Dear Mike,
Someone on the RV Quilter FB forum asked about using a solar generator and sewing with a Singer Featherweight sewing machine. It’s 100-110 volts, 25-75 cycles at 0.4 Amperes. Doable with a solar generator? —Terry B.
Dear Terry,
You know I can’t answer a technical question with a simple YES or NO answer. This is a great opportunity to continue the discussion on just how much energy (not just watts) each appliance uses. So it’s time to break out the slide rule (or calculator if you’re not an engineering geek like me) to determine how much energy is needed to sew something like a sock puppet. Don’t worry, this is easy…
But first, what’s a “solar generator”?
A few definitions are in order, and I didn’t make the marketing hype up, so as Elton John would say, “Don’t Shoot Me I’m Only the Piano Player.”
Of course we all know what a traditional portable generator is. There’s some kind of gasoline or diesel engine with pistons and stuff (yes, I could build one from scratch if given a machine shop, appropriate materials and enough time). Its job is to spin some kind of wires and/or magnets to create current and voltage. Some generators make 3-phase DC (yes, there is such a thing), while others make AC (Alternating Current). But as long as you put gas or diesel in the tank, these portable generators will output 120 volts (or 120/240 volts) of AC for days at a time.
Marketing hype of the solar generator to the rescue
There’s another class of portable power that uses a battery to store energy on board, rather than filling up the tank with petrol or whatever. These are also called solar “generators” by marketing people who want you think they can function like a gasoline generator.
However, in reality they’re just a battery in a box with some kind of inverter to change the 12-volts DC into 120-volt AC. They can be handy, but you need to recharge the battery from another electrical source like a wall outlet or the 12-volt DC outlet in your vehicle.
Here comes the sun (Thanks, George Harrison)
By adding solar panels to this battery-powered so-called generator, marketing types figured they can now call their product a “solar generator.” And it does indeed “generate” electricity from the sun.
Sounds like a clean solution to your sewing machine question, doesn’t it? And if you look at the first page of marketing hype it seems a little too good to be true, doesn’t it? Wow, it’s 1800 watts “with Electric Start.” What does that mean?
Let’s read the fine print on the solar generator
But, if we expand the screen and look further down the page we come up with this little gem of information.
How much energy can it store and how fast can it recharge from the solar panel?
Those are the first two questions we need to answer before we can calculate how many sock puppets or wedding gowns we can sew from one of these solar generators. Yes, I’m going to calculate how much energy it takes to sew a sock puppet, so be prepared.
While it does have a 1800-watt peak (1440 running watts) pure sine wave inverter, there’s only a 60 Amp-Hr (720 watt-hr) SLA (Sealed Lead Acid or Gel) battery inside. Those two pieces of information are critical for us to understand how big of an appliance (like a microwave oven, Espresso coffee maker or sewing machine) it can run, and how long it can run it before running out of battery power.
Charging time….
First, let’s do a quick calculation on how long it would take a 100-watt solar panel (yes, that’s what’s included) to fully recharge a 720 watt-hr (60 amp-hrs at 12 volts = 720 watt-hrs) battery. Now you can’t just multiply the 100 watts of solar power output at high noon by 10 hours of sunlight per day to come up with a number. You have to consider clouds, angle of the sun, shade from trees, etc… In general, a 100-watt solar panel is good for an average daily energy output of around 300 to 400 watt-hrs.
Now that we have everything in watts and watt-hrs, we simply divide the 720 watt-hrs of energy we need to put back in the battery by the 300 watt-hrs per day that the 100-watt solar panel can provide. That’s 2 to 2.5 days (720/300=2.4) of sitting in the sun with nothing being used from the battery just to recharge it fully.
Discharging time…
Since they say in their literature that it can provide 1,440 running watts of 120-volt AC power, let’s calculate how long it can do this from a battery charge. We just have to divide 720 watt-hrs of stored energy by the 1,440 watts we want to use (720/1,440=0.5 hrs or 30 minutes) to find it will flatten the battery to 0% SOC (State of Charge) in around 30 minutes.
Now, I’m not even going to say that you should only take this type of battery down below 50% State of Charge because that will reduce battery life, and make the marketing guys go nuts (guess I just said it…) Not looking like a real powerhouse, is it?
