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Hi guys, thanks for the tons of knowledge and guidance I have already got from here, hoping for some input on my calcs. I don’t have a solar system yet but am trying to live as if I do and see what that means in terms of usage patterns and balancing that with comfort and day to day living with a baby’s needs. Then figure out what I could potentially save and would still need to pay City of Joburg. Its not JUST a financial decision but I like to have a realistic expectation.

I have been running an efergy engage for about 6 weeks and moved loads around so currently averaging about a 45%:55% split between day and night usage.

I currently have an evacuated tube system connected to a 200l geyser which works well but because of the kids evening bath and adults morning showers, the 4KW element kicks in for an hour in the morning. Generally it doesn’t run in the afternoons but I have set it to reach temp at 3.30pm so that there is sufficient hot water for the evening. We will see if this holds true in winter. I may well put in a 2kW element there but essentially kWh usage should remain the same.

Other big loads are a 0.75kW pool pump also only running in sunlight hours and a 2nd geyser which is a 2kW, 150l and services the kitchen side of the house.

I’m on CoJ prepaid tariffs, so for my R1400 a month I’m getting 722.9kWh. So an average cost of R1.94/kWh but soon to increase by 15% or 17% depending what you read. (or even more if the flat monthly fee comes in but that’s a sunk cost because I want to stay tied to the grid)

So after that background (if anyone is still reading) I am really trying to size my system and get a decent estimate on what costs and then savings can reasonably achieved.

Assumptions:

Day time = 9am to 4pm

Night time = 4pm to 9am

24 hours measured from 9am to 9am the following day

Average 24 hour usage = 24.15kWh

Average “day” usage = 11.5kWh

Average “night” usage = 12.65kWh

 

So currently averaging the R1.94/kWh from CoJ and if we add 15% to that we get a forward-looking rate of R2.23/kWh assuming my usage stays the same. There are many assumptions that I would like to have your thoughts on:

Theoretical System: Sunsynk 8kW, 2 x 5.5 Hubble, 20 x 435W CS panels (sized in order to run a heat pump for the pool about 6 hours a day)

Potential PV production time = 4 hours (sun hits roof at 9am and is dipping behind trees at 4.30pm, obviously between these times it is not all at full power so hopefully 4 hours is a conservative guess?)

PV Array = 8.7kW (435W x 20panels)

A = Potential PV production = 34.8kWh (4hours x 8.7kW)

B = Average “day” usage = 11.5kWh (all covered directly by solar/batteries being dynamically used and recharged)

C = Batteries require charging from 20%-100% daily = 8.8kWh

A-B-C = Spare capacity = 14.5kWh to be used to power 2.2kW pool heat pump (also theoretical at this point)

Since the heat pump is the added bonus I’m excluding it from my return on investment calculations. Therefore:

Savings = B + C = 20.3kWh/day

So at the new rate of R2.23/kWh discussed above I save on average R45.27/day.

In my pivot table with the hourly Efergy data I get a slightly more accurate number of R43.03/day saved due to maximums being hit, but the simplified calculation gives a broad idea of my thinking and hopefully you can point out any errors? Of course not every day is sunny in Joburg so maybe I need to apply a factor to that savings number as well?

 

To calculate the return on investment over 10 or 15 years:

Sunsynk 8kW = R34k

2 x 5.5 Hubble = R50k

20 x 435W CS panels = R50k (on a north facing, tiled roof, 25 degree pitch, can fit max 20 panels)

Installation and accessories = R40k? Would appreciate some guidance on this number, but a guesstimate based on a friends quote.

So my outlay will be R174k and I’ll save R43.03/day on average, which is R15 706 per year.

Then lets assume Eskom increases at 10% per year. This assumption is really the biggest one, if Eskom increase more then obviously it becomes more worthwhile so I want to be conservative.

Using the internal rate of return (IRR) function I get:

IRR 10 years

6.21%

 

IRR 15 years

12.99%

 

     

Year

 -174,000

Outlay

1

     15,706.00

Amount saved each year not paying Eskom

2

     17,276.60

 

3

     19,004.26

 

4

     20,904.69

 

5

     22,995.15

 

6

     25,294.67

 

7

     27,824.14

 

8

     30,606.55

 

9

     33,667.21

 

10

     37,033.93

 

11

     40,737.32

 

12

     44,811.05

 

13

     49,292.16

 

14

     54,221.37

 

15

     59,643.51

 

 

So 6% return is better than sticking it in the bank but certainly not amazing. But then added to that there are benefits of avoiding load shedding and the theoretical pool heat pump runs in addition to this. In fact if I can utilize the full spare capacity above on the heat pump, the IRR increases to 14% over 10 years. Getting the system to survive for 15 years then really starts to pay off at a nearly 13% return.

 

So I guess my question is, am I in the general area in terms of expectations? Comments on any of the above assumptions and points would be appreciated.

