Hello from The UK

[KeithG]

Hi everyone,

I'm hoping to be able to provide answers to others in due course but right now I've got my own question.

I'm going through the numbers of an off grid system and I'd just like to confirm something regarding strategy.

It's an off grid system with 4 day battery back up.
Would you size your system such that the annual surplus from summer is larger than the winter deficit, and it then charges the battery (battery is sized with no problems. This is just about summer surplus versus winter deficits.

I only ask because the system I'm investigating seems to be designed with an annual surplus (from summer) much less than the required energy needed for winter. The battery capacities are sufficient, but the energy to charge them is just not available through this system! Which makes little sense to me! unless the batteries were intended to be charged through an additional generator. Maybe it's just a terribly designed system!

Thank you
Keith

[solar pete]

Howdy Keith,

You generally design for the worst months of the year, but it could be its just going to work out better to charge the batteries occasionally from a genny. This would allow you to have a smaller system than doing all the numbers of the worst month, all the serious off gridders around here run with about a 6kva genny and consider it required with all off grid setups, cheers

[KeithG]

Thanks Pete.

[russ]

Quote, "Would you size your system such that the annual surplus from summer is larger than the winter deficit, and it then charges the battery (battery is sized with no problems. This is just about summer surplus versus winter deficits."

That is only for grid tied where the metering agreement with the utility allows.

You need to size to use your batteries where they will last and have a generator for when the sun don't shine for too many days.

[DanKegel]

You need to size to use your batteries where they will last and have a generator for when the sun don't shine for too many days.

+1

Furthermore, would you save a lot on batteries if you run the generator as soon as the sun doesn't shine for, say, half a day? Might make the project pay off a lot sooner.

[russ]

+1

Furthermore, would you save a lot on batteries if you run the generator as soon as the sun doesn't shine for, say, half a day? Might make the project pay off a lot sooner.

What is the cost of diesel in the UK now?

[DanKegel]

What is the cost of diesel in the UK now?

Lots cheaper than batteries, isn't it?

[SunEagle]

Lots cheaper than batteries, isn't it?

I would think diesel is cheaper than batteries so your suggestion of using the gen set a couple of hours (to top off the batteries & run high watt loads) at sun down is smart.

[russ]

Lots cheaper than batteries, isn't it?

Fuel is at least double what you pay and possibly 3 times - what you suggest must use the "new" math?

[KeithG]

Quote, "Would you size your system such that the annual surplus from summer is larger than the winter deficit, and it then charges the battery (battery is sized with no problems. This is just about summer surplus versus winter deficits."

That is only for grid tied where the metering agreement with the utility allows.

You need to size to use your batteries where they will last and have a generator for when the sun don't shine for too many days.

Isn't the sizing a combination of both, in that the batteries should be sized for energy consumption and the array should be sized for power production based on irradiance in order to meet the requirements of the batteries?

My biggest problem is that I've got plenty of theoretical knowledge but very limited, zero, practical experience. I think practical experience really teaches a lot. I'm going to trawl this forum and try to learn from you guys.


ps. Thank you to everyone, I didn't realise there were so many replies. The notifications were going into my junk folder.

[DanKegel]

You said you had a battery that could hold 4 days worth of usage.
Thus the upper amount of surplus you can store is four days' worth.
To your question "Would you size your system such that the annual surplus from summer is larger than the winter deficit", the answer is "No, because the battery isn't big enough to hold that, and you don't have a grid to use as an infinite battery".

Or were you asking "how big should I make the battery"?

[KeithG]

I get you know thanks, I had been thinking about it the wrong way. Also, originally the system (for which I only have minimal and crap data) was a case study as part of my study. It quickly became apparent that much of it was rubbish. None of it made sense. I've since complained to the institution responsible.

As for the query, I've been trying to get my head around how to size a home system with and without generator. Although I know you would rarely see a system without a generator. The basic method I currently understand (losses and other variables not shown below) is something as follows:

1. Identify your load and peak power consumption
2. Select inverter (if necessary)
3. Size your battery bank for x number of days of autonomy
4. Size your array so that the lowest monthly daily average insolation/irradiance is enough to charge the batteries for 1 day's worth of load. Where the remaining energy storage has been, generally, kept topped up from days with an average insolation higher than the worst day.
5. Select appropriate charge controller.

Edit: I should add that my own calculations are quite close to pvsyst simulations but I'm not sure how to use (calculation wise) Loss of Load probability, how alter array size when using a generator and how the number of Days of Autonomy (battery bank size) effectively reduces the size of the array needed.

I've still got a lot to learn but I'm making good progress. I'm more than willing to go do the leg work myself and read anything and everything I need to know. I guess I'd just like to check that as a basic methodology I'm on the right track. The problem is all the methods I've read in books and online don't seem to match up with the real world requirements. I've already started going through as much of this forum as possible. It's a gold mine for advice and information.

If you can point me in a better direction I'd be very grateful.

Cheers

[somecreepyoldguy]

Greetings here from Riverside!

[inetdog]

Greetings here from Riverside!

Creepy, we would really appreciate you making meaningful contributions instead of blatantly padding your post count up to 10.

---- Moderator.