What about sewing machines and sock puppets?
In the reader’s question, they asked about using a sewing machine that pulled 0.4 amps of current at 120 volts. So to find watts we just need to multiply 0.4 amperes x 120 volts and come up with around 50 watts (48 if you’re not rounding up).
To find out how long you could run this sewing machine motor at full throttle, we just need to divide 720 watt-hrs of available energy (down to 0% SOC) by 50 watts to run the motor (720/50=14.4). and come up with around 14 hours of running time.
Now, your mileage will vary because I haven’t included any other losses in the system. So let’s throw away 20% of this for efficiency losses and come up with around 12 hours of run time.
How much energy does a sewing machine need to make a sock puppet?
I asked my wife (who used to do a lot of sewing) how long it would take her to sew a sock puppet like this one. After she stopped laughing at me, she said it would take her around an hour to make one of these. I’m guessing that the duty cycle of the motor would be around 50% of sewing machine run time.
That suggests that each sock puppet would require around 30 minutes of run time (or 25 watt-hrs). We already calculated 14 hours of available power with just the sewing machine motor running. So for my highly WAG (Wild A** Guess), I’m going to say that you could sew around 28 of these sock puppets from a single charge of this solar generator.
But what about the sun?
And yes, considering that the sun could resupply around 300 watt-hrs of energy per day, and each sock puppet requires 25 watt-hrs of electricity to sew, you could be making 12 sock puppet per day, just on the solar panel’s recharging ability. So, ad infinitum, as it were…
So will this work?
Yes, it should work nicely for this type of sewing. And if you’re making wedding dresses that require perhaps 100 hours of sewing time over the period of two weeks, then it could still work as long as you don’t go too much over 12 hours of sewing time per day.
Once you get everything converted into watts of power and watt-hrs of energy, it’s pretty simple to run the calculations just to get a ballpark number of what is and isn’t possible to run from battery and solar power.
Final analysis regarding solar generators
So, things that draw a small amount of power (like a sewing machine) are good candidates for these solar generators. However, trying power an 1,500-watt hair dryer (for your RV barber shop) would quickly deplete the storage battery in 30 minutes. That would be maybe 5 or 6 customers if it takes you 5 minutes to you blow-dry each head of hair.
Why is this stuff important to understand?
Now, this is important to me because I’ve been offered an electric-powered SUV this summer that could tow a small RV trailer. So I’ll be doing all kinds of calculations and gathering empirical data on how many miles I can drive it before finding a charging station, how long it will take to recharge it, and how much it will cost in kWh (kilowatt-hours) to refill a tankful (battery) of electricity.
This is important to you because EVs (Electric Vehicles) are coming very soon, and it won’t be long before you have to decide if you want to buy one. So I’m giving you the intel to make an informed decision of their capabilities while towing an RV, or just driving around town. Gonna be some serious fun, real soon.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.
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“Some generators make 3-phase DC (yes, there is such a thing)”. No there isn’t.
DC does not have a phase. DC generation can have a positive or negative output when referenced to a common.
Now, if you meant to state that DC can be converted into 3 phase AC that would be correct.
I knew this would trick somebody. Actually, your car alternator (or the “spinning generator thingie inside of your Honda/Yamaha/Predator inverter generator) is a 3-phase AC generator/alternator that’s immediately converted into pulsating DC by a bunch of diodes. Then that 12 volts of pulsating DC (from the 3-phase generator) is inverted to 120-volts of 60 Hz AC sine wave power by some fancy electronics. So it (and the alternator in your car) is actually a 3-phase DC generator/alternator/dynamo. Who knew???
Not me – that’s for sure! 😆 Thanks, Mike. 😀 —Diane at RVtravel.com
Mike, I stick by my response to your posting concerning 3 phase DC and respectfully disagree.
Jon, that’s just fine. It was all in fun on my part and I’ll buy you a beer if we meet at a rally. 🍺
Yes, I do know the difference between AC and DC and have worked on all flavors of 3-phase AC power, including the highly dangerous High-Leg Delta. Now there’s some crazy stuff.
