How Long Can Your System "Coast"?

[Symbioquine]

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.



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!

 

[DIYrich]

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.

During normal usage, an off-grid system frequently reaches "full" so that it has capacity to "coast" through extended number of rainy days.

If I had an off-grid system, I would want enough solar to recharge in one day enough batteries for 3 days of usage.

 

[Steve_S]

Interesting Question... Situational & Conditional because there is so many variables.
Maybe I am tad extreme in some ways but again, as applicable to "my" situation. My bank (45kWh) effectively gives me 10 days in scrimp mode.

Usage is average 3.5-4.0kWh per day year round.
Fairly North @ 45Deg so conveniently panels are at that angle too.
Winter solar is k.r.a.p. ! Lucky to get 4 hours mid-winter and pathetic no less. Sometimes the sun off the snow can really kick the solar "when" the panels are clear.
Genset can charge 80% within 7.5hrs and I typically never let bank capacity get below 25%. An evil required in the north but I have a fix and that's coming to an end. Another array tweaked for optimal winter angle...

Now here we are end of Feb. Last 3 days, I hit 100% & Float by NOON with all cells in all packs sitting happily at 3.430vpc +/- 0.005

Now I do have "heavy" days and the large bank is also there to support that as well. If I have laundry going and do vacuuming and other such activities the usage will be higher than the average... After the snows are gone, then it's tools (tablesaw, compressor etc) start getting a lot more use and so along we go. An interesting thing I laughed my ass off about last fall... Had the tablesaw, compressor (3hp), mitresaw all going at the same time and I noticed it was ALL being fed from FLOAT as the bank was taking <1.0A (we were in the middle of building and all tools were actually running with load, I happened to note as I ran into the powerhouse for a minute to get stuff). Put a heck of a smile on my face...

 

[Symbioquine]

During normal usage, an off-grid system frequently reaches "full" so that it has capacity to "coast" through extended number of rainy days.

I'd argue that you should replace "full" with "full enough", but otherwise that's correct. I should have specified that the idea of "coasting" as a system size metric really only makes sense in the least productive time of year.

If I had an off-grid system, I would want enough solar to recharge in one day enough batteries for 3 days of usage.

Most of the late fall and early winter I get that. A good day at the winter solstice only provides ~1.5 days of usage though so I supplement with the generator.

I try to balance the generator usage to stay in the <50% range so there's available battery capacity to capture solar power if it is available. Charging more than that (if I ultimately hit 100% soon thereafter) means I wasted fuel/runtime on the generator when that power could have come from solar.

Genset can charge 80% within 7.5hrs and I typically never let bank capacity get below 25%.

I currently get about 10%/hr generator charging too.

Interesting Question... Situational & Conditional because there is so many variables.
Maybe I am tad extreme in some ways but again, as applicable to "my" situation. My bank (45kWh) effectively gives me 10 days in scrimp mode.

Yeah. My theoretical "autonomy" is about 6 - 7 days (without turning infrastructure things off), but I was hoping folks would answer with how long in days their systems are actually "coasting" (neither getting all the way full nor all the way empty) in practice.

 

[marionw]

Multiple banks of batteries under external software/monitoring control where banks can be switch in or out (online or standby) as required.
With some BMS's (EG4 Lifepower4) you can control the Charge and Discharge MOSFETS with software to effectively control a batteries "online" or "standby" status.

 

[Steve_S]

Yeah. My theoretical "autonomy" is about 6 - 7 days (without turning infrastructure things off), but I was hoping folks would answer with how long in days their systems are actually "coasting" (neither getting all the way full nor all the way empty) in practice.

After Feb, I guess I coast till end of Oct, regardless of what I'm up to (power wise). I did do a test before my current setup and managed to go 9.5 days from 100% to 0% in "normal" mode before the inverter dropped, that was with 0% solar input. After this years finalization I'll have to do that again because I am "done" finally. Last batch of cells, bms', cases etc hit port on the 1st.

 

[Mattb4]

I don't have any snazzy graphs or precisely calculated numbers. In the Winter my system coasts through 2 days of bad weather and on the 3rd it needs supplementing. It can fully charge in one full day of good sun after that. It most often sits at full by noon if I don't kick on the water heater. (takes about 2 hours to kick back off at set temperature 135F).

This Summer I will be attempting to add in A/C without needing grid to back stop it. Should be interesting.

 

[Symbioquine]

Multiple banks of batteries under external software/monitoring control where banks can be switch in or out (online or standby) as required.
With some BMS's (EG4 Lifepower4) you can control the Charge and Discharge MOSFETS with software to effectively control a batteries "online" or "standby" status.

Not relevant. The question I'm posing (updated title to make it clear that it is a question) refers to the abstract reservoir of power a system has relative to production capacity and usage. Whether that reservoir is in one big battery or segmented somehow with a software control scheme, shouldn't matter to that.