Has anyone actually calculated their own return on investment in Gauteng where feeding back to the grid doesn’t make sense?

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Looks like you've done all your calculations. 

Your usage is similar to mine with the only difference that I have 3 aircons that run most days in summer. I also have 200l evacuated tubes and 4kw geyser element that switches on till 4pm from solar so kids can shower in the evening and again at 5am for the adults to shower in the morning. 

Your system should be paid for after 8 to 9 years. 

My setup cost me in total about R240k. Return on investment however did not feature as one of my main considerations when I decided to go solar. Reliable electricity was more important to me. Monthly savings from not paying CoCT for electricity I put away for future upgrades and maintenance cost. It's been running for nine months with no maintenance so far and only used some of the savings to add an additional battery and more panels. Future savings will go towards adding more batteries and possibly a wind turbine to get me some electricity when the sun is not shining. 

I could have taken the cash and invested it or basically prepay my electricity cost for the next 15 years by going solar. I decided to go with the latter. Even my water is off grid so I'm only paying CoCT for rates and refuse. Zero water or electricity charges. Water savings alone for the last 3 years is over R80k (initial cost of under R10k and only one pump replacement last year) with electricity savings currently at R28k after 286 running days. My average CoCT tariff is ±R2.50/kWh with a 13.5% increase expected in June. 

I was done with paying for corruption, incompetence, and subsidising free loaders through my electricity usage. The people going solar are the ones actually paying their monthly bills so the situation is just going to get worse for Eskom and municipalities. Every cent you pay goes towards propping up their failed system. If you have the funds and space for solar, you should go ahead. 

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7 hours ago, Brani said:

This is assuming that you didn't have to borrow R174k and have no interest to pay. If you would invest that money at 6% return, your payback becomes longer.

Thanks, this is how I also originally looked at it, but I quite like IRR because then i can compare that rate directly to the bond rate, money market rate or whatever investment rate you would expect directly. So I would prefer higher than 6% from an investment but there are a lot of other factors involved.

 

7 hours ago, Achmat said:

Water savings alone for the last 3 years is over R80k (initial cost of under R10k and only one pump replacement last year)

Thanks, @Achmat have you done a write up on your water system? I can't seem to find it. That will be the next step for us as well once solar is bedded down.

Edited by Robbo
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6 minutes ago, Robbo said:

Thanks, @Achmat have you done a write up on your water system? I can't seem to find it. That will be the next step for us as well once solar is bedded down.

Not on this site but over on my broadband. Was at the hight of the cape Town drought a few years ago with CoCT going ballistic with their punitive water tariffs which had still not come down. 

I was fortunate in that there is a river less than 1km from where I live with almost crystal clear water less than 5m underground so need to for very expensive filtering equipment needed. 

My well point journey

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To jump in (mainly because I'm looking to pull the trigger soon), can I ask 2 things?

1) do you really need the 8Kw inverter? and the 2 batteries? My usage is on par with yours, and I'm fairly confident I'll be able to get away with a 5Kw inverter and 1 battery.
2) I assume you "night" usage is high because of the geyser?  have you thought of ways to mitigate this?

Interested because I'm hoping (with similar consumption) that my payback period can get drastically cut with a smaller system

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I have a 5 kW Sunsynk and two 2.4 Pylontech’s, the only thing that is not hooked up to the inverter is the electric oven( stove is gas), installed a 2 kW element in the 200 liter geyser( heated to 70 degrees once a day),then the microwave/kettle/dishwasher/washing machine/tumble dryer/ aircon/pool pump are all hooked up to the inverter and with a bit  planning and training the wife, all works great, only negative is, that I have a 4.2 kWp solar array, need to figure out how to mount the maximum for the inverter which is 6.5 kWp panels

 

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27 minutes ago, bobmabena said:

1) do you really need the 8Kw inverter? and the 2 batteries? My usage is on par with yours, and I'm fairly confident I'll be able to get away with a 5Kw inverter and 1 battery.
2) I assume you "night" usage is high because of the geyser?  have you thought of ways to mitigate this?

I've tried to consider all options in various pivot tables but my post was already extremely long so didnt go into it all.

1. Because I want the pool heat pump to be the sweetener of the deal for me, that extra 2.2kW pushed me over to the 8kW inverter. If you dont need that or want aircons you can probably get away with the 5. But as @Brani said there is also an element of oversizing on all specs (output, passthrough and PV capabilities)

The batteries is also quite a close call purely financially speaking. If I go down to 1 battery and therefore down to 18 panels (since only one battery needs charging during the day) my IRR goes up to more like 7%. So not a huge shift for the added safety.

There are many other considerations, like how do you use your power. If in big bursts then 2x 1C batteries also ensures that you dont draw anything from the grid as you would if you have a low discharge rate and the grid is needed to supplement.

