# Thread: Dabbling in Solar Power

1. ## Dabbling in Solar Power

Anyone have a good starting point to begin dabbling in solar power. Want to explore the viability of running the shop trailer off of solar when the grid is unavailable.

2. First step is to figure out your expected load.. Everything else goes from that so it sort of figures the viability of the project.

Lets say you want to run a 1.5HP motor (say a lathe) and a couple of lights (we'll use 4'x2 bulb T8's as they are pretty close to LEDs efficiency wise and a whole lot cheaper to install still).

1.5HP is roughly 1119 watts, if we figure that normally you average ~80% of load (this can vary wildly depending on the tool/use so being pretty conservative) maybe 900watts sustained.
Figure a conservative draw of 0.8 amps @120v per fixture * 2 (assuming one overhead and a similar draw light for some spot lighting) we're at 192 watts

So one tool and decent lighting is around 1100 +- watts (again without knowing motor efficiency/actual draw this is being fairly conservative).

Using a completely arbitrary google search (and not the most efficient) cell: http://www.lowes.com/pd_182581-40683...ductId=3961605
We get 250 watts per 2519 square inches (64.6 * 39)
So lets say 5 of those and you have about 200" (~17') of panels at 64" wide at around \$3400 for the cells.
edit: Lowes is a ripoff http://www.wholesalesolar.com/solar-panels.html - similar for closer to \$1500

You also need to add on a controller+inverter and some buffering capacity in batteries. Maybe 10x the solar capacity in batteries (not a huge buffer, but some) or around 6250 stored watts using deep cycle batteries that's somewhere around \$2,000 worth of batteries (or drop your buffering capacity and turn to whittling when the clouds roll by, its a valid strategy ).

Now if you can get your draw down.. it gets a cheaper in a hurry. Something like http://www.wholesalesolar.com/RV.html#kingoftheroad ("The King of the Road RV Solar Kit") - and NOT saying that that is the best deal but its an easy to compare package - you can get around 4 hours of 500 watts for ~\$1500 (plus some bits for install). The accessories will drop off pretty good at a few points as well (those points have moved up since I last looked very seriously at this so its probably not as bad as my first blink says it would be).

One place I wouldn't cheap is on the inverter, everything will run better (or at all in a few cases I've seen) on cleaner power and they've gotten reasonably priced for good quality electronic ones nowadays.

3. Thanks for the link, Ryan.

But what is the math. HP to watts? Volts, amps, watts relationship? Inches per watt?

4. Warning: math and theoretical numbers ahead.

1 hp = 745.699872 watts - source google.com "hp in watts"
watts == amps * volts
So if you have a 1HP motor running at 120V that's roughly 6.2 amps (745.699872 / 120).

That's the simple math, the truth is of course more complicated.

Motors aren't anywhere near 100% efficient so you won't get 1HP for 6.2 amps at 120v in the real world. A motor with good efficiency might be 75-80% efficient so would actually draw ~1000watts at full load (and more than twice that at startup for surge - which is one reason you want some battery backing). Its really hard to know what the efficiency of a given motor is without measuring it and they do vary very wildly from 80% down to 50% or sometimes even lower (you might) get the data from the manufacturer, but its kind of like a snipe - a rare bird indeed). Generally the heavier the motor is the better it is in this regard for a given timeframe (some newer motors are lighter but use more efficient designs).

Countering that once the motor is running its draw depends on actual load. So for a 1HP saw motor it might draw up to 3000 watts for ~1s at startup, then idle at maybe 2-3 watts until you put it under load (i.e. for the duration ripping a 2x4) or maybe even higher if you take it over the rated load (by pushing to fast). I would expect a lathe to have a moderately low "average" draw just based on hand waving numbers because the motor is mostly idling once it gets the piece spun up unless you really start hogging material off (even then momentum is on your side). Adding in power converters (like a VFD for variable speed on a lathe) also imposes overhead, although I don't have handy numbers in front of me for that, there is non trivial loss there as well.

The numbers this guy: http://www.generatorsales.com/wattage-calculator.asp has are mostly believable as full load numbers, but there is really no substitute for measuring actual draw on your tools under use if you can. The "rated watts" number there is at nominal peak usage so for example the fridge obviously wouldn't take 500w continuous but only while its running (and there is generally a moderately healthy budget on top of actual draw in most of those numbers - source: abusing datacenter power usage by squeezing more computers in than we really should have.. don't turn them all on at once and then looking at other power supply/motor ratings).

You do have to size the system for all the things that can be running at once though.. So if you have a small system you might consider piggy backing some of the RV power feed onto it, but have a transfer switch so that when the table saw is running the fridge doesn't kick in as well and trip the breaker.

You can get a clamp on ammeter that just clamps over the power cord that is accurate enough for rough estimates for around \$50 which imho would be money well spent before dropping a few grand on a solar system that may or may not work (or may be oversized so you spend less than its worth).

On the solar side you need to oversize the panels to absorb loss in the system. Generally you would chain it together vaguely like this:
panels->charge controller[1]->batteries[2]->inverter[3]->power out

1. the charge controller regulates the power to the batteries to make sure that they aren't over voltaged (frying them) and may convert the power from the solar cell type to a battery happy type (i.e AC->DC or 30v to 6v or whatever). Better ones are smart enough to detect bad batteries or other problems. This process obviously introduces some loss; figure maybe 20% would be reasonable (a good system might be less).

2. The batteries provide buffering for when there is no sun (obvious). They also provide backup power for when a motor starts up and you get a surge. Its also useful to put the power through them as ~most (all?) solar arrays have the output voltage vary depending on the amount of sun which if you directly fed that to tools would be bad (brown outs from the clouds).

3. The inverter converts battery voltage/type (xxV DC) to useful power (120v AC). Again some loss, 20% or less depending on the quality of the components.

5. Question... doesn't your RV *already* have extra battery storage and an inverter for it's own internal use? Anyway you could piggyback off of that, maybe upgrade/expand the battery storage? Then you only need the solar panels.

6. Just gathering information, Art. But, yes, the RV has batteries and an inverter. But this is an old coach. Those areas will need a consider boost. Also, the trailer will more likely not be that close to the RV when asked and set up. COnsidering putting the panels on top of the shop trailer. Means batteries and an inverter need to be there as well.

Thanks, Ryan, for getting me started. This may not prove to be viable or cost effective, but one does not know until one does the homework.

7. Carol, over on the Escapees forum under the HDT section (remember those big rigs I Posted in your rv/shop thread?)... there's a guy named Jack Mayer, he has an excellent primer on rv solar setups which you can find here.

8. Thanks, Ned.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•