You probably want a bigger panel - 45 watts is a joke. At current prices, you should get at least 300 watts, you can get panels for 33 cents a watt. See here : https://sunelec.com/. Some are 25 cents a watt. (but various shipping charges, minimum order requirements, etc, and I don’t feel like digging through the site)
If you have a night lamp that burns 20 watts for 12 hours, you’d reasonably need to collect all that energy in about 1-2 hours during the daytime during the worst case month of the year. So you’d need about 240 watts of panels. Rounding up to 300. You similarly need a beefier capacity if you want to intermittently use large power tools.
That will last 5 times longer, and it’s equivalent to a 12 volt, 20 amp hour battery. But since it’s lithium iron chemistry, you can discharge it to 20% safely instead of just 50%, so in practice it holds the same energy. You want a solar charge controller in any case - it needs to be a programmable one if you want to use lithium.
A table saw (or anything with a motor) is a pretty dynamic load. The inverter you reference will probably work well for “small appliances” and electronics, but I have doubts about running anything like a saw motor. Also, remember that you don’t really need a sine wave output for a typical motor. A modified sine wave is just fine in most cases.
I don’t think you will be happy with a single 35AH battery. Remember, the inverter is going to (very simplistically) exchange current for voltage to provide you with 120 VAC for the saw and other power tools. And there will be some loss in the process. I’ve currently got a 2400 watt inverter on a single deep-cycle battery and there’s no way on earth it would run a table saw (or washing machine or anything else with a reasonable motor). YMMV.
As far as the charging capacity of the solar array, I can’t say much. A lot depends on geographic location, orientation, season, and the duty cycle (time in use) of your inverter and battery. Given enough time, you can pretty much charge any set of batteries.
The saw motor is 15amp. Am I thinking about it correctly that the load is then 1800 watt continuous, with the startup draw closer to double? I had looked at a 2k-4k inverter, but I figured 3k-6k is a better buffer. I wouldn’t be running the table saw and miter saw simultaneously. The sine wave for the inverter is in case I want to power a small TV
I know lithium ion batteries may be more efficient, but based on the cost I’m better off just running more batteries I think. Those panels from sunelec seem more aimed at commercial than small residential DIY. Home Depot has a 100 watt kit for about double the price, but really, I think my usage is low enough and I get enough sun to charge so that I wouldn’t have to charge that quickly. Even just running from two batteries that would probably be enough for my power needs, and the frequency is low enough that they should be topped off before the next use session.
More batteries will help, but I don’t know if it will solve the problem. Let’s think about the batteries in a VERY simplistic way:
Let’s say you have a perfect inverter, a perfect battery, and an appliance or power tool that draws 6 amps. This appliance is drawing about 720 watts (watts=volts x amps). To provide 720 watts, the inverter will need to draw at least 60 amps from the batteries (720=12 x 60). Therefore, a 35 AH battery won’t last very long, especially if you are also using it for lighting or other purposes.
Keep in mind that an appliance drawing 6 amps is actually pretty modest. We’re also assuming that the battery voltage doesn’t decrease (it will…very rapidly), that the ambient temperature is perfect for the batteries, and that the inverter is 100% efficient. As the battery voltage drops, the inverter will have to draw MORE current to provide the 720 watts and, eventually, will shut down as the input voltage drops below a critical point. And, of course, a real inverter will have some loss during the inversion process (probably 10-15%).
I tried running a small electric chainsaw on my 2400 watt inverter and the unit gave up after a few seconds of actual cutting and shut down. It wasn’t damaged, but it did not like that electric motor at all.
The starting load is going to peak at 5 times the running load for a second or two. I have an older Onan that handles double the normal 6500 watt output for 5 seconds, they are more of a commercial unit. The beastie has a 950 cc engine, as big as some autos!
I’m a do-it-your-selfer, so what I see is:
Rent concrete saw for 4 hours - $100
Rent trencher for 4 hours - $125
120’ of plastic conduit and wire or 120’ of direct burial cable. I doubt you will be able to run overhead in most places. That 1800 watt motor will draw 150 amps of 12vdc and combined with the inefficiencies of the inverter the battery will last about 10 minutes with no lights or anything else on.
