Sizing a Grid-Tied system (no batteries)

Hi all

Opinions will differ, but that’s what makes it interesting.

When sizing a grid-tied solar system with no batteries, would you use the same principles and calculations as when sizing a hybrid system with batteries?

Obviously with a grid-tied system there is no batteries to fill up before the end of the day, everything comes directly from the sun.
Plus especially if you can not feed back in to the grid, moving as many as possible loads to daytime will be a big advantage.

Let’s leave possibly adding batteries to the system in future out of it for now.

So how would you size it?

I would take your chosen panel and position it in position at your chosen slope and direction and measure the power output at noon.
Don’t waste your time with theoretical outputs.
( Then whilst observing the maximum voltage constraint).
I would size my arrays 50% bigger than this practical ideal to allow for the panels to accumulate dirt, underate dealing with real-world ambient temperatures and aim to still clip the production peaks up to 10-15% on the best days of summer.

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As Phil said. Enough PV for all the daytime loads, plus some extra to account for dirt, cloudiness, and winter. Panels are cheap compared to the rest. I don’t have the experience with this that Phil has, but gut feeling says at least 20% extra feels about right.

On good days your grid limiter will then choke away some of that extra you installed, but that’s the compromise you make.

Then you immediately get a timer switch for the geyser, and the pool pump runs over noon, the dish washer just after breakfast, and so on.

The one “appliance” that just really doesn’t work all that well… an electric car. If you are employed outside of your home, that is not home during the day. But… with these new long range vehicles… I’m beginning to think it might even make sense to size the array such that the car can charge from the excess on Saturday and Sunday. Perhaps not now… but sometime in the near future maybe.

(In other words, I am in favour of oversizing even if it means some waste every day).

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So basically what you guys are saying is, use the same principles to size a system with or without storage batteries. Sounds like I’m on the right track and not missing something then.

Methinks, IF you add batteries, the system actually gets bigger.

Reason being, the normal loads you want powered as discussed above AND the recharge of last nights batteries on top of that.

Yup. Or conversely, the question is: how much smaller should the system be if you don’t have a big battery bank that also has to be recharged?

The answer is: Not much (for most hybrid systems). For full off grid systems the answer would be different, it would be half the size or less.

With batteries, in a system that wants to not import from grid about 80% of the time (making up the % a little bit, but probably reasonable based on my own experience), your panels should be sized to be able to produce your full daily demand most of the time (i.e. when it is not raining) with some margin to account for some overcast/degradation etc as mentioned before. Your batteries should be sized to the demand you have when the sun isn’t out. The more you can manage to shift loads to daylight times (staggered to stay within the capacity of your inverter) the less batteries you’ll need.

And don’t spec the system for temporary changes in demand… We have a small baby that needs a little bit of a warmer room at night. This need will go away soon and then my night-time demand would be less again. Dealing with temporary changes with temporary solutions (buying power) should be more efficient. Guess it is up to me to now decide when the temporary changes in usage would become permanent - I imagine teens are also quite a bit more nocturnal than our current usage pattern.


And wait for when they cannot afford to move on and you accommodate them in their own living space at home.

On that some Notes:
Some will argue they MUST leave.
Others seeing the facts realizing/asking: Why the $*#&% must my child pay someone else’s bond!?
Others may also understand that getting your own property is a lability till the day you sell it. Read, in debt for life?
Versus pool resources (wisely) and EVERYTHING becomes cheaper for everyone … that necessitates more solar panels/inverter wattages. :slight_smile:

And Grandma is also available for babysitting.

Oh for sure, I’m very much against the notion that families should all go their separate ways once a child reaches a certain age. Similarly that parents should grow old on their own.

I’m constructing a bachelors’ flat attached to the main house to have the flexibility of offering a separate living space to any of my parents or my kids one day they are a bit older.

More variables in the mix do make system design (especially peak usage) much more difficult to manage, but it is very much a problem worth having.

Yup, remember before Covid, when we used to think that emigrating to the UK or some such place is no big deal, a visit to the family is merely a short plane trip away? At a time people needed that the most, that turned out to be profoundly mistaken.

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O yes!!! … pointed out to Grandma the other week, that:

  1. She gets withdrawal symptoms if she has not seen the 2.8yo in 4-5 days.
  2. That she literally gets “rejuvenated” with the little soul in our care.
  3. That Oupa en Ouma is MOEG every single time she leaves. Man she is BUSY!!!
  4. That we see the little one more awake per week than her parents the entire week.

Titbit: Man, when wife and I go out (anywhere) with the little one, people think she is ours.
If daughter is with they think she and her mother are sisters.
Me, they then look at me, see the grey beard and wonder. :rofl: :rofl:
Wife was quite “insulted” the other day, lady comments “you are so young.” Wife looked at our daughter, pointing, no, that is our daughter, this is my grandchild. Genes is all I can say.

Being an Oupa is the best thing EVER!

And it is true!
You can give them back!!!
You do NOT have to discipline nothing. NOT YOUR JOB!

Best part is, she somehow instinctively follows our rules. And no, the house is not toddler proof. We go to her, explain why she must not fiddle with xyz, and she steers clear. OBVIOUSLY one keeps a hawks eye out all the time … and Oupa “knows” she is going to do try things when you least expect it.

Thank snot for the electrician who refused flat-out to install the solar system where I thought it was “best” … because of toddlers, his words, ±10years ago. He was spot on. Thank you Herman!

So yes, this Village is raising this little one. :grinning:


Joke is, I am more than happy to add more panels … soon we are 9 + a 2.8 year old on a 5kva system.

Geysers, yes plural, my ONLY "nemesis.

But I have plan a, b, c, d … z.
With A, B, C … Z if none worked.

Forget heat pumps/solar geyser … one day will add a “dedicated” (kinda) grid-tied inverter and more panels and bugger this thing called geyser heating … use the spare in the house the rest of the time, Victron is for LS. Finish en klaar.

AND IF LS comes back with a vengence, I swear I will go 2nd hand Fronius.

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The man is thinking four-dimensional alright!