How Long Can Your System "Coast"?

[Symbioquine]

Well, darn it. Once gain I and others have failed to read someone's mind, and attempted to post a comment that might help someone else reading a thread, and not just the OP. Seems like I gotta keep practicing this "forum on the internet" stuff. Will try harder. And btw, I can "coast" for about three days with no sun or generator, and consider anything more than about five days more than I need. But you do you, and congrats on your nearly a month of coast capability.

Haha, don't beat yourself up. Lots of folks talk about "days of autonomy" (i.e. total kWh ÷ kWh/day) which is what you're referring to there.

"Coasting" (as I'm trying to define it here) is a bit different. It's about the behavior that the system exhibits under real-world conditions.

While you might be able to function for 3 days without any inputs, you probably see stretches (of "coasting") longer than that where the batteries didn't get full from solar, but also didn't get empty enough to warrant turning on the generator.

More broadly, you might also run the generator, but not enough to get the batteries full - just enough to get to the next somewhat sunny day. This is also "coasting".

I think the OP has really no idea what he’s asking nor what the expected answer should be. Also keeps changing the goalpost so to speak. Say what you mean and mean what you say sheesh lol. You’re overthinking it

You can think whatever you like, but the concept I'm referring to isn't complicated. I've just stated the same thing in a bunch of different ways because some folks don't seem to be getting it.

 

[JRH]

I think the OP has really no idea what he’s asking nor what the expected answer should be. Also keeps changing the goalpost so to speak. Say what you mean and mean what you say sheesh lol. You’re overthinking it, although I know the type. Overthinking is a hobby for many….and that’s okay.


It’s like the “design 7 lines sketch”.

That is a great vid…..it reminds me of watching congressional hearing’s…

But back on topic:

Being a somewhat vague and variable word , my interpretation of “coasting” is as follows :

with no power renewal probably 2 -3 days on batts with the changes and additions I’m adding.

With a 10 to 20% partial recharge per day , maybe a week…or so.

With a 30 to 50% recharge per day it would be possible for an ongoing period ..

Above 50 or 60 % , no prob….

100% + charge per day , easy.

100% x 2 or 3 times daily , I could waste energy…

With that said , basically spring ,summer and fall would be easy where I live using my propane for heat…( yes it’s chilly many nites all summer) but winter will always be a serious challenge in my location as there’s too many weather related variables. ….

J.

 

[sumpower]

With the consideration of a property having a large enough lot to do ground mount solar and the advent of cheaper solar panels of high quality, I find it hard to believe that a new system even needs a generator where I live (300 + days of sun). I do not have a generator and have no problem with turning off the hot water heater when needed to keep the SOC on the batteries high.

So a lot of the "coasting"as you describe is very dependant on the local weather conditions (real world conditions) , as well as the geographic location of the installation.

So are you just hunting for others peoples real world experence with how long they can go without firing up that generator and what loads they have to turn off to stay in the "coasting" mode?

 

[Spartan]

I get the "coasting" part. This is where my system is this time of the year. I'm getting enough charge to avoid generator use but not enough to be at full "parameters-met" for my lead-acid battery bank.

For proper battery health, my Solar One HUP battery calls for a full, parameters-met, charge at least once every 10 days. I'm 5 days in since the last complete charge and not likely to get to parameters-met for another 4 days when we'll be getting two days of sunny weather. That should get me to a full parameters-met charge within the 10 day window. Then, I'll likely coast along for another week before the weather cooperates for another full, parameters-met charge until the later part of Spring when I no longer have to think about it.

December to early February is when most of my generator use for battery charging occurs but even this varies from year to year. Typically, I rely on 14 to 20 hours of generator support each year. The shoulder periods of deep winter see a lot more coasting along where I'm not completely full, nor empty enough to trigger the generator.

Mid April to mid October, I have solar abundance and need not even think about power consumption. Sorry, no fancy graphs or hard numbers but right now, I'm happily "coasting" along.

 

[eXodus]

I was trying to define "coasting" as the duration in days between the last time the batteries were full and the next time they get full (without ever getting completely empty).

define "full" define "empty"

I usually do not use the upper part of the LFP curve - I cut my charger at a lower level - that will still charge the battery fully - just not as fast.

