Simple solar power for outbuilding lights and pumping water

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In this post, we’ll show you the simple steps we took to setup our goat barn with solar-powered lighting and running water.

Note: Though titled as ‘simple’, some will no doubt find this complex. Understandable, however, nothing beyond grade-school math or a calculator is necessary for figuring this out. Take your time and try to understand it, ask questions in the comments if you don’t understand.

Our goal

Our goat barn is over 1000′ from our home, and the thought of running power to it gives us heartburn. Not only would that be tremendously labor-intensive, but also expensive and disruptive. We needed power to light the goat barn when we needed to be in there in the dark, and also to support having running water. We don’t spend more than one hour per day in the goat barn, so the true amount of time we would need to light it or run water was small.

Calculating the loads

To determine what we needed was fairly simple. First, we located the 12v LED lights we wished to use. We wanted something simple and common and found these on Amazon. They had good reviews and only consumed 7w while running. We knew we wanted to install four light fixtures but typically would only have two on most of the time the lights would be on. The maximum “load” of these bulbs (the watts times the hours to get Wh) would be about 28 Wh/day, or .028 kWh.

Next, we knew we needed to pump water from our rain tanks into the barn and out through a faucet and utility sink. This too was easily accomplished by using a 12v Shurflow RV water pump, also available on Amazon. This pump has an internal pressure switch which will turn the pump on when the pressure is low (ie, when a faucet is opened). However, we had an old pressure tank laying around and wanted to run the pump less often than every time we opened the faucet, so we hooked up the pump to fill the pressure tank. When the tank reaches pressure, it triggers the pump to turn off and also has plenty of pressure at the faucet. The maximum load of this pump we calculated at about an hour per day (which is very conservative since it runs more like 10 minutes a day). The amp draw is about 6 amps, so we calculated 12 Volts x 6 Amps = 72 Watts for one hour a day equals 72 Wh or .072 kWh.

So far, we need to support less about 1 kWh per day. No problem!

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Batteries

Now, we’d not typically recommend using deep cycle marine batteries for solar applications, because they’re really not designed for multiple cycles of deep discharging – something you regularly do with solar applications, but we had two on-hand, and let’s be reasonable – we needed to support some pretty small loads. So we wired these together in parallel, which keeps the voltage at 12 V but combines their amps. We did this to be sure that we’d never discharge the batteries below a very very small margin of their capacity, which helps them last a long time.

To help understand how this works, picture this… treat your batteries like a bank account. Treat your loads like withdrawals and your solar input like deposits. If you withdrawal more than you can put back in, you have a deficit (a dead battery). You need to size all your components so that the ratio of withdrawals and deposits keeps the battery happy.

Our system would take about 100 watts per day from the batteries. We need to put at least that much back in. Now.. to figure out what kind of solar panel to get, we needed to know about how many hours of sun we could expect on average. This is called “solar insolation”. There are many useful maps online that show what average hours are for any area. Ours is approximately 5.5 hours. This means that the average amount of usable sun hours per day, across all days of the year and average weather – would be 5.5.

Though we get 5.5 hours of sun a day on average, we can still go a week or so of no meaningful sun in our part of the world. We want to make sure our stuff works when this happens so we might need to support up to 5-7 day withdrawing  100 Watts of power, but with no deposits (no solar). These 5-6 days are called “Days of Autonomy” (DOA), or how many non-sun days we want to run without recharging.We also had to keep in mind that our batteries had to be adequately sized so that we could

We also had to keep in mind that our batteries had to be adequately sized so that we could withdraw 500-700 watts of power from the batteries without significantly discharging the batteries. This is why we used two because the amount taken out of each would be small. With something like a deep cycle battery, you shouldn’t really discharge them more than maybe 20% or you risk killing the batteries. Some solar batteries support much deeper discharges, but not these. The gist is that you need to make sure that after taking all you plan to take from your batteries, you still need to have the right amount of energy remaining. The percentage of how much of the battery energy you can safely take is called the “Depth of Discharge” or “DoD”. Our DoD would be 20%.

If we had been buying new batteries, we would have needed to buy batteries where 20% of their capacity was enough to supply 500-700 Watts. Solar batteries are measured by Amp hours. We have watts. How does that work? Well… take your watts, divide by the voltage of your system and you have the Amps.