Mike, can an EV be topped off when connected to campground power? Would it come with a 50 amp style plug? If so, that would be a great way to see the country and fuel up while at camp. Just would have to plan certain amount of mileage per day. Can’t wait to read about your experience with an EV towing a camper.
Yes, you can get an EV charger adapter that can be powered from a NEMA 14-50 receptacle, which is the standard 50-amp campground pedestal outlet. That’s around 12kW of power, so it would be able to completely recharge a 100kWH EV battery in around 8 to 10 hrs or so. However, note that if your RV is plugged into an unmetered pedestal, that 100kWH of electricity could cost the campground $10 to $40 in extra power fees (depending on their own cost per kWh). So not a lot of campgrounds will be happy when you “fill up” your EV from an unmetered pedestal. One thing I’m talking to EV design engineers about is writing software that would enable a toad to use regenerative braking to recharge the toad EV’s batteries. Now, I don’t think the regenerative braking recharge effect can be used for full recharging while being pulled from your Motorhome, but it could be possible to use it like a diesel exhaust brake while descending long grades.
It’s funny you mention this idea, because I had the same thought experiment one day around the campfire while talking to a neighbor who had an EV toad behind a bus, and was complaining about charging the EV that had sounded so great when they bought it. I COULD implement the software you talk about, but suspect the EV regen system is not meant to run high-duty without possible damage…
Dragging the motors for regen near-fulltime to charge quickly is possibly bad for the system, and not very great for efficient charging anyway. Regenerative braking is less efficient than putting the gas in your generator and charging that way. I agree that’s a non-starter.
But, if we use ‘waste’ energy as the system was meant to harvest, I think it’s a good idea… With the toad as companion to the TV’s braking, duty cycles are shorter, and if set to only regen equivalent to stopping the toad’s weight it seems pretty identical use-case to operating the EV normally. If you set the regen to maximum braking to assist stopping the TV, you get into increasingly “possible system overload” territory. I’d love to hear from an automaker’s opinion of this.
One other thought: I’m not sure explicit software would even be needed — if the toad is operating it’s brake lights triggered from the TV, it’s possible the EV’s regen system is triggered by the same signal and would already “see” that as a braking event if the ignition is on — it “might” already work as a sort of bug. Based on my initial discussion, I don’t think this IS the case, but could be (or made to be).
Mike, I am very interested in your reports on your future EV SUV. More so because my son-in-law works for one of those manufacturers, whose vehicles are due to be on the road in June. Thanks for your continuing excellent and interesting advice!
Always enjoy reading your articles. Looking forward to your EV results. Especially range towing and not towing.
The range of an EV with and without an RV trailer in tow is going to be a really important topic soon. And I’m curious about EV toads as well. Interestingly it’s basically sock puppet math once I figure out the main variables.
You’ve come up with several unique terms, as necessary, over the years, Mike. But I think “sock puppet math” is the most interesting and creative one yet. 😆 —Diane at RVtravel.com
It’s now in my electrical vocabulary, so better be prepared to see it again….
Got it. 😆 —Diane at RVtravel.com
So I’m thinking “SPEU” pronounced like “spew” for Sock Puppet Energy Units. What do you think?
🙄 😆 —Diane at RVtravel.com
SPEU? We’re RVing in the land of “Smoots” now… 🙂
Why not “SPEW CHUNKS, EW!” ? – (Sock Puppet Equivalent Wattage Converted Haphazardly Unneccessarily, Not Knowing Squandered Energy Waste)
Mike I always enjoy your articles and the way you break it down so it’s easy to understand. I was an industrial mechanic for 35 years and worked with many electricians, I have a “basic” understanding of electrical applications and have successfully completed many home electrical projects. I can read and follow electrical schematics but ladder diagrams confuse me, but I get by. Anyway keep up the good work and I await your next lesson.
Hey Mike! I asked my quilter friend about this. She uses a featherweight too. She says if your sewing you would also need an iron. My guess is an iron would be equivalent to the blow dryer and would deplete the charge rapidly.
Oh yes, an iron is likely 1,200 to 1,500 watts. So that’s not gonna work with a small battery powered “generator”. Now that you know how it works, you can easily see what will or will not run on a battery system.