Solar Panel Size

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sryan

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omaha, ne
I am putting in a solar panel in to power some communication equipment. I have 2 12V 250Ah batteries in series. The needed amperage to power the equipment is 7A. How long will the batteries last and what size solar panels do I need? Trying to find what rules to go by, I know perfect world calculations are not reliable in the real world for solar
 
You need to add more specific information. How constant is the 7A load, and is that peak usage or normal usage? Or in other words, how much is the energy use of the load, per day? What's the location? (How much sun you get affects how much solar panel you need.) How critical is it for the equipment to be online 24/7/365?

When you know those things, PV Watts is a decent bare bones tool for estimating how much energy you'll get from a PV panel given a rough location and the direction it faces.

You'll also need a charge controller, and the quality of the charge controller is key to how long the batteries will last. Also, assuming these are lead-acid batteries, you don't want to discharge them more than about 20% per cycle. In other words, hopefully your energy draw is no more than about 1kWh per night.

There's a PV subforum here by the way, maybe the moderator can move this there.
 
A 7A load at 28.8V (full charge, depending on battery type) is 201.6W. Assuming that's the continuous average load (since that's the number we need), that is 4838Wh/day. Your batteries store about 6000Wh, but only about half of that is usable without shortening the battery life. Even less capacity is usable in the winter when the batteries are cold. That means your batteries store about a day's worth of energy. You'll need larger batteries or huge panels to compensate for the lack of storage. Omaha, NE has a minimum of just over 9 hours of daylight in the winter, so you need to accumulate 4838Wh in 9 hours, which is 537W. But your panel ratings need to be about double that to account for the low irradiance and off-angle lighting early and late in the day, so you would be looking at about a 1kW array. However, and I know from experience (I'm a Husker) that it's often overcast for days in a row in Omaha in the winter. Since your batteries must be fully charged every day to avoid the possibility of power failure, you'll need huge excess panel capacity to deal with extended cloud cover. You'll also need to keep the panels at a steep angle to shed snow. All things considered it's not unrealistic to size your panels at about 8-10X the initial calculation. That's roughly a 4-6kW array to handle your load year-round with those batteries. It would be much cheaper to invest in more batteries so you can use a smaller array (perhaps 1-2kW) and ride through periods of cloud and snow cover with your storage capacity. Remember that only 1/3 to 1/2 of the rated battery capacity can really be used, especially in cold weather.

For a different approach, a solar power calculator online predicted that a 1kW array in Omaha would produce 91kWh during the month of December (the lowest month), which is an average of 2.9kWh/day. So the array can be expected to produce about three times its rated power in kWh daily. Since you need 4838Wh daily, that requires about a 1.6kW array. However, this calculation does not account for the duration of bad weather - only averages over month-long time spans. This means that to use an array of 1.6kW (or perhaps 2-2.5kW to be safe) you would need a battery bank that stores enough power to get through any cloudy periods. That's probably about ten times the storage you currently have.
 
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A larger array is going to do very little good if the batteries aren't adequately sized for the load. That's just paying for power that won't get used. On the other hand, nowadays batteries are not really cheaper than solar panels. So it may make sense to just add more solar panels to ensure that the batteries charge more on a cloudy day. But then again, that probably lessens the life of the batteries if it means you're cycling them deeper.

I've gotta believe that 7A is not going to be anywhere near the average draw, although until the OP provides a little more information it's a wild guess how big the array really needs to be. 10x the average power consumption would indeed be in the ballpark, if the array has decent exposure and orientation. If I were to make an assumption that the batteries are already appropriately sized, i.e. they won't discharge more than 30% during, say, a 4-day cloudy spell, then I would say that two 200-250W modules would probably be adequate. These could probably be gotten for less than 50cents/watt on clearance.
 
You should analyze the load better. You could get a DC Wh/Ah meter fairly cheaply and log for about a week.
You don't say if 7A is the maximum or if it draws 7A around the clock.

A combined wind/solar is superior for the obvious reason that your power source is not daylight limited. If it's for some radio equipment atop a tower, the height makes it advantageous for wind generation output.
 
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...power source is not daylight limited. If it's for some radio equipment atop a tower, the height makes it advantageous for wind generation output.
Although putting a wind turbine on the radio antenna tower could cause both vibration and RF interference problems.
And it might rule out using a tip down tower for the turbine if the radio service cannot be interrupted for scheduled or unscheduled turbine maintenance.
 
Although putting a wind turbine on the radio antenna tower could cause both vibration and RF interference problems.
And it might rule out using a tip down tower for the turbine if the radio service cannot be interrupted for scheduled or unscheduled turbine maintenance.

It may have to be a permanent magnet induction machine to avoid all the electronic noise and placed somewhere on the tower that doesn't interfere with radio trajectory. It would dramatically extend the runtime and lifespan of battery by not having the load supported entirely by battery every single night. Noise consideration affects both PV and wind when you add voltage regulator unless you use an inefficient linear drop regulator.

Bad weather tends to increase wind while sunny clear days are usually calmer. Having BOTH seems like a major improvement in reducing down time. The battery reserve capacity would need to be a lot more when it has to account for several cloudy rainy days in a row. All my assumptions are based on this place being way too remote for utility connection or fuel powered generator.
 
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