Further I also stay out of the dropping part of the curve. My best guess is that I leave 10% on table in my normal "Full to empty" operations.

But it gives me a lower buffer - which I can use in emergency situations. Since my all my settings shut everything of - before the batteries are truly empty - there is still some juice left.

I do not sweat it when my batteries are not getting full. Usually just adjusting load, cooking with a butane cooktop instead of electric.
Using less A/C etc.

 

[Nobodybusiness]

We'll call the time where a system is neither all the way full nor too close to empty: "coasting"

IMHO, time spent coasting is one measure of how well an off-grid system is balanced in terms of production capacity, storage capacity, and usage.

Lots of ways to look at this, but;

• If your production capacity is too large you'll spend a lot of time full and not getting the full value out of your solar panels (or wind/hydro/etc).
• If your storage capacity is too small (relative to production) you'll spend a lot of time either almost empty or full and either not have power to use or room to store power for the future use respectively.
• If your usage is too large or too small (relative to storage and production) you'll also spend a lot of time either almost empty or full and either not have power to use or room to store power for the future respectively.

So, post your numbers/graphs. I'm feeling pretty good about ~26 days "coasting" without generator and ~33 days including generator use. 43 kWh storage, 5.75 kW PV, Avg ~7 kWh/day usage.

View attachment 198977

Obviously, you can game this metric by limiting usage to match production so I bet some folks "coast" all winter... Kudos to you, tell us about it!

Right now I’m at almost 8 days.

182 hours 38 minutes.

I imagine at some point it will charge to full again soon depending on the weather.

I’ll keep you posted of longest run.

 

[Primal1]

I run a tiny system that I am back here to figure out upgrading. I have lived off grid on 400 watts of panels and 215ah of lead acid batteries for the last 5 years. No generator. I have gone 3 days in a row without hitting float several times. Somedays I flip the Inverter off early due to poor sun. Seeing as the batteries are still holding up I think I have the minimum dialed in. Now I need to figure out the logistics of adding 200AH of LiFePO4.

 

[Daddy Tanuki]

So my “coasting period was (past tense) about 60 days, 30 on either side of winter solstice. This was due to only having 28 kWh of battery. Once I brought the two new 22 kWh packs on line I now no longer coast.

Once I bring my water boiler project online I expect to go back to coasting. That is provided I can curb my desire for more panels and cells.

The actual plan for the boiler is to burn through excess heating a 450 gallon tank and using it for domestic hot water as well as partial heating for the cabin/cottage so I am guessing I will go back to coasting through the winter.

 

[Rocketman]

I think everyone has the wrong idea about winter- if you are not getting enough sun in the winter to keep the battery full - go south - that’s what those big black tires on your MotorHome are for.

Winter in Arizona or Baja provides plenty of sun…

Oops - I think I missed the point.

In my MotorHome I don’t have a big enough battery 544ah - 12v - only eight of the big blue cells… and only 1400w.
I can have one bad solar day, but if there is another the generator gets it’s exercise.

 

[Daddy Tanuki]

I think everyone has the wrong idea about winter- if you are not getting enough sun in the winter to keep the battery full - go south - that’s what those big black tires on your MotorHome are for.

Winter in Arizona or Baja provides plenty of sun…

Oops - I think I missed the point.

In my MotorHome I don’t have a big enough battery 544ah - 12v - only eight of the big blue cells… and only 1400w.
I can have one bad solar day, but if there is another the generator gets it’s exercise.

yep thats the biggest issue with a motor home, lack of space. though if I owned a large-ish motor home I would build a hinged frame of aluminum on the side to replace the roll out awning and it would be made of solar panels with sealant between the panels to stop water.... use it as a sun shade, same as the roll out awnings... a place for your fold out table and some lawn chairs and get the solar goodness at the same time.

 

[pollenface]

There's probably only a handful of days in a year when I don't reach absorption voltage due to heavy rain.

Excess panels are cheap insurance.

Like having it and not needing it is better than needing it and not having it