Using existing panels to power off-grid

[1Watt]

Hi

I recently moved home and at the bottom of the garden we have an outbuilding that I'd like to turn into a mancave but has no power (and no practical way to add grid power).

The previous owner had a solar setup of some type as there are 4 x 400W panels on the roof, but no other equipment remains in the building.

The panels are 32.2v & 9.38A each. x4 = 128.8v / 37.5A (I confirmed the voltage is ~correct using a meter)

I'd like to install an MTTP, battery and inverter to replace the missing equipment, I've watched some videos and done some reading but I'd appreciate someone with experience offering advice on my choices below.

For the MPPT I was thinking of using a Victron BlueSolar Charge Controllers MPPT 100/50 - it seems fine for amps (50) but it's maximum PV open circuit voltage is 100V, can I can around this but re-configuring the panels in parallel (64v instead of 128v)? Or should I be looking for an MTTP that can handle the panels in series (which would be helpful as it avoids getting onto the roof) ?

I'm planning to buy a '12V Lifepo4 Battery 100AH 1280Wh Lithium Battery', there seems to be lots of generics on Amazon - are these ok or should I be going for a known brand?

And for the inverter I was planning on this one:

Output Waveform	Pure Sine Wave
Rated Power	2500W
Peak Output Power	5000W (<10ms)
Frequency	50Hz ± 2Hz
Conversion Efficiency	> 90%
Input Voltage	12V DC (10V-15.5V)
Output AC Voltage	230V ± 5%
Low Voltage Alarm	10V DC ± 0.5V
Low Voltage Cut-off	9V DC ± 0.5V
Machine Size	360 * 160 * 105 (mm)
Net Weight	3kg

I'd really appreciate any feedback at all. Thanks

 

[DIYrich]

If you have 4 panels, you can either put them in series for 128v @ 9a, or in parallel for 32v @ 36a. Or 2s2p for 64v@ 19a.

 

[1Watt]

If you have 4 panels, you can either put them in series for 128v @ 9a, or in parallel for 32v @ 36a. Or 2s2p for 64v@ 19a.

Great, thanks for confirming that.

Do you think the mppt I suggested would be a good fit in that case?

 

[yodamota]

since they are going to be on your roof. I don't see a reason to maximize voltage. Id put them 2S-2P. The VOC is 41V. So that would be 82VOC which I believe is safe using the victron 100/50amp

here are the panel specs for everyone

 

[MichaelK]

Couple of problems I can see right off the bat, poor choices for equipment. Let's look at your panels. With four 400W panels, assuming you stuck with 12V, you'd be generating (4 panels X 400W)/12.5V charging = 128A. Even with 85% de-rating for normal real-world production, that's ~109A. So, you want to pair a 50A controller to an array that can put out at least 109A. Not good!

A more sensible choice is to upgrade immediately to 24V, which will bring your real-world amps down to ~54A. Still a bit high for the controller selected but not seriously too high.

Battery is way too small for your application also. If you assume you don't want to charge with over 1/4th of C, that's just 25A for a 100Ah battery. Ideally, you'd want a 200Ah battery at 24V. That will happily accept the high charging amps that those panels will output.

Since that controller only has a 100V limit, you MUST wire your panels no more than 2S2P. If you get a 200V controller instead, you could wire all four panels in series, and buy a smaller gauge (cheaper) copper wire running from the array to the controller.

With the larger batteries, you'll have a system robust enough to feed power tools, or a full-size refrigerator. I myself have a Schneider Conext SW4024, which I am VERY happy with. It an run all my power tools, a cement mixer, and a 1.6hp air-compressor.

 

[1Watt]

Couple of problems I can see right off the bat, poor choices for equipment. Let's look at your panels. With four 400W panels, assuming you stuck with 12V, you'd be generating (4 panels X 400W)/12.5V charging = 128A. Even with 85% de-rating for normal real-world production, that's ~109A. So, you want to pair a 50A controller to an array that can put out at least 109A. Not good!

A more sensible choice is to upgrade immediately to 24V, which will bring your real-world amps down to ~54A. Still a bit high for the controller selected but not seriously too high.

Battery is way too small for your application also. If you assume you don't want to charge with over 1/4th of C, that's just 25A for a 100Ah battery. Ideally, you'd want a 200Ah battery at 24V. That will happily accept the high charging amps that those panels will output.

Since that controller only has a 100V limit, you MUST wire your panels no more than 2S2P. If you get a 200V controller instead, you could wire all four panels in series, and buy a smaller gauge (cheaper) copper wire running from the array to the controller.

With the larger batteries, you'll have a system robust enough to feed power tools, or a full-size refrigerator. I myself have a Schneider Conext SW4024, which I am VERY happy with. It an run all my power tools, a cement mixer, and a 1.6hp air-compressor.

