Small Solar Off-grit setup - calculation check?

Okty

Okty

Expert Member
Joined
Aug 25, 2009
Messages
2,343
Reaction score
2,381
Considering getting a small solar off grid system for my house, and wanted to check if my calculations are correct. Just for clarity, I will still be connected to the Eskom grid, but will have a spate off-grid system.

Wh needed for 24 hour period: 4868 Wh
Hours of sunlight per day: 6 hours
Watt to generate during sunlight period: 811 W

300 Watt panels needed: 3
Ah needed for batteries: 406 Ah (12 Volt batteries, deep cycle etc.)
Batteries needed: 3 x (200 Ah @ 80% efficiency)
Solar Charge Controller: 3 x (150Ah @12 V)
Inverter: 1 x 1500 W

I am not sure about the Charge Controllers though, if a battery is rated at 200 Ah (160 Ah if taking 80% efficiency into account), then 150 Ah might be too low.

Thanks
 
Some off the top of my head for you -


-----

Figure on 3hrs in winter @ rated panel rate.
5hrs in summer.

ALWAYS buy for your winter needs.

If you need 5KW daily (which is low)

I'd look at 6 x 325W panels - around R2000 each incl. vat so winter is catered for.

eg http://www.solarsolved.co.za/index....t-solar-panels-519-520-605-606-607-641-detail

6x 325 x 3 (winter) = 5850W / day


6 Panels hookup = either

8.6A / 1950W - Serial (Close circuit will be higher, check panel rating, inverter needs to be able to handle the open circuit rating)
51.6A / 325W - Parallel

I would suggest if you're going offgrid to get a hybrid inverter.
The Axpert 3KW would probably suit you, although I'd get the 5KW for some headroom.
Around R15,000- R20,000 depending where you order.

Batteries - you will need 48V, so 3 isn't going to work. 4 minimum if Dead Acid..
If you have money, I'd suggest rather spend it on Lithium.
These aren't too badly priced -

2.4KW - @ R14,720
http://www.solarsolved.co.za/index....4kwh-lithium-ion-battery-excl-brackets-detail

Could get away with two of those, but I'd suggest 3 to be safe, and then the batteries would last longer.

You'll also need mounting and cabling, but thats cheap - R1-2k total
 
Okty said:
Considering getting a small solar off grid system for my house, and wanted to check if my calculations are correct. Just for clarity, I will still be connected to the Eskom grid, but will have a spate off-grid system.

Wh needed for 24 hour period: 4868 Wh
Hours of sunlight per day: 6 hours
Watt to generate during sunlight period: 811 W

300 Watt panels needed: 3
Ah needed for batteries: 406 Ah (12 Volt batteries, deep cycle etc.)
Batteries needed: 3 x (200 Ah @ 80% efficiency)
Solar Charge Controller: 3 x (150Ah @12 V)
Inverter: 1 x 1500 W

I am not sure about the Charge Controllers though, if a battery is rated at 200 Ah (160 Ah if taking 80% efficiency into account), then 150 Ah might be too low.

Thanks
Click to expand...

As matter of interest, why you making the move to solar? I started looking it up myself, but in my use case, its cheaper to drop the money into the bond and save on interest.
 
Thanks Isheed, will have a look.

One thing though, 3 hours sunlight seems a but rough for winter?
 
calypso said:
As matter of interest, why you making the move to solar? I started looking it up myself, but in my use case, its cheaper to drop the money into the bond and save on interest.
Click to expand...

My bond is almost payed off. Two main reasons, one to save on some electricity, eskom and municipality increases making a noticeable difference.

Secondly, our municipality doesn't pay Eskom, so Eskom going to load shed us again, severly, ie punish those of us who actually pay. Randwater also droping our supply pressure due to non municipality payment.

So yeah, want an off gridd system that keeps the lights, fridges, internet etc on.
 
Call it 5kWh you need as storage

If going for Lead Acid, you want to use about 50% DoD. If lithium, 90% DoD.

