# Need help Designing 10KW S PV System on Paper

Solar Expert Posts: 44
Heyllo,

I am gaining knowledge in this field slowly and steadily. I already did a 2kw Solar PV System and its running well and fine.

Now, I want to go a bit further, but this time just for my knowledge. Please help me.

SYSTEM

A 300W x 30 = 9,000W load is to be run for a time period of 16 hours. This system is completely off-grid

Out of which an 8 hour will be day time. Since Sun light is available from 8AM to 5PM. Now we need a backup for 8 hours.

9,000 Watts x 8 hours = 72,000 Wh (At 240 VAC Current)

Total current = 300 Amperes for 8 hours.

Since the above is in 240VAC, at 48VDC it should be 5 times more. = 1500Ah

This would require a 20 Batteries of 150Ah to stay above 50% DOD.

Is it correct?
«1

## Comments

• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Need help Designing 10KW S PV System on Paper

welcome,
interesting scenario, but far underestimated and for the record this is a huge setup.

you can't count on full pv power for 8hrs a day and 4hrs may be more realistic on average for most times of the year depending on your location. go back to basics and look at your required loads for the day as a whole. this is 9kw x 16hrs for 144kwh. for a 48v battery bank this is 144kwh/48v=3000ah. now a battery bank can't drop below 50% soc so this would normally double the bank capacity, but the catch here is up to about 5hrs of the loads will be fed direct from the pvs. if no sun then count on needing the full 48v 6000ah battery bank for 1 day allowing to be no greater than 50% dod (same as 50% soc) on the battery bank. now if you get full sunshine and your pvs are capable of delivering the load power required for both charging and load requirements it can be reduced to about 3/4 of that capacity or 4500ah, but the sun doesn't always shine and more than 1 day autonomy would up the bank size proportionally.
• Banned Posts: 17,615 ✭✭
Re: Need help Designing 10KW S PV System on Paper

Hold the phone: you want to run 9kW load for 16 hours off grid? That's 144,000 Watt hours. Don't count on the daylight of 8 hours supplying it because that has to recharge the batteries when there is no daylight and you will likely get closer to half 8 hours of good sun, no matter how long your days are.

What you'd be looking at:
48 Volt system with 6000 Amp hours of battery
37,400 Watts of PV, divided up to feed ten 60 Amp charge controllers
A couple of Outback Radians or Xantrex XW 6048 stacked to provide the 9kW output power
A few million dollars to pay for it with.

I sure hope you slipped a digit somewhere and were thinking of supply 9kW hours of load total over 16 hours, not 9kW for 16 hours.
• Solar Expert Posts: 2,381 ✭✭✭✭
Re: Need help Designing 10KW S PV System on Paper

That is about double what my houses uses in July with a pair of 3 ton AC units running nearly 7X24 at close to a 75% duty cycle. It would be a real challenge to get it off grid.
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

Well, there is no error. WE are setting up a office which is out of the city limits. Getting a 24/7 3-Phase Grid Supply of 10KW Rating has an expense of around 10k USD and the per unit cost is 10 cent plus taxes. The voltage is not stable and its not actually 24/7 suuply. Electricity supply is not continuous.

I know the price expected for a solar setup must be around 18-20K USD but then, I will not have to pay anything and I Will have Gensets to support and a stable Power Supply.
• Super Moderators, Administrators Posts: 31,688 admin
Re: Need help Designing 10KW S PV System on Paper

Assuming you are somewhere around New Delhi India, a fixed solar array will output around (noon time equivalent hours of sun per day):
```Month      Solar Radiation (kWh/m2/day)
1      5.56
2      5.90
3      6.65
4      7.01
5      6.91
6      6.34
7      5.28
8      5.25
9      5.89
10      6.17
11      5.74
12      5.56
Year      6.02
```

Battery bank, assuming 2 days of storage and 50% maximum discharge at 48 volt bank:
• 9,000 watts * 1/48 volt battery * 8 hours * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 7,059 AmpHour @ 48 volt battery bank

The size of solar array to supply rated output in January would be:
• 9,000 watts * 8 hours * 1/0.52 off grid sys efficiency * 1/5.56 hours average sun Jan = 24,903 Watt Array minimum

Add a couple of large AC inverters, backup genset (you need that anyway), plan on replacing the batteries every 5-10 years or so (depending on batteries and how well they are taken care of), etc... Not an inexpensive system.