Hi @MichaelK

Thanks so much for the detailed feedback, I feel like I've been schooled

The SW4024 isn't available in the UK but there are lots of similarly specced units so I'll source one of those.

 

[MichaelK]

The SW4024 isn't available in the UK but there are lots of similarly specced units so I'll source one of those.

Seems like Schneider does make a 230V, 50Hz version? Is it just that it's not marketed in the UK?

https://solar.se.com/au/wp-content/uploads/sites/3/2021/10/SW-230V-Installation-Guide_975-0637-01-03_ENG.pdf

 

[Hedges]

Victron and Schneider are both good stuff.
You started out mentioning components, separate MPPT and inverter.

What loads do you need to power? Any big motors (which are difficult to start)?

How much power daytime, how much night time, and how many months/years to you expect to use it? How many days per year?

There are inexpensive and expensive hybrid/AIO. Some 24V, some 48V.
Prices probably range from $500 to $7k, something for any budget.
MPP, EG, Deye, SolArk (expensive Deye for US market), Outback, SMA, others.

You may also find used deals, depending on what your local market including eBay is like.

Lithium batteries are probably the way to go for daily cycling. Pay attention to maximum current, which needs to support surge starting motors.

Essex freezes. Lithium don't like that (when charging). Need to keep warm. Maybe look for a battery with electric heaters. Definitely with a BMS that prevents charging when too cold.

Sometimes lead acid batteries still make sense, either sealed or flooded wet cell. They do work in the cold (if not discharged deeply and frozen to the point of rupturing.) A fraction the number of deep cycles of lithium. Might be a good choice for weekly use, could last a number of years.

 

[1Watt]

Hi @Hedges

What loads do you need to power? Any big motors (which are difficult to start)?

I have generally fairly low power requirements:

iMac with dual screens which I measured at 400W peak but ~100W in general and ~5W in standby.​

TV 240W peak / 130 average.​

Lighting approx 50W max​

So 700W would likely be my peak load, but for the sake of future-proofing I'd like to have 1500-2000W of load capacity.

I'd plan to use it 2-3 days each week for 4-5 hours at a time, I would mostly be using it during the day and hardly at all at night.

I read this "LiFePO4 batteries can typically operate within a temperature range of -20°C to 60°C", it hardly ever reaches 0°C in Essex, UK so I figured they would be ok, is that not that case?

I'm definitely looking at budget options if possible, I just moved house the stamp duty tax was eye-watering.

My main goal is to use it as my photo-editing maccave and home gym.

 

[Hedges]

Down to 0C, no problem charging at modest rate (like 0.1C). Warmer, can probably charge 0.5C

No motors to run, you can find low cost all-in-ones, 24V or 48V. With only 2000W, 12V is reasonable too, but most are at least 24V. The cheap ones may or may not be so reliable, but cheap to start and you can replace if they do fail.

Hardly at night, could use second hand automotive starting batteries, if PV panels will generally supply the power needed. Batteries are the most expensive part, but you need little to no storage. Just shut off inverter when you leave so it's idle draw doesn't drain them. Not sure, maybe component MPPT SCC separate from inverter is easier way to leave SCC running but inverter off. Then you could easily do 12V.

PV panels are cheap, especially used. Put in extra to make up for cloudy times, but have SCC limit charge current to something FLA or AGM likes, maybe 0.12C to 0.2C.

 

[1Watt]

Thanks so much for or the all of the advice guys, I. think I understand what I nee now, but I still have a few of questions if anyone is able to help.

Here are the parts I'm currently planning on buying:

2 x Renogy Core-12V 100Ah Deep Cycle Lithium Iron Phosphate Battery £640.00
1 x Victron SmartSolar Charge Controllers MPPT 100/50 (specs attached) £235.00
1 x ECO-Worthy 3000W Off Grid Pure Sine Wave Inverter 24V to 220V £290
2 x circuit breaker 40A (one for each string) £10EA
1 x circuit breaker 80A before charge controller £10
1 x switch/fuse from batteries to inverter 200A ??
1 x cables

Batteries wired in series = 24V
Panels wired 2S2P = 82.4V (open-circuit) / 46.8A (charge controller limit is 100V / 50A)

Does that seem reasonable? Am I missing any parts / desirable extras?

Regarding fuses, do I need a fuse on each parallel string and then another where they combine, or just a fuse for the whole array before the solar charger?

Eg each string is (12.28 ISC x 2) x 1.56 = 38.32A (40A fuse?)
Combined strings = 76.64A (80A fuse?)

And then another fuse between the battery and the inverter, should this be 200A? Not sure how to calculate this one.

Lastly, what gauge of copper cable should I be using between the solar charger -> battery / battery -> inverter, I couldn't figure out how to calculate this, my cable lengths will be 12-18 inches max.

 

[MisterSandals]

That cheap audio breaker pictured is not a good choice at all.