Therefore you need about 10 kWh of PbA storage, call it 5.5 kWh for LiPo/Li-Ion

For PbA, you need (10[kWh:VAh]*1000[Wh/kWh]/12[V]) = 833 Ah total. 24 V is much better in my opinion, and depending on which inveter you can source even 36 V is better. 48 V is best. For compatibility's sake lets say you are using 200 Ah cells, arrange them 2S2P for 800 Ah which is close enough and at 24V nominal for the inverter.

Check if you can source some e-bike or drone batteries as they may infact come out cheaper than going the PbA route.

Winter weather call it about 4 hours equivalent sunlight normal to a correctly oriented panel face in Joburg, 3 hours for Capetown.

Therefore you need about (5[kWh/day]/4[h/day]) = 1.25 kW worth of panels for Joburg and 1.67 kW for Capetown. To be honest panels are the cheapest part, if I were you I would get about 2kW in panels total.

The inverter is the part you dont skimp on. Get a decent one that is pure sine wave and can charge the batteries from solar (via MPPT not PWM - what year is it??) and via grid. That way you can buy an el cheapo generator to charge your batteries if you need to if is really bad weather for more than a couple days

I can do a proper analysis and simulation for your area if you want so you know what angle to point the panels and if it is worth the effort to make them adjustable, how many hours of sunlight you'll get per day each month etc. - you're welcome to drop me a PM
 
Thanks for the agentrfr, especially the inverter MPPT part.

PM sent.
 
For where Okty stays he asked specifically for winter (June-Aug)

How to use: Expected daily insolation * size of panel W is how many Wh you can genreate: Eg. 4 hours a day for a 1kW panel is 4kWh generated per day on average (panels only run at 80% efficiency when they get hot, so take that into account too not done above)

Bruh, it's moerse hot where you stay.

Here are results:
Data is normalized about the 21st of each month (so you can compare to winter/summer solstices) per m^2 of a flat panel. I suggest panels are adjusted on the 9th of each month to the optimal value(s) for that month. Or for winter just use July and point it that way at the start of winter

June: Optimal Azimuth: Due North. Optimal Elevation (Degrees above horizon): 32.5 degrees. Clear sky kWh: 8.0097. Weather clearness index (22 years avg): 0.7. Expected daily insolation: 6.36 hours equivalent
July: Optimal Azimuth: Due North. Optimal Elevation: 35.75 degrees. Clear sky kWh: 7.9656. Weather clearness index: 0.71. Expected daily insolation: 6.55 hours equivalent
July: Optimal Azimuth: Due North. Optimal Elevation: 46 degrees. Clear sky kWh: 7.7877. Weather clearness index: 0.67. Expected daily insolation: 6.58 hours equivalent
 
Last edited:
agentrfr said:
For where Okty stays he asked specifically for winter (June-Aug)

How to use: Expected daily insolation * size of panel W is how many Wh you can genreate: Eg. 4 hours a day for a 1kW panel is 4kWh generated per day on average (panels only run at 80% efficiency when they get hot, so take that into account too not done above)

Bruh, it's moerse hot where you stay.

Here are results:
Data is normalized about the 21st of each month (so you can compare to winter/summer solstices) per m^2 of a flat panel. I suggest panels are adjusted on the 9th of each month to the optimal value(s) for that month. Or for winter just use July and point it that way at the start of winter

June: Optimal Azimuth: Due North. Optimal Elevation (Degrees above horizon): 32.5 degrees. Clear sky kWh: 8.0097. Weather clearness index (22 years avg): 0.7. Expected daily insolation: 6.36 hours equivalent
July: Optimal Azimuth: Due North. Optimal Elevation: 35.75 degrees. Clear sky kWh: 7.9656. Weather clearness index: 0.71. Expected daily insolation: 6.55 hours equivalent
July: Optimal Azimuth: Due North. Optimal Elevation: 46 degrees. Clear sky kWh: 7.7877. Weather clearness index: 0.67. Expected daily insolation: 6.58 hours equivalent
Click to expand...

Thanks very much for the info - seems my initial estimate of 6 hours was not far off :)

However, I took 4 hours of equivalent sun hours into my calculations (lets say it includes panel inefficiencies etc).

