Here comes the sun.

August 1, 2010 12:01 PM

I'm doing some preliminary research into what it would take to run a garage on solar power. This, of course, means using a solar array to power the heavy duty shop equipment that runs on 220.

I'm thinking of a pair of lifts and an AC compressor. I'm pretty sure that the tire mount and balance equipment is 110. So far, I'm seeing lots of installer and manufacturer websites that tell me about how great solar power is for my home and how much I'll save on my electric bill and what kind of government incentives there are for installing panels. And that's all great, but I need some help crunching numbers to determine what size array I would need to power an electricity-thirsty shop for 8 hours a day.
This would be in Philadelphia, installed on a North-facing structure.

So, I basically need a quick run down on the capabilities of solar power compared to the demands of an industrial application.

Thanks everyone!

posted by Jon-o to Technology (7 answers total)

The first thing you need to do is calculate how much power you use. Something like a monthly power bill could be used. The voltage of the shop equipment doesn't really matter, it's how much power is actually used. If you don't have a power bill, you could find a list of the equipment used, what percentage of time it's used, and stats on how much each specific tool uses and extrapolate it from that. We random people on the internet don't really have enough information to calculate that any degree of accuracy.

If those power hungry tools are actually run for a substantial part of the 8 hours, I think it's likely not feasible to depend only on solar.

Here's a rough example:

About $800 per 250watts. You would need 10 panels to power a 2kw circular saw. That's under completely optimal conditions - bright summer day, sun shining directly on the panels, no clouds. Panels can produce between 5%-50% of normal power in overcast conditions. $8000.

You'd need a high capacity inverter to convert the power to AC unless you have DC equipment. For the amount of power you need, that would likely be several thousand dollars.

Unless you only run the equipment in optimal conditions you would also need a hefty battery bank. I'm not sure how much this would be, but I'm sure it would be a considerable amount if you needed to be able to have power for several overcast days in a row. You'd also need extra solar powers so there would be charging capacity even while the equipment was used.
posted by Vulpyne at 12:17 PM on August 1, 2010

The first question to answer is grid connected or not?

Grid connected means that you're hooked up to the utility supply, if you need more power than your solar system is producing then you get the balance from the grid. If you produce more than you use, then your meter runs backwards. In this case you don't need to worry about voltages or anything other than sizing your solar system to approximately the average power you use. This is a bit tricky for the novice but any good solar installer will help you given your historical power usage from your utility bills. If the system is sized correctly you will approximately zero out your electricity bill. If you want to take a stab at calculating this yourself then the best place to start is with PVWatts from the National Renewable Energy Laboratory. This online calculator will tell you how many kWh a solar system of a given size will product at your location. So if you know the number of kWhr you used in the previous 12 months you can iteratively workout how big an solar system you need. Note that you lose a huge amount of energy with a north facing array, probably about 50% (compared to south facing solar system of teh same size). BTW, if there is a power outage you will not have power, the solar system only runs when the grid is live.

Off-grid is a whole other matter. In this case you need to provide the 'buffer' that keeps your garage running when there is no sun. This is typically a large battery bank. A fully integrated system can have a solar system, batteries and a gas generator and then trade-off each of the elements to maintain a power supply. These systems are considerably more complex (and expensive) compared to grid-connected systems. They can be prohibitively expensive for very large power loads. OTOH, when the apocalypse comes, you'll still have electricity.
posted by Long Way To Go at 1:56 PM on August 1, 2010

Response by poster: A couple quick notes:
The system will be grid connected and I meant Air compressor, not AC compressor in the original post.
Another obstacle is that the business will be in a new structure and I won't have any consumption history to judge my solar needs.
posted by Jon-o at 3:28 PM on August 1, 2010

Quick back of the envelope calculation: a mid-size car might weigh about 3200 lb and the mass of the moving part of the lift might be another 1000 lb or so; let's call it an even 2000 kg and say that it lifts 2 m in the air. Let's also just say that losses due to conversion from electric to mechanical energy and friction come to 50% efficiency -- I have no idea how close that is but this is a very rough estimate. 2000 * 2 * 9.8 / .5 = about 80 kJ of energy to lift once. Now here's where the issue comes in: if we say that a lift takes 20 seconds than that means it's drawing 80/20 = 4 kW of power while operating, which is quite a lot. But if you also assume that it only has to lift a car once an hour (and that lowering is free) then the usage averaged over the whole working day is only 80/3600 = 22 W, less than that of a light bulb.

Now I don't know anything about the actual numbers for one of these, or how many times per hour something is lifted, etc. But I don't think the details are going to sway things too drastically -- you've got a very heavy load that's also got a very short duty cycle, such that the total electricity used is not that much. It seems to me like this would be the perfect thing to simply leave on the grid and not even worry about trying to power it from solar.
posted by Rhomboid at 5:31 PM on August 1, 2010 [1 favorite]

The folks at Backwoods Solar have been doing this for decades and are good at "reality check" kind of questions. (Not affiliated, just a good experience when I needed a PV-powered well pump.)
posted by werkzeuger at 5:54 PM on August 1, 2010

Since you're grid connected, your system doesn't need to cope with the peaks, which is good. Do you have a budget in mind? I would be tempted to install say, 3-4kW array at first (if that's in budget) and make upgrades as required, see if you can get an oversized inverter etc. to cope with more power. Panels are cheaper every year.

A super quick google has say a 3hP (2kW) motor for a car lift, assuming 100% current draw, six times an hour, for 1 minute, is only .2kWh, so 2kWh for 10 hour day.
Power tools can be 1500W ea, air compressor may be what, 5-10hp for a shop? (3.5-7.5kW), lights another kW or two.
Heaters for the winter? Gas / electric? etc.

I guess what I'm saying is you need to add up all the bits and pieces that you think will be plugged in, guess how long they will be running (minutes / hour) to get some average kW numbers, and size an array based on that guess. The nice thing is that you don't have to cope with peaks, and days that you're closed, you're still feeding power back into the grid.

Also, panels must be facing direct sun as long as possible during the day. This may mean some sort of bracket to elevate the panels on the roof to the correct angle.
posted by defcom1 at 6:04 PM on August 1, 2010 [1 favorite]

It doesn't really matter that some of your stuff is 220v.
Since you will be grid connected, you should probably just have a stab at what your total usage will be and scale a system to deliver that. I'm assuming this is so you can say you are carbon neutral or something?
If you underestimate, buy some carbon offset credits.
If you are really stuck for an estimate, call up some competitors and ask what their power bill is.
posted by bystander at 11:28 PM on August 1, 2010