 

[1Watt]

That cheap audio breaker pictured is not a good choice at all.

Could you point me the direction of a appropriate part?

Should I be using DIN rail mounted breakers like this?

Or would one like this be appropriate?

 

[MisterSandals]

Those look like they are made for what you are doing, though i am not familiar with the brands.

 

[Hedges]

PV string fuses only needed if 3 or more in parallel.

Batteries are > 50% of cost in this list? (PV panels not included)

How may Wh/day do you plan to draw from batteries?
If system just sits there at night not powering anything, has PV when in use during the day and batteries just buffer for momentary high loads, consider two used automotive starting batteries from the junk yard. At least to get the system running, and only replace if inadequate.

 

[yodamota]

I will admit, I appreciate having a circuit breaker for our solar panels. Being able to disconnect the power is a relief for me. Also makes it easy to do maintenance on the system

 

[1Watt]

PV string fuses only needed if 3 or more in parallel.

Batteries are > 50% of cost in this list? (PV panels not included)

How may Wh/day do you plan to draw from batteries?
If system just sits there at night not powering anything, has PV when in use during the day and batteries just buffer for momentary high loads, consider two used automotive starting batteries from the junk yard. At least to get the system running, and only replace if inadequate.

I was thinking forward to times when its cloudy/raining and i still need power. I did consider the lead acid battery option but i was confused about their charging properties, or rather depth of discharge, it reads like they cant store anywhere near their rated power?

 

[Hedges]

If you want power from batteries, that will cycle them. Lead-acid typically has 1/5 to 1/10th the cycle life of lithium.

Automotive starting batteries are for shallow very high current use.
Golf cart batteries are for deep cycle use. ("Marine deep cycle" not so great.)
Some AGM and Gel batteries are for repeated deep cycle.

People usually say to not cycle lead-acid below 50% SoC, but that is so you get hundreds to thousands of cycles. If only for a rainy day, you can do 80% on true deep cycle batteries.

I have SunXtender AGM, should be good for 650 cycles to 70% DoD. I only cycle them during grid failures, so expect 10 year life. For daily cycling, lithium would now be cheaper.

If you can get started with a couple used automotive batteries, free to maybe $100 the pair, that at least delays spending 650 GBP on lithium (which may get cheaper later, or may not.)

Charge controller for lead-acid should have a battery temperature sensor and adjustable settings. Of course get one which will also support LiFePO4 for the future.

 

[SamDeleted]

I did consider the lead acid battery option but i was confused about their charging properties, or rather depth of discharge, it reads like they cant store anywhere near their rated power?

These are going really cheap ,

4x 6v 180ah dry fit gel traction, 4.3kwh total , 700 cycles to 70%dod (makes for 3kwh usable capacity) . That's 1kwh more usable battery capacity that the 2x 12v /100ah LiFePo4 you're looking at



£1000~ worth of batteries when new , only £150+p&p


, he's a good seller (check feedback) , very reputable I have bought from him myself

Exide Sonnenschein GEL Battery Traction Block GF 06 180v Autopol for sale online | eBay

Find many great new & used options and get the best deals for Exide Sonnenschein GEL Battery Traction Block GF 06 180v Autopol at the best online prices at eBay! Free delivery for many products!

www.ebay.co.uk

 

[SamDeleted]

These are going really cheap ,

4x 6v 180ah dry fit gel traction, 4.3kwh total , 700 cycles to 70%dod (makes for 3kwh usable capacity) . That's 600wh more battery capacity that the 2x 12v /100ah LiFePo4 you're looking at



£1000~ worth of batteries when new , only £150+p&p


, he's a good seller (check feedback) , very reputable I have bought from him myself

Exide Sonnenschein GEL Battery Traction Block GF 06 180v Autopol for sale online | eBay

Find many great new & used options and get the best deals for Exide Sonnenschein GEL Battery Traction Block GF 06 180v Autopol at the best online prices at eBay! Free delivery for many products!

www.ebay.co.uk

A good way to compare batteries is by looking at their over all lifetime capacity Vs cost

Doing the maths those renogy LiFePo4s take a slight edge. at least based off manufacturer quoted figures (5,000 cycles on Lifepo4 is yet to be proven in the real world)

4x Sonnenschein Gel Traction batteries 06v 180ah:

£200 ÷ 2,100kwh = £0.095 per lifetime usable KWh

4 x (6v x 180ah) = 4.3kwh
4.3kwh X 70%dod = 3kwh usable capacity
3kwh X 700 cycles = 2,100kwh lifetime capacity

2x Renogy 12v 100ah LiFePo4:
£679.98 ÷ 9,600kwh = £0.07 per lifetime usable KWh

2 x (12v x 100ah) = 2.4kwh
2.4kwh X 80%dod = 1.92kwh usable capacity
1.92kwh X 5000 cycles = 9,600kwh lifetime capacity