So, as a 'starter pack', I am thinking of getting the following:
6 x 260W Solar panels http://www.solarsolved.co.za/index.php/products/solar-panel/ae-260watt-solar-panel-detail
5kVa Inverter/Charger http://www.solarsolved.co.za/index....5000watt-48v-off-grid-inverter-charger-detail
3 x 2.4 kW Li-Ion battery http://www.solarsolved.co.za/index....4kwh-lithium-ion-battery-excl-brackets-detail

Can always later on add another battery pack to the system.
 
Okty said:
Thanks very much for the info - seems my initial estimate of 6 hours was not far off :)

However, I took 4 hours of equivalent sun hours into my calculations (lets say it includes panel inefficiencies etc).

So, as a 'starter pack', I am thinking of getting the following:
6 x 260W Solar panels http://www.solarsolved.co.za/index.php/products/solar-panel/ae-260watt-solar-panel-detail
5kVa Inverter/Charger http://www.solarsolved.co.za/index....5000watt-48v-off-grid-inverter-charger-detail
3 x 2.4 kW Li-Ion battery http://www.solarsolved.co.za/index....4kwh-lithium-ion-battery-excl-brackets-detail

Can always later on add another battery pack to the system.
Click to expand...

Yeah looks close enough for a start. Just check with them that the inverter is happy and tested with those batteries

Just to check, you are aware that your selected inverter is Off-Grid, right? I.e. you are not grid tied with your panels, and any excess power you generate will be "wasted"

Also those panels are 30 VMp, so you would connected them as 3S2P to match the inverter's optimal MPPT range of 60-115 V

Otherwise post some pictures when it's up :D
 
agentrfr said:
Yeah looks close enough for a start. Just check with them that the inverter is happy and tested with those batteries

Just to check, you are aware that your selected inverter is Off-Grid, right? I.e. you are not grid tied with your panels, and any excess power you generate will be "wasted"

Also those panels are 30 VMp, so you would connected them as 3S2P to match the inverter's optimal MPPT range of 60-115 V

Otherwise post some pictures when it's up :D
Click to expand...

Hi

Yes, already did the calcs for the panels, i.e. 3S2P, same principle as hooking LEDs :p

I'll double check the inverter, but from its data sheet it can receive a solar and AC input, and you can priorities which input charges the batteries.

Yes, don't want to add any energy back into the grid (I believe it is actually 'illegal' here to do that).
 
*breathes*...

I started investigating this again after the last price hike, and the 3rd random power outage in my area...

And it started to look feasible to at least take the lights off the grid... until I saw those battery prices :(
 
ToxicBunny said:
*breathes*...

I started investigating this again after the last price hike, and the 3rd random power outage in my area...

And it started to look feasible to at least take the lights off the grid... until I saw those battery prices :(
Click to expand...

Yes, Lithium batteries are still very expensive.

I was first looking at getting the so called "deep cycle" lead (crystal, gel etc) batteries, but looking at their cycles per DoD, a few things became apparent:

You can deep cycle them, but then you reduce their life cycles dramatically. For instance, the one from First National Batteries will give you about 200 cycles if you DoD it to about 80% (phoned them to confirm). :/

So, you are basically forced to go about maximum 50% DoD on them, and even then the cycle life is not that great (about 3 years if cycled daily). Then take into account each battery weighs about 60kg, will have to buy link cables and battery controllers for parallel operations etc.

I found one crystal lead battery that actually had decent cycles for about 60 % DoD rate (would last about 5 years if cycles everyday), but the price difference between them and the 3 Lithium packs I wanted to buy was about R5000.

Those Lithium batteries on the other hand weighs about 28kg each, includes cables to plug and play them into parallel operation and each box has its own battery controller included. Also, their cycles are just over 4000 @ 90% DoD - that's about 10 years for a daily cycle (yes, the battery capacity will degrade over time, but that's also relevant for lead batteries)!
 
Okty said:
Yes, Lithium batteries are still very expensive.

I was first looking at getting the so called "deep cycle" lead (crystal, gel etc) batteries, but looking at their cycles per DoD, a few things became apparent:

You can deep cycle them, but then you reduce their life cycles dramatically. For instance, the one from First National Batteries will give you about 200 cycles if you DoD it to about 80% (phoned them to confirm). :/

So, you are basically forced to go about maximum 50% DoD on them, and even then the cycle life is not that great (about 3 years if cycled daily). Then take into account each battery weighs about 60kg, will have to buy link cables and battery controllers for parallel operations etc.