I would start with an extension cord and see how that works out. You can suspend it if you don’t want it laying on the ground. If the ends are inside your house and shed you can just leave it out. Take it in and wrap it up if you’re going away for a few days (local codes and conditions may not allow this). Make sure to use GFCI protection and it should be a good quality cord intended for outdoor use. It’s a good idea to have 100 ft. extension cord available anyway so it wouldn’t be a waste of money if you don’t have one already and you end up running a permanent line later. See how that goes over the summer. Maybe you’ll be fine with it. Use LED lighting in the shed so you don’t overload the line with your power tools.
You will want a power switch in the house no matter what you do. It could just be the breaker if the line is dedicated. Just remember to turn it on before you walk all the way out to the shed.
I think that’s where the solar comes in - it should be continuously charging right? I guess I’m thinking that there will be very little continuous usage. A chop saw is active for maybe, 5 seconds at a time? A table saw could be 20-30 seconds? The battery only needs to be large enough to support the usage until the battery is recharged.
I believe you and this seems right, but I swear I’ve seen video of it working (yay youtube). Am I missing something? Check out this vid(starting at around 4:56). That’s a smallish chop saw running off of a single 35 AH battery using a 2k-4k inverter.
How do I know what the starting load for the tools would be? They say 15amp, but it doesn’t say the starting load. It’s kind of not an option for me to do trenching myself. Me cutting up the concrete patio around the house has been vetoed so I’d need to pay someone to do it. Once I do that we’re talking permits, etc.
Generators are a solution, many are noisy, but a few are made to be quite.
I would say a low cost solution would be a low power solar or just battery lighting (even disposable batteries and LED lighting) for most everyday use, and run the extension cord for when you need it.
Running and ‘de-running’ the cord is about the same as starting, stopping a generator, cheaper and more dependable, however the generator has additional benefits.
An additional solution would be run a 12V cable to your shed instead of the solar panel.
You are using an extension cord now, right? The disadvantage is reeling it in and out, which would be a pain.
Can you do your own trench for most of the way, without the going through the concrete? Have whatever plastic piping and use the heavy duty extension cord through that?
Then have a shorter extension cord for the connection from your house to the buried extension cord.
You could have a smaller battery for just lights.
Personally, I’d go for the generator, but that’s because I live in a place that attracts typhoons and we lose power a lot.
It’s against code to have an extension cord inside a plastic pipe. (Plus on most heavy-duty extension cords, the ends would be too big to fit thru the pipe. Unless you get a bigger pipe than needed, which will increase your cost.)
Besides, the proper UF (Underground Feeder) cable is cheaper than an equivalent extension cord. That’s what you would use to actually wire this building up properly.
How wide is the section of concrete you have to get past? And how thick is it? If it’s something narrow like a driveway, it’s not that hard to run the cable underneath it. That saves a lot compared to cutting through the concrete, and then repairing it afterwards.
Believe me…I’m not a tool of the generator manufacturers
A reasonably-sized generator, similar to what many contractors use on job sites, is not very expensive ($300-$400). And you can always sell it if you decide to do something else, like install underground service to the shed.
The biggest issue I see with either using a heavy-duty extension cord for everything or installing a big photovoltaic system is the likelihood of frustration. You’ll have a nice shed with your power tools and you will find yourself having to unreel 100’+ of extension cord any time you want to do something in it, including just watching TV. Or, you will want to build something and find yourself having to stop after using the chop saw for just a few cuts. Then you have to wait a few days for the batteries to charge up.
A modest solar/batteries system will probably provide you with all the lighting you need, as well as power for a TV and probably even some small power tools (like a hand drill). But I think you’ll need either a generator or a good heavy-duty extension cord for your big toys.
This is just my opinion, based on my own frustrations over the years.
The concrete is a normal patio so probably about 4" for about 8 feet, then surrounding the house it’s probably several feet thick and at least 6 feet in length. That section is raised two steps off the ground and is a solid porch that surrounds the entire house. Cutting through it is possible, as I’ve gotten estimates for someone to do it - but seems like overkill and I’d have to persuade the other decision maker in the house.
