Calculation of Solar Output from PV

[thinwater]

In my experience, clouds and shadows are everything. I certainly understand the fine calculations, but the sun variables are much bigger. And are the panels clean? Wiring losses? Are you at the rated temperature?


My challenges were always in the winter, when the nights are long (more lighting and heater hours) and the days were short (with low sun and lots of overcast weather).


Regarding the boom shadow,swing it out to the side at anchor. It looks funny but makes a big difference if there are panels under it.

[hzcruiser]

Originally Posted by noelex 77

Rules such as this are useful, but it is important to realise that solar insolation has considerable variability depending on the location and season.
[...]

Even in locations closer to the equator the 3x rule is very a rough approximation and you can expect considerable variation depending on your location and season.

Solar power is great, but uniform output is not one of the strong points.

It's an approximation, yes, and that's what people generally want to know. An "exact" formula is impossible to provide as it depends on too many factors including the upcoming weather.

Nevertheless we need to start somewhere and if it turns out after one year that the panels deliver way less than expected, an upgrade is required.

[rgleason]

Thanks John, will redo calc using absorption voltages.

[sailorboy1]

My 290W solar panel did 120AH yesterday here in Port Washington NY (Long island). It's been doing this fairly regularly on sunny days if I pull my boom over out of the way.

[valhalla360]

Originally Posted by noelex 77

At my current location in winter the maximum peak sun angle is only around 6° above the horizon, but in summer daylight is almost 24 hours day.

If you start talking about extreme conditions, yes rules of thumb often fail. So if you are above the artic circle, you need to account for that.

If you are the more typical between the 45th parallels, the rule of thumb works pretty good.

[valhalla360]

The other issue is bulk charging vs the final topping up.

If the solar array is large relative to the battery bank, you may find much lower total amp-hr into the bank as it quickly gets the bank up to 80-90% charge and then much of the output during the peak is scaled back because the batteries can't absorb it.

[john61ct]

PV solar output per day is a **completely** separate metric from what a lead battery bank will absorb.

One really has very little to do with the other.

[valhalla360]

Originally Posted by john61ct

PV solar output per day is a **completely** separate metric from what a lead battery bank will absorb.

One really has very little to do with the other.

If the batteries won't absorb what the panels put out, your panel estimate doesn't mean much. It's very much related.

30yrs ago when panels were expensive and people rarely had huge arrays, it would have been odd to run into the issue but today when 400-600w arrays are common place and you see a fair number of 1000-1500w arrays, it's a legitimate issue if you skimp on battery bank size.

It can also come into play if you commonly motor for a couple hours in the morning and the alternator does the bulk charging before the panels really start to put out power.

[newhaul]

Originally Posted by valhalla360

The other issue is bulk charging vs the final topping up.

If the solar array is large relative to the battery bank, you may find much lower total amp-hr into the bank as it quickly gets the bank up to 80-90% charge and then much of the output during the peak is scaled back because the batteries can't absorb it.

that point is when you crank up the stat on the holding plate refer unit to take advantage of the "excess" power production.

[valhalla360]

Originally Posted by newhaul

that point is when you crank up the stat on the holding plate refer unit to take advantage of the "excess" power production.

Depends how much you want to manage the system manually. You can manually tilt the panels to follow the sun all day also to boost output.

If it's a set it and forget it system, it starts to get complicated if you need it to automatically pickup and shed loads based on state of charge.

[john61ct]

Originally Posted by valhalla360

If the batteries won't absorb what the panels put out, your panel estimate doesn't mean much. It's very much related.

My point is that they are two completely separate issues.

Of course when designing a practical overall system, both factors need to be taken into account and balanced, especially when insolation conditions will vary a lot.

But as pure metrics, the PV potential can only be measured with full available loads demanding every watt they are capable of putting out.

Of course switching from a 300AH FLA bank to a 800AH LFP bank will in practice change how much of that potential your system can take advantage of.

But that has no bearing on the theoretical maximum output of the panels themselves.

[valhalla360]

Originally Posted by john61ct

My point is that they are two completely separate issues.

Of course when designing a practical overall system, both factors need to be taken into account and balanced, especially when insolation conditions will vary a lot.

But as pure metrics, the PV potential can only be measured with full available loads demanding every watt they are capable of putting out.

Of course switching from a 300AH FLA bank to a 800AH LFP bank will in practice change how much of that potential your system can take advantage of.

But that has no bearing on the theoretical maximum output of the panels themselves.

Sure but it's a theory with no practical use if you don't consider the whole system.

[sailorboy1]

why do these always become the same tic for tat battles over small details

Just accept that boat electrical power storage and generation doesn't care about theory or perfect and go about making it work for you. I get amazed people will spend $1000+ on solar and batteries to save $1-2/day on diesel costs. Just to find out they are going to motor or run the generator etc. anyway.

[a64pilot]

Originally Posted by valhalla360

The other issue is bulk charging vs the final topping up.



If the solar array is large relative to the battery bank, you may find much lower total amp-hr into the bank as it quickly gets the bank up to 80-90% charge and then much of the output during the peak is scaled back because the batteries can't absorb it.

This is why I often never see the 1/3 rule. Yes my panels will supply it, but in order to fully recharge my bank, the acceptance rate of the bank is lower than what the panels can make towards the end of the day. I cheat, I will run a generator to kick start things in the morning.

I’d say that if your day to day limit on power production is what the panels can make, then it’s likely you are short cycling your bank.

It’s just reality, if you are taking every Watt your panels make and can put that in your bank, then you don’t have enough Solar or are using too much power, or just accept a shorter life out of your bank, which may be the least expensive and most logical thing to do depending on situation.

Just don’t be upset if your not making the 1/3 rule in power, watch your panel voltage, if is often at the max voltage, it’s likely that your battery acceptance is limiting power the bank can absorb, cause remember the amp counters don’t count what the panels can make, they only count what the bank can accept.

[a64pilot]

Originally Posted by sailorboy1

why do these always become the same tic for tat battles over small details

Just accept that boat electrical power storage and generation doesn't care about theory or perfect and go about making it work for you. I get amazed people will spend $1000+ on solar and batteries to save $1-2/day on diesel costs. Just to find out they are going to motor or run the generator etc. anyway.

A lot of people here are preparing, are still working and have the $$ to spend, if it will save $$ once they leave.
Plus there is way more to it than fuel costs, average built in generator likely cost at least 2$ an hour just based on cost of the unit and life expectancy, figure maintenance maybe another 1$ and fuel another 1$. So figure just as a SWAG $4 an hour to run a built in, it’s just a WAG actually, but another WAG is a Honda is half that?