Megaphat Information Networks

My Solar Shed


The Job

At home we have a tool shed with no power. The shed is about 25 feet from the house. I really do not want to run an extension cable to the shed every time I need power. In fact I really do not want to run extension cables from the house at all. I cannot bury cable to the shed as that requires permits and an electrician to sign off on the work for the town inspector to approve. Instead, a solar powered shed on the other hand, all mine!

At work sometimes we need to provide off-grid power. This means that somewhere on a mountain top we need to set up a wireless network relay system but there are no power lines to be found anywhere. Generators are not doable because that means someone will need to go up there every 3 days and refill the tank or switch tanks. We want a set it and forget it type setup. So we opted to go with Solar for those projects. This means that we needed to take the total power consumption per hour, multiply it by 24 to get the power for a single day, then multiple it by at least 3 days (bad weather) THEN pad that by maybe an extra day. This will give us the specification for our solar power configuration.

The Challenge

A solar shed on the other hand you may think "oh that's easy" alas, not so simple. My shed has multiple uses with many different power requirements. For example the sheds interior lighting takes 2 Watts per hour, but we use the lighting maybe an hour per month. The exterior lighting has 2 parts, trim lights (pretty blue that runs 24/7) and motion sensor lights. The trim lights take 5 watts per hour. The motion sensor lights take 30 watts per hour (15 each) but they run for 1 minute when motion is detected and they both have 2 different and distinct zones they cover.

I always use Monocrystaline panels. They cost a bit more but the power production on them is superior to Polycrystaline. They collect better in low-light and cloudy situations. So my peak sun hours in Winter time is about 3 hours but about 5 in the Summer.

Q- Could I just take the average of 4 hours and call it a day?
A- Nopes. The minimum is what I need to use otherwise you will overestimate the amount of power you are receiving and when you don't get anywhere near that, you'll be like WTF so, 3 hours it must be.

Q- But if I plan based on 3 hours and I get 5 hours, won't I be losing potential energy?
A- Yes but if I oversize my batteries, that should cover it and I will be able to store any additional energy that the panels can collect.

So let me explain this a bit better. My minimum daily potential for peak sun hours is 3 hours. That's in the winter.

The Math

So now I need to consider my usage. I will not always be using power tools (15 Amps) but when I do, I cannot imagine that I would even be using a full hour. Let's see how this works out based on a project, let's say ummm.. gutting and redoing the garage interior.

Table Saw Usage (Cutting Wooden Rafters)
Time per cut: 20 seconds
Number of Cuts: 30
Total Time: 600 seconds (10 minutes)

So the Table Saw is 120VAC @ 15 Amps. That is 1,800 Watts of power required. Based on the calculation above, I would not want to calculate for less than a one hour minimum. To be safe, I would upsize my inverter by 200 Watts (also easier to find a 2kW over a 1.8kW inverter) just in case of any surges from the device.

The Materials

This means that I would need at least a 2,000 Watt inverter, so I got a Renogy 2kW (4kW peak) inverter. So now I need a battery bank that can handle this load. Ok so for 1.8kW I am using 12VDC batteries, that is 1800/12 = 150Ah needed for the bank. Ok so I got a deal on 2 batteries rated 75Ah each and hooking them in parallel will keep them 12VDC but double their current. Sadly they are Sealed Lead Acid batteries which have a horrible rate of recovery when you hit them hard with pulling power so I need to oversize my battery bank. At this time I am not oversizing my battery bank since this consideration slipped my mind when I made the purchase. I should have purchased 2x 100Ah but noooOOOooo...

I would certainly be better off with ANY kind of Lithium battery system. Lithium handles massive surges far better than any commercially available battery chemistry available right now and SLA is the old timer chemistry which has horrible charge rates, horrible recovery rates and horrible life span, but $180 for 100Ah SLA vs $390 for 100Ah LiFePo4, for now I am not investing too much into energy storage for the shed so I'll deal with the massive dives.. for now. Especially considering how often I will be using maximum power output on this system? Better yet, other than providing shed light, it will be rare that I power on the inverter. Maybe 4 times a year? So yea, there's that.

So let's get this started. I needed to source the panels, so of course I headed over to my A to Z company to find some prime Monocrystaline panels. I ordered 2 at first but when I saw they were different sizes, I was concerned. The specification listed was for the physically larger panel, but the smaller panel had stronger potential but at a cost, slightly lower current but it was certainly within acceptable limits. I contacted the manufacturer and they informed me that newer technology is allowing them to create panels with smaller physical footprints and even the smaller panel I received is considered old news. Well heck! I ordered 2 more!

Here are the first 2 panels installed. As you can see I was a bit anal about the cabling. After the next 2 panels went on , i was a bit more careless about the cabling but that will be changed wwhen I get the junction box. They are even smaller than the smallest of the first pair and still they are more powerful! As you can see from left to right, we have a large 36 cell panel. Next we have a medium 27 cell panel. They are both rated at 12VDC but the larger delivers more current and slightly more peak voltage. The 2 smaller ones however deliver as much power as the largest with a smaller footprint. I am not complaining.

I did start with this wonderful little waste of money as a charge controller. It was advertised as a 30 Amp MPPT 12/24VDC controller, it was far from. It was at best a 10A PWM, unable to fully charge the batteries and no matter what the instructions say, was not programmable. Sure it has a USB port for power but a $5 circuit can do that as well. I ended up replacing it with an EPEver Tracer 40A MPPT controller which is an amazing improvement in so many ways! Sure it has no USB charging port but as I said, a $5 circuit can cover that. To be honest for the price and the power, I would buy these MPPT controllers every day of the week, but I usually don't need 40A for many of my installations. I've used them before but I didn't think I would need one for my shed, until I threw the 15A circular saw into the equation. Even then, 40A is a bit much considering that there's no way that 20-26A is anywhere near 40A, but hey better safe than sorry.

The cabling I am using for the power is 4 AWG. Again a bit of over kill but the 2 things I needed to be sure of are (1) as little resistance as possible is present to ensure that I receive as much power as possible and (2) absolutely NO heat will be coming from the cables as this system will be self-sufficient with little to no maintenance/supervision. The requirement of this voltage and current is about 12 AWG. Since I already work with 4 AWG and have all the tools for that gauge (actually 2/0 all the way up to 22 AWG but I have lugs for 4-6 AWG) I figured why not.

Currently the main circuit panel is anything but that. It's an open busbar (I know I know) but I do have plans (sure I do) to replace it with a breaker box. The busbar has breakers of various sizes connecting to it for various feeds. I am currently using 2 different types of marine breakers of various ratings from 30A to 200A. I opted to use breakers over fuses for obvious reasons (if you don't know, you won't understand).

Some of the feeds are DC so power conditioning was important. Using regulators was a given. Tycon Power systems has some very good commercial-grade units such as DC step-up and step-down applications. But for the AC requirements (and oh yes, there is that table saw) I initially decided to try and save some moola and picked up this little (a-hem) gem. It currently resides in my garage awaiting refurbishing as the fans failed within the first 30 days. Additionally it's supposed to be rated for 2kW run time with 4kW peak of pure sine wave conversion. It's not. It's about 1kW/1.8kW peak and I'm pretty sure it's a modified sine wave because everything that I plug into it has a buzz.

Without hesitance I picked up this Renogy 2kW/4kW peak PSW inverter. It does its job just fine. Actuallly the inverter complains that my battery bank is insuffient if I am running more than 1 heavy tool at a time for a extended duration, but that is to be expected especially since my SLA battery bank really cannot handle such heavy loads. But I really am much happier with this unit.

As this project nears completion (in other words, never?) I will post more about it including the final schematic.


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