I found one crystal lead battery that actually had decent cycles for about 60 % DoD rate (would last about 5 years if cycles everyday), but the price difference between them and the 3 Lithium packs I wanted to buy was about R5000.

Those Lithium batteries on the other hand weighs about 28kg each, includes cables to plug and play them into parallel operation and each box has its own battery controller included. Also, their cycles are just over 4000 @ 90% DoD - that's about 10 years for a daily cycle (yes, the battery capacity will degrade over time, but that's also relevant for lead batteries)!
Click to expand...

Yeah I know..

But the pricing of batteries means I have to park the idea for now again, the payback period is still just a bit too long to justify it at the moment.
 
ToxicBunny said:
Yeah I know..

But the pricing of batteries means I have to park the idea for now again, the payback period is still just a bit too long to justify it at the moment.
Click to expand...

Disagree. Actually at the moment its cheaper to buy batteries (if you can afford them).

R14,720 for 2.4KW LFP Battery including VAT.

2.4KW * 80% = 1.9KW usable including rough loss for charging in /out.
1.9kw * 3650 (10 years cycle warranty) = 7000KWhr (will be longer, but we'll work on that as a rough guide)

14400 / 7000 = R2.05 / KWhr (including vat)

Its less than domestic block 2 tariff at the moment, which is R2.34/KWhr

Its marginally more expensive than domestic block 1 tariff of R1.92/KWhr.
Although, if you include the daily fee (R5 / day or R150 / month). R1.92 x 600 + R150, the Block 1 tariff is actually R2.17 at cheapest, R2.9 ish at worst.

CoCT is basically charging MORE than battery power costs this year.
Each year will see more increases. Its literally cheaper to go battery powered already.

*This is very simplistic, and doesn't include finance charges etc, but if you can afford to, its a no brainer.
 
lsheed_cn said:
Disagree. Actually at the moment its cheaper to buy batteries (if you can afford them).

R14,720 for 2.4KW LFP Battery including VAT.

2.4KW * 80% = 1.9KW usable including rough loss for charging in /out.
1.9kw * 3650 (10 years cycle warranty) = 7000KWhr (will be longer, but we'll work on that as a rough guide)

14400 / 7000 = R2.05 / KWhr (including vat)

Its less than domestic block 2 tariff at the moment, which is R2.34/KWhr

Its marginally more expensive than domestic block 1 tariff of R1.92/KWhr.
Although, if you include the daily fee (R5 / day or R150 / month). R1.92 x 600 + R150, the Block 1 tariff is actually R2.17 at cheapest, R2.9 ish at worst.

CoCT is basically charging MORE than battery power costs this year.
Each year will see more increases. Its literally cheaper to go battery powered already.

*This is very simplistic, and doesn't include finance charges etc, but if you can afford to, its a no brainer.
Click to expand...

Yeah its the problem of if I can afford it....currently its just outside of the realm of what I can reasonably get my hands on...

I obviously need to spend a fair chunk more time on the idea, but I was contemplating just taking all but the large scale power draws onto a solar system of some form... or at least building up to something like that, though I would need a decent Inverter that can handle the full final load (or do I have this concept wrong?)
 
ToxicBunny said:
Yeah its the problem of if I can afford it....currently its just outside of the realm of what I can reasonably get my hands on...

I obviously need to spend a fair chunk more time on the idea, but I was contemplating just taking all but the large scale power draws onto a solar system of some form... or at least building up to something like that, though I would need a decent Inverter that can handle the full final load (or do I have this concept wrong?)
Click to expand...

Panels and inverter are cheap.


- 5KW offgrid capable Axpert is < R13,000 (peak draw is 8kw).
If you need more sustained draw, can parallel them for higher levels.

- Panels R2000 odd for 325W panels

If you say use 20KW / day x 30. = 600KW / month

Would likely be something like this in terms of hardware (will depend on location etc, as irradiance levels differ). I'm in CPT, so calculations below for that. If up north, will be a lot better in irradiance terms.

20kw per day usage. So you want to generate at least 20KW / day.

