Do anyone know of an online site where you can calculate usable power from a given sized solar cell?

Watt’s Law may hold the answer you seek. (W = Watts (power), I = Amps, E = volts) W = E x I, E = W/I, I = W/E.
746 Watts = 1 horsepower.

^ I barely got a working understanding of Ohms law. I understand the bigger the wattage the more it should do. Will, say, 100w solar panel always have the same amps, or does the amperage change, if that change, does the volts then change? Wait, if watts are the power, what does the volts represent or are used for?

I thought Ohms Law stated that if you sit in one spot quietly chanting “Ohmmmm” to yourself long enough eventually your wife will come in and tell you to stop wasting time and do something productive for a change; like install those solar panels you bought and stashed in the garage months ago.

You need to understand a couple of basics first. There are 2 main things that define a system: Power and Energy (Three, if you include your location)
Power is the rate the system will deliver. energy. It is measured in Watts or kW (kilowatts) A small drill is about 100 Watts, a small electric heater is 1500 Watts or 1.5kW. A laptop charger might be 50 Watts . .
Energy is Power times time. Usually it is in kWh (kilowatt hours.) If you run a 100 watt bulb for 15 hours that is 1.5kWh. If you run a 1.5kW heater for an hour that is also 1.5kWhr.

Got it? Read it again. And look up the specs for the items you want to run. The info will be on every appliance you use. It’s the fine print most people ignore. Read it and report back. Then we will go to the next step: sizing a system based upon your location and needs.

The first question you have to ask is what size battery bank you need to run the things you want to run. Then you can start deciding the size of your solar array. It will change depending on where you live.
I’ve got a huge battery bank but we’re powering 2500+ square feet of living space and hubby is mr gadget so we’ve got a lot of electric stuff to run.
There are a lot of solar sizing apps if you have a smart phone.

@LuckyGuy Then we will go to the next step: sizing a system based upon your location and needs.
OK, here would be the specifics, mind you; this is the worst case scenario.

• Small fridge 115v, pwr input 85w, 1.2a (might be ditched for propane fridge, but let’s go with it for now)
• Laptop 20v, 5a (4 hours total usage daily but likely less)
• Occasional charging of one 18v NiCad batt and one 20v lithium ion battery (once a week at worse)
• Once a week charging of 240 lithium ion batt.
• Small fan 50w 5a total 5 hours frequently in hot weather but not daily.
• 1500w heater with thermostat in winter months for 4 hours a day, not contiguous (might be ditched for propane heater)
• 250w halogen shop light, 3–5 hours in the evening winter, 2 hours or less summer.

That is the worst, of heaviest load the panels (keeping in mind, not all the items will be drawing at the same time, at most 4 of them) would have to handle if nothing changes, whadda ya got, my ears are open.

First, a few random facts;

- Amperage is Voltage divided by Resistance; I = E / R
– If Voltage (E) is constant, and Resistance® decreases, Amperage (I) increases
– Too many amps is bad; it can set wiring on fire. Put in terms of Ohm’s Law, a short-circuit is basically an attempt to divide by zero, which sends Amperage high enough that you better hope you trip a breaker or blow a fuse before things break/melt/combust.
– Wattage is Voltage times Amperage; P = I * E
– Most things you plug in are 115V so Watts = 115 * Amps.
– Things that can run off of a car’s lighter socket are 12V so Watts = 12 * Amps.
– If you raise Amperage while keeping Voltage constant, Wattage will increase.
– You can rearrange that to Amps = Watts / Voltage.
– One kilowatt-hour is, as the name implies, enough energy storage to run a 1kW load for one hour, or a 0.1 kW load for 10 hours, or a 10kW load for about 6 minutes

Okay, now that that’s off my chest, let’s get to answering. Off the top of my head, it looks like all but two of those loads could run comfortably on a car battery even if you had them all going at the same time. Recharging batteries doesn’t take terribly much, and the two fans and small fridge are relatively light loads. And if you’re only charging those tool batteries once a week, the drain is practically negligible.

I think that replacing the electric heater with a propane one would have a dramatic effect on how big a system you would need. As it takes a considerable amount of energy to raise the temperature of an area, you can rest assured that a heater capable of rendering a small shop habitable in the winter will require a battery at least ten times the size of one that was adequate for the other loads.

Well, the other loads minus one. That halogen shoplight is your second biggest load, though not nearly what the heater is. If you could find some DC lighting (likely 12-volt, possibly 24-volt) of comparable lumens (light output), that would probably cut about a third off the size of the system required. If not, well, we’re still only around half a kilowatt; one-third of what the heater alone would draw.

Aside from the halogen light and the heater though, the loads you’ve listed thusfar are pretty small, and aside from the fridge they are relatively intermittent as well. I’ll get back with more concrete numbers after I get some sleep and can think a little more concretely.

As an aside, you could do a bit of off-grid with a Power8 workshop too.

@Hypocrisy_Central Let’s assume the system will use 12 volt car batteries (typical size 42 amp hour , Figure 500 Watt hours )

The biggest load by far is the electric heater: 1.5kW x 4 hours per day = 6 kWhr /day . 6kWhr divided by 12 volts = 500 amp hours / 42 amp hours per battery = 12 batteries I’d ditch that and go with propane.
Let’s now look at the next item 250 Watt light for 2 hours = .5kWhr /12 =~ 42 amp hours = 1 car battery . It’s possible but that is the next biggest load. Can you switch to LEDs?
Laptop = 100 watts 4 hours = 1 car battery OK
Fridge = 85W figure 8 hours = 1 car battery OK
The other stuff is small .= 1 battery.

