Solar System Sizing

C

Coleflo

Member
Location
Albany, GA, USA
Occupation
Electrician
I am new to solar. I have a customer that wants to run an outdoor shooting range on solar. There will only be a few lights and receptacles but i want to be able to run 30-40 amps occasionaly. I want to be able to run 20 amps for 4-5 hours at a time. How do i go about sizing the system?
 
Hire a solar company. 😁

Seriously this type of question goes rather beyond the scope of a forum post. But the second point, after infinity's question, is that solar systems are generally sized for the energy they need to produce, not the power. That is, for kWh, not kW. So the max number of amps you want to draw is mostly irrelevant. You need to size the system based on the amount of energy you think will be consumed over a period of time.
 
jaggedben said:
Hire a solar company. 😁

Seriously this type of question goes rather beyond the scope of a forum post. But the second point, after infinity's question, is that solar systems are generally sized for the energy they need to produce, not the power. That is, for kWh, not kW. So the max number of amps you want to draw is mostly irrelevant. You need to size the system based on the amount of energy you think will be consumed over a period of time.
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I'm not disagreeing with you but could someone design a system given what the OP has outlined in post #1?
 
infinity said:
I'm not disagreeing with you but could someone design a system given what the OP has outlined in post #1?
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If the OP's numbers are accurate (big if), then assuming a 120V system, looks like enough to get started. Daily load is 120V * 20A * 5 hours = 12 kWh; peak load is 120V * 40A = 4.8 kW. Next step would be to get info on the solar resource available; battery size would be driven by the longest deficit in available PV power that the system needs to accommodate.

Cheers, Wayne
 
infinity said:
I'm not disagreeing with you but could someone design a system given what the OP has outlined in post #1?
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Yes, but the devil is in the details. A lot depends on how adamant the customer is that the system has sufficient power available at all times. The more unlikely scenarios he wants to cover, the bigger the system has to be. Put another way: 100% is unattainable; how many nines would you like to buy? :D
 
infinity said:
I'm not disagreeing with you but could someone design a system given what the OP has outlined in post #1?
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Not really, as you can see from the discussion. As mentioned, at a minimum one should at least know solar resource (location and shading factor) so one can plug it into pvwatts (https://pvwatts.nrel.gov/) to find out how many kW of solar one needs to get x amount of kWh. I would want to be pretty assured of an accurate actual kWh load for the month I'm sizing for, can't really judge the OP info one way or the other there. Also I might want to know if I'm sizing for use throughout the winter or if it will be primarily summer use and winter matters less. And then, finally, there's arguably a 'safety factor', i.e. once you size the system for expected use, how much of a multiplier are you willing to pay for to assure you have power in the less expected scenario? Or is that not critical?
 
jaggedben said:
And then, finally, there's arguably a 'safety factor', i.e. once you size the system for expected use, how much of a multiplier are you willing to pay for to assure you have power in the less expected scenario? Or is that not critical?
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Hence, how many nines would you like to buy? :D
 
ggunn said:
Don't feel bad; PV and battery sizing for off grid systems is a non trivial exercise and there is no universally correct answer.
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Just to add:

Aside from design and installation challenges, ownership of an off grid system requires a much deeper understanding of how all this stuff works than does a simple grid tied set-it-and-forget-it PV system. The company I last worked for stopped doing off grid PV systems altogether, not because they are more difficult to design and build than grid tied systems (though they are), but because of the interminable service calls we received after the fact. These were largely due to operation and maintenance issues that came up in systems we built because the customer did not thoroughly understand how to keep their systems working properly.
 
Another key design question: does the customer want a 'pure' PV system (subject to the variability of the PV supply) or would they be happy with a system that has and regularly uses a generator, and uses PV to reduce fuel consumption.

Designing a _battery_ system to run 120V at 20A for 5 hours and 240V at 40A for 1 hour is pretty straightforward. You'd need 22 kWh of energy storage and an inverter capable of supplying 10kW.

Designing a PV system capable of supplying 22 kWh per day, most days, is pretty straightforward. Go to PV Watts mentioned above, and enter the parameters to see what it would take.

Selecting a generator to supply 22 kWh per day when PV doesn't work is actually pretty easy, and that is a small generator easily sourced.

Now the hard stuff.
1) Are the power consumption numbers realistic? How much power do the lights really need? You say 'shooting range', but that could cover lots of ground.
2) Optimizing the PV system to minimize total cost. Do you select a PV system that at most will produce 22kWh in a day, knowing that most days you will have to run the generator a bit? Or do you oversize the PV system so that it regularly produces the 22kWh, but most days there is excess production that just goes to waste?
3) What is the _reason_ for using PV? Is grid power expensive to bring to the site or does the customer just want the good feelings of using solar?

If bringing grid power in is cheap, then just connect to the grid.
 
winnie said:
If bringing grid power in is cheap, then just connect to the grid.
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This. I don't know how much the all in average off grid kWh costs these days, but not all that long ago it was >10X the price of a grid power kWh.
 
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