WHEN he was going to use power,
Should he put more money in panels/batteries because he used the power at night, or panels/charge controllers because he planned to use power in the daytime when panels were producing...
Charge Controllers being considerably less expensive than batteries or panels.

The same subject came up in a thread here yesterday (MUCH more polite!) and today, so I want to know what others think and are doing...
---------------------------

With REALLY SMALL 12 Volt systems, the types used in Vans, Trailers converted to campers, maybe a cabin,
Roof space seems to be a limiting factor,
4Ea. 100 Watt Panels, 30 Amp Charge Controller and 2 Batteries seems to be real common, so I'll run with that as an example.

View attachment 4210

Example, Before System Losses,

5 Peak Sun Hours,
4Ea. 100 Watt Panels, 400 Watts/Hour, 12 Volts, 33.33 Amps/Hour.
2 Ea. 12 Volt, 100 Amp Hour Batteries, 200 Amp Hours.

Batteries Discharged 50% or 1,000 Amp Hours.

1,000 Watts @ 12 Volts = 83.33 Amps
30 Amps/Hour x 12 Volts = 360 Watts/Hour From Charge Controller,
360 Watts/Hour x 2.78 Hours = 1,000 Watts/1kWh
2.78 Hours To Recharge Batteries

5 Peak Sun Hours (or more) Per Day, Minus 2.78 To Recharge Batteries,
Leaves 2.22 Sun Hours @ 360 Watts An Hour, 799 Watts Left.
799 Watts @ 110 Volts Left On The Table.

Click to expand...

My system will be like the first diagram you designed only with a starting battery added, so I will have a lot more left on the table I suppose. (That is, If I am driving that day, right?) ( I am using a slightly modified version of Will's blueprint: https://www.mobile-solarpower.com/simplified-400-watt-fewer-wires-and-alternator-charging.html)

How did you come up with 110 volts? Still learning...

I didn't know it only took 2.78 hours to re-charge batteries. But what if it's raining/cloudy? Aren't you supposed to figure in 2 to 5 days of autonomy (no sun)?

FWIW, since panels are so cheap I've sized the array for crappiest time of year to meet our base load while we're in "conservation/bare essentials mode" and the grid is down as I dont particularly want to be sitting in the dark in the middle of winter and there is some medical machinery that absolutely has to have power before anything else.

It means we'll be horribly overpaneled for 3/4th of the year even though the panels will be tiled 50 degrees to optimize for winter solar angle. It's also why the batt bank is on the large-ish size to soak up some of the overproduction and give us 3-5 days of autonomy. Being this overpaneled was a big push to go grid-tie just to get some benefit from it.

Santa said:

My system will be like the first diagram you designed only with a starting battery added, so I will have a lot more left on the table I suppose. (That is, If I am driving during daytime that day, right?) ( I am using a slightly modified version of Will's blueprint: https://www.mobile-solarpower.com/simplified-400-watt-fewer-wires-and-alternator-charging.html)

How did you come up with 110 volts? Still learning...

I didn't know it only took 2.78 hours to re-charge batteries. But what if it's raining/cloudy? Aren't you supposed to figure in 2 to 5 days of autonomy (no sun)?

Click to expand...

All ideal/hypothetical, perfect sun, no losses, etc.
110 volts from inverter, there is one in every diagram. Use the extra as 12 or 24 volts if you want to.
No sun is batteries/backup power. No amount of panels/charge controllers can make the sun shine.
I even tried the Indian Sun Dance in the rain swinging a charger over my head, that didn't work either

About how fast you can recharge LiFePo4s,
I'm CAUTIOUSLY creeping the amperage up on LiFePo4 batteries, they will suck up amps FAST.
I've killed so may lead/acids I'm gun shy, so I'm sneaking up on how fast you can dump amps into them, but 'Dump' is the correct term, they REALLY take amps fast!

I'm watching my charge rate,
BATTERY LIVES MATTER!

Maast said:

FWIW, since panels are so cheap I've sized the array for crappiest time of year to meet our base load while we're in "conservation/bare essentials mode" and the grid is down as I dont particularly want to be sitting in the dark in the middle of winter and there is some medical machinery that absolutely has to have power before anything else.

It means we'll be horribly overpaneled for 3/4th of the year even though the panels will be tiled 50 degrees to optimize for winter solar angle. It's also why the batt bank is on the large-ish size to soak up some of the overproduction and give us 3-5 days of autonomy. Being this overpaneled was a big push to go grid-tie just to get some benefit from it.

Click to expand...

That's why I have a big freakin' alternator with voltage regulator, something that can REALLY throw amps for a generator instead of a 110volt generator.
No sense in going 110vac, then back down to 12, 24, 48vdc.

Everyone does thing differently, and I'm generally as different as they come!

JeepHammer said:

That's why I have a big freakin' alternator with voltage regulator, something that can REALLY throw amps for a generator instead of a 110volt generator.
No sense in going 110vac, then back down to 12, 24, 48vdc.
Everyone does thing differently, and I'm generally as different as they come!

Click to expand...