Maybe around \$50,000 USD for solar panels and \$70,000 USD for industrial "fork lift" type batteries (could last upwards of 10-20 years), plus mounting hardware for array, wiring, inverters, backup battery chargers, etc... Probably \$200,000 just for material in the US (as a first guess). I would not begin to guess what the costs in your region would be--But something more than 10x what you were hoping to pay for off grid solar power.

I understand that energy and fuel costs are not cheap... But utility power at \$0.10 per kWH (USD?) is not a bad price. You could design a battery+AC Inverter system (with backup gensets) first that used utility power first, then add solar panels as it makes economic sense later.

In the US, off grid solar power costs around \$1 to \$2+ per kWH or about 10x the cost of utility power. This puts a huge incentive on you to conserve. Computer systems, lighting, Air Conditioning all use huge amounts of energy and can be reduced by a lot (50% or more) with the correct choices in equipment and construction.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭
Re: Need help Designing 10KW S PV System on Paper
emcc wrote: »
Well, there is no error. WE are setting up a office which is out of the city limits. Getting a 24/7 3-Phase Grid Supply of 10KW Rating has an expense of around 10k USD and the per unit cost is 10 cent plus taxes. The voltage is not stable and its not actually 24/7 suuply. Electricity supply is not continuous.

I know the price expected for a solar setup must be around 18-20K USD but then, I will not have to pay anything and I Will have Gensets to support and a stable Power Supply.

Think again.
\$10,000 to connect to the grid is not that much for 3 phase power.
There is no way you can duplicate that in off-grid supply for even double the money, and there will be on-going maintenance/repairs/replacements all of which you will be responsible for.

If the problem is unreliable grid power than what you need is some form of Uninterruptable Power Supply to protect critical loads against blackouts. This would be much less expensive than trying to supply the whole load from batteries.
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

hm.. ! Well from what I had calculated the price of the system, it was not near from what you have calculated. The batteries here with 5 yr replacement is approx 230USD of Capacity 150Ah 24V.

Adding 50 Batteries would be 7500Ah, the total price is 11,500USD. (At max)

Here, a cell of 1.8 watt cost approx 0.56USD, for 24,000 we need approx 14000 Cells. 7840USD (at MAX)

Adding Inverter/Charger = I guess it would be another near 2000USD (or may be less?)

Installation I was quoted at Another 4k USD if I get it done from a professional company. Or if I hire a professional installer and give her the workers I have in my office. It would not be half of what is quoted.

Total comes approx:

11500
7840
2000
4000
25500 USD Approx.

I am not saying its not cheap or is very reliable. But if I install this system, keeping in mind the 5 year battery replacement, at which I will have to pay half of the amount to replace it under warranty.

The Utility Bill I have to pay would be approx 620USD Per Month.

\$26,000/\$620 = 42 Months or 3.5 Yrs

After 5 years, If I replace batteries under warranty at half of cost = \$5750 or \$6000 at max

Again, paying \$620 per month = 10 Months !

Sorry for the long post. Please correct if My math is wrong
• Banned Posts: 17,615 ✭✭
Re: Need help Designing 10KW S PV System on Paper

Well I estimated 6000 Amp hours @ 48 Volts for the power needs. That calls for forklift batteries or 2 Volt cells, as you really do not want lots of small batteries paralleled to get the power.

So four parallel strings of 1500 Amp hours 24 Volt forklift batteries is eight of them @ \$6,536 each, or \$52,288 worth of batteries not including shipping, taxes, enviro fees.

Three parallel strings of twenty-four 2050 Amp hour 2 Volt cells for seventy-two total @ \$1,165 each is \$83,880 worth of batteries not including shipping, taxes, enviro fees.