20KW generation = +-6KW panels @ 3hrs of full generation in winter.
Summer you'll be throwing away energy, winter, should break even with 6KW though assuming roughly 20kw / day use.

So winter sized 6KW per hour generation = 18 panels.

18 x 2000 = R36,000
Mounting roughly (R150 / panel) = 2700
Inverter = R13,000
Batteries - R14,400 for 2.4KW
You'll need roughly 14KW of storage for winter if using 20KW/day

7 x 14,400 = R100,800


So a full offgrid system that will last 10 years will cost you about:
36000+ 2700
13000 + 13000 (5 years life for inverter to be safe)
100800
======
R165,500

For a 6KW panel / 5KW inverter / 14KW battery system.

600kw month *12 * 10 = R165,500

Or a full cost of about R2.2986 per KW for the entire system.
Again, thats cheaper than CoCT now. Next year will be more expensive...

If you calculate in the finance costs for the R165,500 vs the CoCT or Eskom yearly increases, you'll find its probably going to turn out cheaper (dependent on your bond rate).

If up North, then the numbers get better. This is a rough guesstimate, but you can see where costs are headed.

Lithium prices are dropping, but even at current pricing it still makes sense to jump.
 
lsheed_cn said:
Panels and inverter are cheap.


- 5KW offgrid capable Axpert is < R13,000 (peak draw is 8kw).
If you need more sustained draw, can parallel them for higher levels.

- Panels R2000 odd for 325W panels

If you say use 20KW / day x 30. = 600KW / month

Would likely be something like this in terms of hardware (will depend on location etc, as irradiance levels differ). I'm in CPT, so calculations below for that. If up north, will be a lot better in irradiance terms.

20kw per day usage. So you want to generate at least 20KW / day.

20KW generation = +-6KW panels @ 3hrs of full generation in winter.
Summer you'll be throwing away energy, winter, should break even with 6KW though assuming roughly 20kw / day use.

So winter sized 6KW per hour generation = 18 panels.

18 x 2000 = R36,000
Mounting roughly (R150 / panel) = 2700
Inverter = R13,000
Batteries - R14,400 for 2.4KW
You'll need roughly 14KW of storage for winter if using 20KW/day

7 x 14,400 = R100,800


So a full offgrid system that will last 10 years will cost you about:
36000+ 2700
13000 + 13000 (5 years life for inverter to be safe)
100800
======
R165,500

For a 6KW panel / 5KW inverter / 14KW battery system.

600kw month *12 * 10 = R165,500

Or a full cost of about R2.2986 per KW for the entire system.
Again, thats cheaper than CoCT now. Next year will be more expensive...

If you calculate in the finance costs for the R165,500 vs the CoCT or Eskom yearly increases, you'll find its probably going to turn out cheaper (dependent on your bond rate).

If up North, then the numbers get better. This is a rough guesstimate, but you can see where costs are headed.

Lithium prices are dropping, but even at current pricing it still makes sense to jump.
Click to expand...

My Problem is more Durban costing than anything else... I'm paying R1.41 per Kwh of power, and its difficult as fsck to beat that price currently...
 
ToxicBunny said:
My Problem is more Durban costing than anything else... I'm paying R1.41 per Kwh of power, and its difficult as fsck to beat that price currently...
Click to expand...

If you can feedback to grid in Durbs, then Solar is still worth it, its the storage that costs the money.
Cape Town is already cheaper to be offgrid. Thats going to bite them, as the larger taxpayers are fully capable of going offgrid completely - its simply more reliable than being ongrid.

If you revisit the calculation, as pure generation -


18 x 2000 = R36,000 (6KW of panels)
Mounting roughly (R150 / panel) = 2700
5KW Inverter = R13,000 x 2 (1 per 5 years life)

64700 would get you at least 20kw generated in winter/ 30kw in summer per day.

Say 20kw @ 6 months + 30kw @ 6 months or 182 * 20KW + 183 * 30KW = 3640KW + 5490KW or 9130KW/ year
over 10 years - 91300KWhr generated.

91300KW generated for R64700 cost, or R0.70/KWhr

Lifetime of panels is 25years+, so reality is that cost will be less.
 
You must log in or register to reply here.
Back
Top
Sign up to the MyBroadband newsletter
X