We've got a contractor grade diesel genset for now and plan to eventually replace that with a MEP-902A but eventually fuel runs out. After the PV system build I've got a few ideas on thermoelectric generation using the wood stove.

I actually couldnt go as big as I wanted so I have a slight shortfall, I ran into regulatory limits of becoming a "commercial producer" and I got close to running out of roof space anyway. Pesky dang roof vent pipes...

Well Winter Solstice really sucks ! Sun is up @ 09:00 and down by 16:00 and I don't see real power generation till at least 10:30 - 11:00 and it only lasts till 14:00 IF lucky... now I do have a fixed mount @ 45 degrees and only 2kw of panel (4s2p) feeding my Classic-200 which can output 79A. Twice a year I see amazingly good production (March & September) and surprisingly I ended up giving away my 12,000 BTU AC unit because I haven't needed it (thanks t home design it never get's over heated in here). Right now I am having to use the genset to charge the FLA's (damned winter & ice storms (happening now) which seriously rubs me wrong on many levels. Of course the FLA's take a heap more juice to charge up and stay up than LFP does, so as you can imagine I am extremely anxious to get that cursed ShunBin pack {24v/350AH) online (final parts delayed till Jan.02 )

Leaving watts on the table ? ! There is an irony If ever there was one... When the sun is good my system can recharge darn fast and then hit float quick, in summer time I was seeing float by 11am (Mar & Sep by 10am !) When deeply discharged I would still see float by end of day on a good clear day. Now I am faced with how to get more juice in winter, do I add 4 more panels at 28 Deg. which can be shutoff in peak production time (I'm already at SCC limits @ max production time) or add a linked Midnite Classic SCC ($900) and a set of 4 330w panels ($1000), do I try to build an adjustable rack for 8, 60 Cell panels (would be tough in my situation)… Hmmmm Maybe run a lower level MPPT Controller through the Inverter's Solar Charge Controller port (max 50A through it) setup for winter use ? Well I have time to ponder it as the budget has no room for much ATM because of the ShunBin fracas and I am not building a damned thing outside in below zero temps (just too old, broken & crusty for that nonsense).

I am supposed to get a fairly new (3 yrs old) Onan LPG fueled genset from an RV to use as my backup genset to use instead of the current Champion 7200/9000 gasser which has a miserable carburetor that must be choked to start (cannot auto-start it & and I have not been able to get an automatic choke carb for it that would allow for that). Now that would cost me $1,000 so cheaper than adding compensatory SCC & Panels but then it eats fuel and makes pollution (not a part of my plans, in any way).

Sorry I wandered off a bit.
Steve

You must be a fair way north. I'm at 21 south so always plenty of sun here even in winter.

gnubie said:

You must be a fair way north. I'm at 21 south so always plenty of sun here even in winter.

Click to expand...

Look up "Algonquin Park" on Google maps. North it is. But not as North as to where I used to work for a while which was at the top of Frobisher Bay in Iqaluit. You know, it can hit -50C without windchill up there ! Wanna see a bottle of Vodka freeze in < 2 minutes, stick it outside on one of those days.

JeepHammer said:

I even tried the Indian Sun Dance in the rain swinging a charger over my head, that didn't work either

Click to expand...

You have to do it naked in a full moon with a bottle of whisky in the other hand.

How come no one considers turning computers on to absorb excess power? My computers account for over half my electric bill. I have plenty I can turn on if needed that are off nearly 100% of the time.

One of the businesses I'm developing will allow you to generate revenue from excess computing capacity. In this realm, you sell metered services online. E.g., you can sell number crunching services for AI.

I see I'm not the only one with 'Issues'!

From suicidal ground squirrels to ice/hail storms, when you own it it's all on you...

I did like everyone else, increased panel count until I could charge stubborn batteries,
Now the batteries aren't nearly as stubborn, and the panels are getting better...

SOMEONE KEEP ME FROM TELLING A, "IN MY DAY" STORY!

erik.calco said:

How come no one considers turning computers on to absorb excess power? My computers account for over half my electric bill. I have plenty I can turn on if needed that are off nearly 100% of the time.

Click to expand...

I already run my desktop most of the day.

JeepHammer said:

I see I'm not the only one with 'Issues'!

From suicidal ground squirrels to ice/hail storms, when you own it it's all on you...

I did like everyone else, increased panel count until I could charge stubborn batteries,
Now the batteries aren't nearly as stubborn, and the panels are getting better...

SOMEONE KEEP ME FROM TELLING A, "IN MY DAY" STORY!

Click to expand...

"suicidal ground squirrels" = do I want to hear?? Yes, we want a story about back in our day...

erik.calco said:

How come no one considers turning computers on to absorb excess power? My computers account for over half my electric bill. I have plenty I can turn on if needed that are off nearly 100% of the time.

One of the businesses I'm developing will allow you to generate revenue from excess computing capacity. In this realm, you sell metered services online. E.g., you can sell number crunching services for AI.

Click to expand...

are you talking about bitcoin mining?

Santa said:

are you talking about bitcoin mining?

Click to expand...

No, but that is certainly one option ... and example of monetizing excess computing power.