I don't know where you're getting batteries so cheap, but it sounds like quite a bargain even though they aren't suitable at 150 Amp hours each (you do NOT want 50 parallel connections) and you seem to have left half of them out as you've only accounted for 7000 Amp hours @ 24 Volts, not at 48 Volts.

I believe I mentioned something along the lines of 38kW array. At \$1 per Watt that's \$38,000 more right there. Add another \$5,000 for the ten charge controllers. I'm seeing \$96,000 so far, and that's without inverters (another \$8,000 for two OB Radians - since you may need 3 phase they may not be suitable so you might need something different and more of them) to say nothing of wiring, mounting hardware, fuses/breakers/disconnects and all the other incidentals. A large part of which would probably be paying someone qualified to install it properly and get it all working.

On the bright side the forklift batteries will probably last 20 years, same as the rest of the equipment.
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

Hm.. Well that left me confused. I got a quote from a Solar Solution Installer here, he was giving me a complete Solar Setup of 100KW including installation, batteries and everything at somewhere near 145K USD. How come a 24KW System alone can cost 96K?

There is something wrong with the batteries. The batteries we get here are not the Forklift Batteries you mentioned but the usual Tubular Batteries rated @ 150Ah or 200Ah @ 12v or 24V. These batteries cost around 230USD
• Banned Posts: 17,615 ✭✭
Re: Need help Designing 10KW S PV System on Paper

There's no such thing as a "100kW" installation or a "24kW" installation in the off-grid world. That's grid-tie terminology and tends to refer only to the size of the array.

You need to supply 144kW hours per day. That's like ten times as much as my house uses, or 60 times as much as my off-grid cabin.

The problem with your batteries is you need to configure them for 7,000 Amp hours @ 48 Volts to meet the power demand of 144kW hours per day. That's two in series for each string to get the 48 Volts, then (6,000 / 150) eighty parallel strings to get the 6,000 Amp hours with only 50% maximum depth of discharge (you can not take batteries down to 100% DOD). There is no way that eighty parallel battery connections are going to share current evenly. Some will do most of the work, others will do practically none, and the net result is that all will die prematurely because they won't be used equally.

Even so, that amounts to 160 of these batteries @ \$230 each, or \$36,800. And it won't even work for long.
• Banned Posts: 17,615 ✭✭
Re: Need help Designing 10KW S PV System on Paper

Forgot to mention that with those batteries you'd probably want to limit DOD to 25% (unlike the forklift batteries which can go to 80% so 50% daily is not so troublesome).

144,000 Watt hours on 48 VDC = 3000 Amp hours, so that 6,000 Amp hours with the forklift batteries going to 50% or 12,000 Amp hours with the more conventional batteries limited to 25% DOD.
• Super Moderators, Administrators Posts: 31,688 admin
Re: Need help Designing 10KW S PV System on Paper

It also appears to be that you are quoting "bare cell pricing"?
Here, a cell of 1.8 watt cost approx 0.56USD, for 24,000 we need approx 14000 Cells. 7840USD (at MAX)

You can find completed panels that are in the \$1.00 per watt range -- and you have to include the cost of the racking for mounting and such.

None of here who have answered are in the solar business--So, our suggestions for actual costs are guesses based on US (or Canadian) pricing. We have no idea of the costs in India.

The 24,000 to 32,000 Watt Array is a first pass guess on your power needs and where the system will be installed. You have more exact information than us--And you have to allow for variation in power usage and the amount of sun (aka "weather"). If you were fully off grid, I would be suggesting ~66% to 75% of system design rating as your "base load". You do have a generator + electrical utility to support "dark days" and the occasional heavier than normal loads... But, it is just a trade off between cost of batteries+panels vs utility/fuel costs (and variability in your needs and weather).

As long as you can get materials that meet your cost needs--Great. But some things (like 50 parallel strings of 150 AH batteries) are a bit more difficult to see working "reliably" for many years to come. Batteries do not "play well" together when connected in parallel (difficulties in sharing current, the occasional open/shorted cell failure), and the costs for wiring up such a massively parallel power system (each string should have its own breaker or fuse), etc..