And yes, I am using an extension cord now. That means that I have to unplug each tool as I switch to something else, and if I want power inside I have to run the cord under the door to the shed, plus having the wire dragged across the yard through all manner of foliage. It works, but is not idea. I know I can probably setup a system where I plug the cord into one main at the shed, then distribute the power to all the other things throughout the shed, but before I set that up I want to settle on a more long term solution.
That seems like a reasonable risk, but that is why I was thinking of multiple batteries in parallel. On the video I linked, the guy seemed to be able to quickly recharge to top off the battery after each intermittent use. Is that not a reasonable expectation? I could increase the panel to 100w rather than 45 to help this along, no?
I know, it sounds weird. It’s a raised porch that surrounds the entire house and is covered by the roof on the first floor. There are two steps down to a secondary patio that was probably added after the house was built. The raised part seems to be joined to footings on the house, but then my ignorance comes into play about home construction. There is also a small amount of slope so the covered part may be thicker due to that. It’s hard to explain but hopefully that makes sense.
Well, you know better than I do, but making it that thick would be completely unnecessary (unless you’re parking an armored car over it) and a waste of concrete. More likely, there is a layer of crushed gravel below four inches or so of concrete.
I have very little experience with photovoltaic systems, so I will have to defer to the other experts on this board.
But I do have a lot of experience with secondary power supplies (batteries of various types) and in the construction industry. I think a lot of contractors would be using a simple battery and inverter set-up if it worked well for them. But I don’t see this at all. I have seen guys hook up an inverter to their truck’s battery to do a quick job, like run a circular saw for a few seconds or do some drilling.
A typical table saw motor (1/5-1/2 HP) draws a lot of power. You can charge a storage battery fairly quickly, too, but I really don’t think that you are going to be successful doing this when you want to make more than a couple cuts or sand something for more than a few minutes. It will be like using a cordless circular saw…make a cut or two, swap for a new battery, make two more cuts, then wait for both batteries to recharge.
I’ve used my 2400 watt inverter and a battery to do some drilling (corded Craftsman drill) in a remote spot and I can watch the battery voltage drop as I drill. After 10-15 minutes, the battery drops below the minimum input voltage for the inverter and I’m done for the day. It takes at least a couple hours on my Schumacher intelligent charger to completely recharge the battery.
This type of cycle may fit your planned use, but it does limit what you can do.
IANA expert, but IMO this is the part where the math falls apart.
If you have a 1500 watt saw and a 45 watt solar panel it takes 1500/45=33-1/3 times as long to recharge as it does to consume. And that’s assuming 100% efficiency from end to end. With more typical efficiencies down around 25-30% from end to end, you’re looking at roughly a 100x factor between run time and charge time.
So run the saw one minute and it takes 1 to 2 hours to recharge that consumption. Yes, you can run the saw for several total minutes depending on how much total battery capacity you buy.
But thinking of the overall process as continuous recharging is mostly invalid. Think more of it as draining the battery/ies completely over the course of one work session followed by recharging from empty over the next day(s). During which day(s) you can’t use your toys hardly at all.
Said another way, the length of your work session is controlled by how many batteries you buy. How frequently you can even have a work session is controlled by how many solar panels you buy. You *can *get from 1:100 use:recharge cycles to 1:1. But you need more like 4500 watts of solar panels to do so. And correspondingly upsized everything else. And only work on bright sunny days within an hour of local noon.
Your math checks out and is solid. And even if you could fit enough solar panels onto the shed roof, that represents a wasted resource. You want to tie those panels to the electric grid so that excess power they produce can be sold to someone else.
A small inverter, panel, and a few motion activated LED lights that turns off 20-30 minutes after someone leaves the shed is far more feasible. Assuming he uses 3 bulbs to really light that place up to find those bolts at the bottom of the workbench, and they draw 8 watts each, and he’s in there an hour daily, he needs to generate just 24 watt-hours. Even on the worst day of the year, that 45W panel will produce it’s rated capacity for about an hour, typically. All he needs to do is downsize the inverter and he’s good to go.
I’d get maybe a 1000 watt inverter, those are cheap, and it’s enough to hook up an outlet in there. He could charge a drill or run a TV as well. Realistically, most home handymen aren’t using their big tools most days anyway. That cordless drill and other simple tools get far more use than big circular saws and things.
