Realistic expectations for a grid tied system

Energy Sources Solar
1

Hi all

The current thinking in taking steps to lower my Eskom bill, is to install a grid tied solar inverter with no batteries, not because I think having batteries isn’t important, but because no matter how I look at it, I won’t be able to afford batteries as part of a system at this stage.

This is a how long is a piece of string question, but please bear with me.

Granted all my heavy consumers is already on timers and already time shifted to daylight hours, geysers, pool pump, borehole pump, stove, oven, etc. realistically, how much can I bargain on saving on my monthly Eskom consumption? Granted of course the system gets sized correctly in the first place, to be able to produce my monthly consumption.

I’ll already be happy if I can cut my Eskom usage by 50%, should I expect more, should I expect less?

Just to complete the picture, we are talking rural direct Eskom customer, 3 phase supply, fixed cost average of R3700 per month, then a average monthly usage of around 1100, 1200 kWh.

If a system have to produce 1200 kWh per month, I get to 40 kWh per day and if working on 5 hours (cause winter), I get to 8 kW.

So the current thinking is a Fronius Symo 8.2 8 kW inverter, with 11 or 12 kWp of PV. The thinking is to oversize the PV array by 140 to 150%, because some panels will have to be mounted on a flat roof.

2

Do you need 3 phase or is this the minimum setup from Eskom?

3

I need 3 phase for my deep borehole pump. I have investigated switching over to a single phase pump and though possible, it will cost a big chunk of money which can go towards a solar setup.
Plus in the end the fixed cost for a single phase connection isn’t that much better, around R2500 per month.

What I don’t need is a 40A per phase connection, but that’s apparently the lowest plan Eskom offers.

4

In the long run you will save more if you have batteries. That’s because the grid-tied system will draw just enough from the panels to service the load.

If you have batteries then excess PV gets stored in the batteries for use after the sun has gone down, or when demand exceeds PV. In the latter case if there is charge in the batteries then the inverter can use that instead of drawing from the grid.

They are the most expensive part of the system, but they do mean you can do more and have a cushion when the grid goes down.

Oh, and if you have batteries then your system will keep running if the grid connection to your house is down. A pure grid-tie system with no batteries stops working when the grid is down.

5

Thank you, I’m well aware that a grid tied inverter also goes down when the grid goes down.

A year ago I would have told you it will almost be pointless not to install battery backup because load shedding, but for the cost difference I am willing to live with the couple of “short” Eskom outages per year until I can get there one day.

Because of that R3700 fixed pole fee per month, I eventually want to go completely off grid, but at this stage it’s unfortunately just a bridge too far to start with a smaller system which includes batteries.

The 3 phase of course complicates things and makes it more expensive, but as said I already investigated the switching everything to single phase route and that in itself will almost cost the same as a 10 kWh battery.

6

Must say, I hazard to say LS is over.

As such, a pure grid-tie inverter makes a lot of sense.

Fronius is a excellent idea.
Me, I wonder, what if one goes in lower cost, like Solis, to further cut the initial costs, to get the ball rolling to see “on site” what the effect will be?

One can always sell it later when a battery system comes into play … or connect them to a Victron inverter, same as the Fronius, if that is one day needed.

I’m always considering Plan a, b, c … z, then Plan A, B, … if I run out of letters. :slight_smile:

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7

I don’t think it is over. Eskom’s financial year starts in April. They don’t do much maintenance in the first half of the year. And they came off the successful return of multiple units that were down for maintenance PLUS much reduced demand. I think we will see a return in the summer months, although probably not at the high levels we saw in previous years.

8

If you are looking at a 3 phase grid tied system, you have to verify that your load is balanced first, because most can only supply balanced power. That is if your power draw is 5kW on L1, 4kW on L2 and 1kW on L3, it will be able to generate 3x the worst - which is 3x 1kW. If your load is not balanced you are better off with a hybrid inverter that can handle unbalanced loads.

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9

This is actually the only thing which has been bothering me for which up to now, I couldn’t get a clear answer looking at both the Fronius and Solis, how it deals with unbalanced phases.

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10

While implementing support for the 700-series of Sunspec information models, I did notice that in information model 701 (which has the power measurements and other things), there are now individual watt measurements per phase, while in the older model 103 (for three phase inverters) there is just one total (although it does have a per-phase current measurement).

That sort of suggests that in future – at least it is provided for in the spec – inverters might be able to power phases differently.

Right now, however, I am not aware of a single three phase PV-inverter that does this. All of them feeds the phases equally, and if you add a limiter, it is limited to the lowest phase times three.

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11

My plan A is the grid tied and yes, I’m looking at Solis as well, though first prize will be Fronius.
Plan B is to add Multi’s and batteries,
Plan Z is to add more batteries to be able to show Eskom the finger one day when I’m big.

The other Jaco actually suggested I look at the Victron Multi RS 48/6000 inverters to start with, I never knew they can run like a PV inverter without batteries, then there’s no need to buy extra inverters at a later stage, I can just add batteries.

The big thing bothering me with the Multi RS however, is it not supporting parallel operation at this stage. One of my little principles in life is to never buy a product based on the promise of future software upgrades / feetures.

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12

Another consideration for you is start up current: Don't under estimate start up current

13

My simple rule for myself: If it is not NRS certified, not interested.

Paralleling: (I know I’m gonna get flack)
So I parallel today.
4/5 years later, still under warranty, even if barely, one of the units break.
Sorry, no more of that model around.
Need to replace all the units if I want to continuing paralleling.
Or get a 2nd hand unit if one can find.

An idea to ponder on:
More than one inverter, running independently, but off the same battery bank.

Each inverter dedicated to a selected range of items:
One 3kva for like 2kw geyser.
3kva for lights, internet, alarm etc.
Bigger inverter for bigger loads.
So what one goes.

14

Paralleling isn’t on the cards right now, but who knows one day when I go off grid, I might just need a bit more to take care of the peak draw.

But your point of availability 4, 5 years down the line goes for 3 phase as well and there I unfortunately don’t have a choice.

15

Indeed Eskom are guardedly optimistic for spring and summer. They need to see some more units come back online, and EAF at 70% before they start partying like it’s 2025.

16

Herewith another option to deal with your 3 phase…

Just click on the link to see the item. (Facebook :anguished:)

17

I don’t understand this limitation. My phases are not balanced. Inverter supplies different loads on different phases? I am probably misunderstanding the issue though.

Example from today:

image
image703×377 23.6 KB

Phase A seems to be mainly a fridge.
Phase B is laundry and pool pump and heating. Pool pump / heating is off today but laundry was in use.
Phase C is kitchen and geyser (think the dishwasher and geyser was on between from 13h00 for a bit).

The above is a Sunsynk Three Phase inverter.

18

Yes, you have a hybrid inverter that supports unbalanced phases. (Almost all) 3 phase Grid tied inverters (ie ones with no battery storage) cannot deliver unbalanced.

The technical reason is that 3 phase sums to DC when balanced. If you want to be able to do unbalanced - you have to add storage in the form of caps and inductors to reduce the ripple on the PV side in order to meet regulations.

19

Ah ok, so if I were not to have batteries I would have the same limitation?