We try to "error" on the conservative side--Regarding estimated output, costs, and safety. If our estimates and your knowledge of local costs/support indicate the project is viable for your needs--That is wonderful. And you have good starting point.

If this is your first large off grid/backup power system, hiring a local engineer/company with knowledge in the field will be a great help (and added expense). After the first time, with you watching what is going on and asking questions, you will be better off building out your next system (and/or doing your own maintenance on this system).

Let us know how things go (and if you have any further questions and/or answers). It sounds like a very interesting project.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

Hm, that makes sense to me now.

I will have to check my data/cost quotes again. Will come back again.

Thank you folks.
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

One thing, if I get a grid tied system, can we avoid batteries in that case?
• Solar Expert Posts: 2,381 ✭✭✭✭
Re: Need help Designing 10KW S PV System on Paper
emcc wrote: »
One thing, if I get a grid tied system, can we avoid batteries in that case?

Yes but it won't help you with outages unles you make a hybrid system with at least some batteries for critical loads.
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

Ok, for a Hybrid system, again I have to design a 30kW Array to support 9kW?
• Banned Posts: 17,615 ✭✭
Re: Need help Designing 10KW S PV System on Paper
emcc wrote: »
Ok, for a Hybrid system, again I have to design a 30kW Array to support 9kW?

No. The battery bank size will depend on the inverter capacity and/or how much off-line time you need. It is recommended that you have at least 100 Amp hours of battery for every 1kW of inverter capacity to keep the ripple to a minimum.

How much can you depend on the grid? 50%? More? Less?
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

Honestly I dont know. Because Currently, we have a load of 40KVA running at the same site. This is of 3 induction motors, 3-phase Rated @ 415V. But the voltage actually from the grid is around 380V or something.

So we are running the 40KVA load from Genset only. Because Induction Motors cannot run at that voltage for our purpose.

BUT since we will be running Desktops, Fans and Lights out of this Solar Array, I dont know if we will have any issues with a voltage of 380V. But we want to keep the cost as low as possible.

The office will be working for 16 hours + 2 hours. If we go for Grid Tied system, we just want, that the load we use in evening, is fed back to Grid in the morning to reverse the meter, if its possible.
• Super Moderators, Administrators Posts: 31,688 admin
Re: Need help Designing 10KW S PV System on Paper

More or less, first look at your loads. Both peak power (say 9kW peak to support starting motors, lunch at a cafeteria, perhaps night time peaks for a call/engineering center supporting world wide customers, etc.--That is the maximum power supplied by the inverter/generator). And then there is the average power--everything running normally. Computers, lights, Heating and Ventilation, water pumps, etc... That may be a 3 kW load (say times 16-10 hours per day for two shifts)--That is the gallons per day of fuel, kWH per day electrical, AH/kWH of battery storage, and kWatt of solar array required to support.

Remember, full noon-time equivalent sun hours per day (one method of measurement based on the average noon-time sun of ~1,000 Watt per square meter) is only ~5-7 hours per day for your area (again, if I have your location correct) with a fixed array facing the equator, tilted to latitude.

So, if you had (for example) a 9kW average load off grid with ~52% system efficiency (conservative average) with 5 hours of sun per day, then the array needed to support that amount of power for 5 hours of sun per day:

9,000 watts * 18 hours per day * 1/0.52 system efficiency * 1/5 hours of sun per day = 62,308 Watt Solar array to "meet" required loads

Or if it is 9kW peak loads and 3kW average load over 18 hours:

3,000 watts * 18 hours per day * 1/0.52 system efficiency * 1/5 hours of sun per day = 20,769 Watt Solar array to "meet" estimated "average" loads

So, yes, it is very likely to have a solar array several times larger than the estimated average load.

To size a Grid Tied system--You can use ~77% system efficiency, and because the solar power is only an "assist", the array and inverter do not have to supply any loads. The Utility will do that. The GT system only assists by reducing power consumption (utility meter turns slower). Or even turns meter backwards in middle of sunny day (credit on your utility bill) and during less sunny times, the meter turns forwards.