2. Yes geyser mainly for sure. This is a tricky balance between living the life we are used to and not wasting. A 3kW oven also doesnt help but we do also have a gas hob. I have tried cutting things earlier and later but you have to keep the wife happy 1st, all costs aside! So there is room to improve. During winter it will be interesting how the EV tubes work and how much power is needed to supplement.

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Very good feedback from ThatGuy, but also let me add one extra point

Your planned system is not static and both your requirements and the possible solution to those requirements will change.

I started out last year March with my main system, consisting of a Goodwe 4.6 with four 2.4 Pylontechs.

Few months later I moved both geysers over to evacuated tubes, to remove them off Eskom or my batteries at night.

Few months later I also changed the hob to gas and added two more 3.6 Pylons, to take me completely off Eskom at night, which was not part of the original plan.

My usage stats below. The May 2020 usage was a mess, as I was getting used to the system and switched back to Eskom on geyser usage and had much higher loads.

Then, my current usage, with better planning, more family education and changed usage patterns. Very little Eskom usage, mainly to sync the inverter.

May 2020.JPG

May 2021.JPG

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On 2021/05/13 at 11:13 AM, Robbo said:

Potential PV production time = 4 hours (sun hits roof at 9am and is dipping behind trees at 4.30pm, obviously between these times it is not all at full power so hopefully 4 hours is a conservative guess?)

There may be options here. I have 6 panels facing North and 6 facing East. I don't get as much juice at midday, but I get more early in the morning. One of my neighbours just went for an east-west setup and will have a longer but flatter generation curve than an installation with all panels facing North.

The batteries can also help to smooth out differences between available PV and total demand.  Though obviously if demand > PV by too much then eventually something has to give.

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On 2021/05/13 at 11:13 AM, Robbo said:

Has anyone actually calculated their own return on investment in Gauteng where feeding back to the grid doesn’t make sense?

I did an estimate before I bought, and now keep a record of what happens from month month. This doesn't match my estimations. I initially figured I'd break even round about the 8 year mark, but I'm now not so sure I'll get that (of course if the cost of electricity gets ramped up steeply then I might, but it's not a foregone conclusion). 

Reselling doesn't make sense in Johannesburg so I haven't even tried. Remember that it requires the installation of a special meter - at your cost - and going onto a post-paid tariff with R several00 of fixed fees per month. Eliminate that from your calculations and if things change then you get a windfall. I don't expect them to change. I don't think the utilities (anywhere, not just in SA) like home solar, and whilst they can't stop you having it they don't have to actually reward you for having it and so they won't.

You did mention something that is worth something but it's hard to put a number to: the soft issues of knowing that load shedding is not a problem and that your fridges will always be on and your security system will be running. That doesn't reduce your electricity bill, but it is worth something. 

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Hi Robbo, as ThatGuy said, your time is worth actual money, and I want to add, it can work for you.

Plug into your calculation:

Inverter R34k? How about R3k? Repurposed obsolete UPS from a company basement. Match panel DCV to do without electrickery.

2 x 5.5kWh Hubble R50k? Built 14kWh storage battery from 1800 recycled 18650 lithium-ion cells, maybe R5k. Manual BMS means indefinite life by easy testing of strings and replacing individual cells.

20 x 435W CS panels? = R50k? Make do with 10 such for R20k by swinging E-W on high pole mount and get max from sunrise to sunset. OK, DIY engineering.

R40 installation? As said, DIY.

Solar geyser R plenty? Use excess PV 200VDC straight into standard 220V AC element. (OK, needs capacitor and diode, R200)

Sorry to mess your figures, just want to point out that Solar makes for a great DIY project that pays for itself real quick and provides free electricity from then on.

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11 hours ago, Abe53 said:

Sorry to mess your figures, just want to point out that Solar makes for a great DIY project

Not at all, these are the options that we all want to hear. I'm more a finance guy than an engineering guy so for me paying a professional is the way to go on the big ticket items. But nevertheless there are more options out there as you point out. 

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Good Calcs @Robbo

Fortunately or unfortunately both me and the missus work from home so cant afford to not have power during meetings. This made it I least to make our solar investment. 
 

On 2021/05/13 at 11:13 AM, Robbo said:

A-B-C = Spare capacity = 14.5kWh to be used to power 2.2kW pool heat pump (also theoretical at this point)

 

How different would your ROI look if you could sell your excess capacity at R0.94 a KWh?

 

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On 2021/05/15 at 2:43 PM, Muchachos said:

How different would your ROI look if you could sell your excess capacity at R0.94 a KWh?

Massive difference! Again assuming that the 0.95c/kWh increases at 10% per year as the electricity cost goes up, I work it out as 17.79% over 10 years and 22.65% over 15 years. Then it really does become a no-brainer.

Of course in my situation I wouldnt get to run my pool heat pump, but for those able to sell back it makes a lot of sense.

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