After Feb, I guess I coast till end of Oct, regardless of what I'm up to (power wise).

Maybe I wasn't clear enough. 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). I'm guessing that your batteries are regularly full between Feb and Oct.

I'd guess there are times between Nov and Feb where you have stretches of more than 10 days where the batteries are not getting all the way full. The question was "How long was the longest stretch like that you've seen for your system?"

In case it's not obvious to other folks reading the thread, this idea of coasting is interesting because those are the times when production is in balance (on a rolling average basis) with usage. If that coincides with the time of year where production is worst, then it suggests that your production and storage capacity are well sized to your average usage.

 

[Mattb4]

It helps if you live where it is rare to have more than 1 or 2 really crappy days in a row. Partly lousy days can still develop some power if you have enough panels. Places like the West coast of Washington State that have only 1 or 2 days in a row of good weather is a whole 'nother ball game. I grew up in the PNW and we would travel across the mountains to Eastern Washington just to see the sun (or that strange yellow orb in the sky as I used to refer to it as a child).

 

[Steve_S]

I'd guess there are times between Nov and Feb where you have stretches of more than 10 days where the batteries are not getting all the way full. The question was "How long was the longest stretch like that you've seen for your system?"

In that context, January, about 3 weeks where I was between 70-30%, getting some sun but "blah" dim sun.

 

[Q-Dog]

We had about 2 weeks in the middle of November and 2 weeks at the end of January where we had very low production. BUT we just cut back on usage when it is low and increase loads when we have more power. We try to keep enough reserve in the battery to get the fridge and a couple of lights through an overnight power outage.

Obviously things would be different for us if we were off grid. I ran the dehumidifier a few times this week just because the battery was full.

 

[fratermus]

I'd guess there are times between Nov and Feb where you have stretches of more than 10 days where the batteries are not getting all the way full. The question was "How long was the longest stretch like that you've seen for your system?"

I sized my system to make sufficient power to meet minimums year-'round. In the past 30 days there were two days (consecutive) where the bank stayed in Bulk all day (did not reach Absorption voltage) but did meet needs. I think that would meet your definition of coasting. IIRC the longest run like I've seen over the past 5 years was 3 consecutive days.

Like Q-Dog once my minimums are met I tend to ramp up discretionary consumption.

 

[OM617YOTA]

Around here, any system built to have any kind of ability to function in winter will be vastly over-sized in summer.

Any system that "coasts" in summer will be immediately and woefully undersized as soon as summer ends.

My modest system gets to full around noon in the summer, can "coast" quite a bit in the shoulder seasons, and is basically just a battery bank for 3-4 weeks in the middle of winter. There was a period this past winter that took 3 weeks to harvest 4kwh.

 

[Madcodger]

During normal usage, an off-grid system frequently reaches "full" so that it has capacity to "coast" through extended number of rainy days.

If I had an off-grid system, I would want enough solar to recharge in one day enough batteries for 3 days of usage.

This. Great, succinct advice. All I'd add is "in winter, when sun availability is worst." Yes, one will sometimes have longer stretches of poor/no sun but that's where a generator (or grid, if connected) comes into play. Some try to completely avoid non-solar power but that leads to excessively oversized battery banks. A few hours of generator use each week, timed to run when the modest noise is not a problem, will keep a properly sized battery bank in good shape during even tue worst of winter, with plenty of power for normal household use. It's not like it needs to run continuously or even every day.

 

[ScrotusGobbleBottom]

game this metric by limiting usage to match production

Isnt this the plan?? lol

 

[Danke]

I was surprised this winter that even on a cloudy, rainy day, I was producing about a 10th of the normal power. I will eventually have a lot of panels installed so I may not need as many batteries as my original plan called for.

 

[upnorthandpersonal]

Depends on the season really. I guess I coast all winter long since the battery never gets full (it's been about four to five months of that). In summer, it's mostly full every other day or so at least. I have about 12 days of autonomy, soon around 16 without any generation and without turning off comfort.

 

[Symbioquine]

This. Great, succinct advice.

Not sure why (almost) everybody thinks I need "advice" here. I'm at ~48.6 degrees N so I know about seasonal variations in production (though maybe not as intensely as @upnorthandpersonal) - and yes my system is arguably oversized for the rest of the year too. That's not the point of this thread.

The point is to share your actual real life numbers about how long you've been able to "coast". This is about the intersection of two forms of frugality - $$$ spent on right-sizing your system vs conserving power when it matters most.

• If you're using the generator and coasting for long periods of time, then you're probably judging correctly how much to run the generator such that it doesn't cause you to subsequently waste solar/wind/etc productivity.
• If you're not using the generator and coasting for long periods of time, then you're probably really good at reducing your usage to match what your system can actually keep up with.

 

[Madcodger]

Not sure why (almost) everybody thinks I need "advice" here.

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.

 

[NC_hydro]

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”.