A grid tied system, for all the world really looks like a car's electrical system. The "battery" is the utility system (really for all the world like a giant AC battery bank), and the car's alternator is your GT system. When the engine is off, the battery supports all of the loads. And when the engine is running, the alternator supports most/all of the loads, and pushes back energy to "recharge" the car's battery (of course, the utility does not "recharge", your "excess energy" is simply sent to nearby buildings in parallel with the normal utility power flow).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭
Re: Need help Designing 10KW S PV System on Paper

Is that 380 Volts from the grid consistently low, or does it fluctuate a lot? It's about 100 Volts below what we'd have for 3 phase here. It might be worthwhile looking in to some 3 phase transformers to get the right Voltage. They're not cheap of course.

You have some options for taking the motors off the gen and powering them from the grid if you can get the Voltage right or from solar if you can predict accurately the power demands or through a continuous UPS unit to make use of the grid and not the solar.

The office equipment should be the easiest to handle as it is a most likely a light and consistent load (no big start surges). Again, utility Voltage through a transformer to run directly or to recharge batteries of a UPS or take it all off grid.

But you're going to need some hard numbers on the power demands of everything and the available supply in order to make the choices.
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

Voltage is anything between 330v to 420v. Yes it fluctuates a lot. We are not going to power the Motors with SPV. Its just computers, lights, coffee machines fans. That all constitutes to 10kW Load.
• Banned Posts: 17,615 ✭✭
Re: Need help Designing 10KW S PV System on Paper

That kind of Voltage fluctuation is a major problem.

So let's see. You could keep the motors on the generator, running on an "as needed" basis. Expensive to operate, but so would be correcting the grid power or replicating it with off-grid source.

I'm going to guess your utility does not allow grid-tie, or at least not with buy-back of surplus power you might generate.

The office equipment is not going to need 3 phase, so there's a big savings right there. However a 10kW load is not light by any means. Double check on conservation here, as the less power you use the less power you have to buy (whether from the grid or from solar).

At that point it just may be possible to create a system that can power your office equipment from batteries, and have those batteries charged from the poor-quality, fluctuating Voltage, unreliable grid. It might not need solar at all. It might also be possible to pull recharging power from the generators running the motors if they are not over-taxed in that duty.
• Solar Expert Posts: 173 ✭✭
Re: Need help Designing 10KW S PV System on Paper

Which part of India you are?

We are about to setup Solar panels on our office too but after talking with nice people here , I have understood that I need to cut down the loads and reduce the VA by using AFC SMPS,etc.

http://forum.solar-electric.com/showthread.php?18678-Trying-to-setup-Solar-energy-at-office

on a long run, I am sure when we have panels/UPS or inverter system, it is MUCH cheaper than a Generator as fuel costs are going higher and higher..

Dont even think about going off-grid here in India... Let it be a hybrid system Grid as main source and batteries as backup1 then Genset as backup2.

If its your own office, try conservation first.
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

There are some other things which pushes us to install Solar System.

I have discussed it my team of techs, the best we can do is to lower the usage max upto 4kW for crucial loads.
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Need help Designing 10KW S PV System on Paper

60% is a massive reduction, good going! that can be a GAME CHANGER!

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

Yup, we changed the whole planning of electrical design. Hopefully we'll start installing the PV very soon.
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Need help Designing 10KW S PV System on Paper

so what is left to supply?

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

Inverter. Since nothing is available locally. Everything has to be imported. What we are confused about is the Inverter only.

Because they say 600V input Max. So if we install all the solar arrays in series or some to achieve a 400 or 500VDC, how will it justify the current generation to charge battery?
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Need help Designing 10KW S PV System on Paper

Is it an Inver/charger? brand? specs?

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Solar Expert Posts: 44
Re: Need help Designing 10KW S PV System on Paper

I forget, I had written it somewhere the exact model number. But its from Xantrac.
Sign In or Register to comment.