100 W per day x 7 DOA = 700 W
700 W / 12 Volts = 58 Amps

Now multiply the Amps by the hours you need them. This is where it gets tricky because we don’t need our energy all at once. The most we will ever need at once is about8.33 Amps. How did I know that? Because, our total wattage, while everything is running is 72 Amps for the water pump plus 28 Watts for the bulbs or 100 watts total. Our system voltage is 12V (the voltage of the batteries, the soon to be solar panel, etc). 100 / 12 = 8.333.

If we ran all our loads for one hour, we would withdrawal the power at a rate of about 8.3 Amps per hour (8.3 AH). Assuming we need that for seven days, we’d need a battery that could support 8.3 AH for 7 days with a total of 58.1 AH.

Now… remember, we can only take 20% or so, so we actually need a battery that has a capacity 5x as much to get what we need out of 20%. 58.1 * 5 = 290.5 AH. Most solar batteries are measured in AH at 20 hours. Forget about what that means for now, but that is the number you want to compare when looking at your total AH needs vs the battery capacity. So, to summarize, to support 58.1 AH of need, we need a 290.5 AH battery. That gives us all the storage we will need to support 7 days of 1-hour per day usage and still not kill our battery.

Solar Panel

We needed a panel that provided as much resupply of watts to our depleted batteries as we’re taking out, plus a little room for margin. We were going to be taking out about 700/week, so we need to make sure we could at least put that much back in. So, we have 5.5h of sun per day on average, and 7 days to collect the sun during that week, that means we had about 38.5 sun hours per week to harvest about 700 W of power.  You shouldn’t just divide 100 w by 5.5 hours because there are a few more elements to consider. Namely, how many days we’d want to be able to run without any sun. We can get a week or so of no meaningful sun in our part of the world. We want to make sure our stuff works when this happens, so we might need to have 5-7 days of withdrawing 100 Watts of power, but with no deposits.

Since that represents best-case scenario and the weather and sun isn’t constant, we didn’t want to just divide 700 W by 38.5 sun hours and figure on an 18 W solar panel. It might work but would more often than not be insufficient. We decided on a 100 W solar panel from the great folks at Alt-E Store. They’re super-helpful, have a great YouTube channel, and are eager to help.

With a 100W panel operating at let’s say, 85% efficiency, we could potentially collect 3,272.5 Watts of power in seven days, or 467.50 Watts per day. Since we only should use 100 watts per day, this left us plenty of buffer and room to grow a little. We added this mount to a schedule 40 iron pipe placed 3′ into the ground and were ready to go.

Charge controller

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A charge controller is an important piece of the puzzle. Some try to be cheap and avoid them to their potential peril. A charge contoller manages the incoming solar power and charges the battery until the battery is “full”, at which time it prevents over-charging of the battery. They also often have a ‘blocking diode’ of sorts that prevents the energy in the battery from flowing into the solar panels when there is no sun (i.e. at night). We purchased this charge controller for that use. It is important to note that you need to have a charge controller that can support the charging amps you’ll be putting into it. Those charging Amps are a measure of the panel watts divided by the panel voltage (100 W / 12 V = 8.3 A). Your charge controller should be support slightly higher than your maximum charging Amps. Ours is 10.5 so we’re good.

Miscellaneous

To add some additional security and also to make things more organized, we purchased a marine battery terminal block on Amazon. We landed all our circuits positive wires to this block and all the negatives to the negative block it came with. This also gave us the ability to add fuse protection to all the circuits using auto fuses.

From there we simply wired everything together and turned it all on!

Using this for rainwater collection and pumping

We collect rainwater from our goat barn into IBC totes, some 3″ PVC pipe, a Rain Harvesting First Flush Downspout Water Diverter Kit, and a few misc pieces such as the Leaf Eater Advanced Rain Head and a stainless steel filter. We then use the RV (Shurflow) water pump mentioned above and pump the water through a standard household water filter and into a surplus pressure tank that we had on-hand. The pressure tank can be turned on/off with a valve

We then use the RV (Shurflow) water pump mentioned above and pump the water through a standard household water spin-down filter and a carbonb filter into a surplus pressure tank that we had on-hand. Oh… and we also have found that a 1/2″ PEX/SharkBite check valve is essential to make this work well – prevening the water from draining back into the tanks and keeping the pump primed.

The pressure tank can be turned on/off with a valve in-case we don’t want to bother with it. It can help the motor run less often by storing pressurized water. The pump has to run for longer periods of time, but less often. This can be handy for say… filling the pressure tank during peak sun hours then using the pressurized water during non/low sun hours.

Here are a few pictures:IMG_0238

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