So if you drop the heater, you can get by with 4, 42 Ahr car batteries, for energy storage to run your shed for one day without sunshine.
Obviously if you eliminate some loads, the system will be smaller.
For now, let’s go with the above system: 4 car batteries. 4×42 amp hours x 12 volts = 2000 watt hours per day

Now you need to charge them. The Dept of Energy has maps available to determine how much solar energy is available on average for your location. If you live in a cloudy place like Seattle or Buffalo, for the same load you will need 2–4x panels than if you lived in Arizona.

You need 2000 watt hours to charge your system. A system that tracks the sun is better than one that is fixed, but tracking is more expensive. Let’s figure fixed, and also figure 4 hours of good sunlight per day. That means you need a 2000 watt hours / 4 hours = 500 Watt of power. The solar panels need to deliver 500 Watts . You can buy five 100 Watt panels or two 250 watt or any other combination.

Then you need the controller/regulator to keep the batteries at the same level and an inverter to covnert the 12 Volts to 120 VAC. It looks like a 1 kW inverter is plenty.

I broad brushed but I hope this give you some idea of what the system requires.

I love you engineering types. Glad you exist because you figure out the nitty gritty of things. Solar power is progressing sort of like computers did. In the 70’s and 80’s you had to be an engineer to figure out how to make a picture of a flower. Now it’s pretty much plug and play.
Solar is pretty darned close to that now too. It’s good that you guys understand all those details but it’s not necessary to understand it to that degree in order to have a working system.
Make sure your batteries are big enough for your needs. How many amp hours do you want? (Our system can do 90 amps.)
Can your solar array produce enough amp hours?
Do you have the appropriate inverter?
Do you have a backup generator for times that the weather won’t allow for maximum production from solar?
Once you’ve answered those questions you have the information you need to purchase the appropriate equipment.

@Judi I know you have an awesome system (you showed us a picture).
People should know the basics so they understand the tradeoffs. If the OP had demanded the heater, his system would be 3x the size and cost of what he “really” needs. The numbers really help.
The next step is understanding the location and the site. How much sun does it get? Are there trees? Does he have a home where the buffalo roam and the sky is not cloudy all day? Or does he live in a place similar to mine where supposedly there is a big, hot, yellow thing in the sky that only a few people have ever seen? (And that was on the NASA channel.) ;-)
A contractor would do all that dirty work – at a cost.

My back of the envelope study gets close and helps answer big questions. However, true optimization is as personalized as DNA.

Note we did not discuss the efficiency of the collectors and how much space is available. We know he needs 500 watts . He can get it by buying many cheap lower efficiency unit that take up a lot of space Or he can get it by buying the latest and greatest high efficiency units that take half the space but cost twice as much.
There are so many options.

And then there are the government incentives. 20%? 30%?

Today on the radio I heard that solar panels cost 80% less today than they did six years ago. Anyone have any idea if this is an accurate statement?
How much has their efficiency increased in the same timeframe?
Are the batteries the limiting factor now? Are “engineering types” working to make them smaller, longer lasting and more efficient?

@rojo , I know that Obama’s first energy secretary had a laser focus on better batteries, but I think there’s a new guy now so I’m not sure. Tesla has come out with a pretty cool looking battery that might be perfect for @Hypocrisy_Central ‘s needs. It’s clean, which is awesome compared to our room full of batteries designed to power a train. It also mounts on a wall so may require less space.
The cost HAS come down substantially but I’m not sure if it is as much as 80%.
@LuckyGuy , the Tax Credit is 30% until the end of 2016. States and utilities might have their own rebate programs in addition.

@rojo The Department of Energy has been funding all kinds of R&D programs to increase efficiency and battery storage. Here are some efficiency numbers from 2013. You can see they are in the 14 to17% range. (These numbers might be biased so go the DOE site for actual data.) There are experimental units as high as 26%. NASA uses them.
About 10 years ago the efficiency was in the 8 to 10% range. Big progress in a few years. Unfortunately China is doing most of the making and selling.

Batteries are also improving. I’m guessing @Judi ‘s battery bank weighs more than 1500 pounds (am I close?) You want to look at energy densities for different battery technologies. So far there is no ideal solution. Each one has tradeoffs., number of charge cycles, number of deep discharges, energy leakage, peak current delivered, mass, volume, cost, exotic raw materials, etc. No technology excels in all characteristics. The DOE and industry is working on it.

@LuckyGuy, we have 24 2 volt batteries. Jeff says they’re about 5000 lbs.

@jerv I think that replacing the electric heater with a propane one would have a dramatic effect on how big a system you would need. As it takes a considerable amount of energy to raise the temperature of an area, you can rest assured that a heater capable of rendering a small shop habitable in the winter will require a battery at least ten times the size of one that was adequate for the other loads.
The space is realistically around 6×9 ft, and I have no expectation it will be like Palm Springs when the heater is used. To make it comfortable for a few hours if it takes solar panels as large as the side of a barn to do it, I would surely just go propane, it may cost continually, but there will be heat.

That halogen shoplight is your second biggest load,..]
Even at roughly four hours a day at the most?

Let’s assume the system will use 12 volt car batteries (typical size 42 amp hour , Figure 500 Watt hours )
I figured on using deep cycle marine batteries, is there a difference than car batteries?

@LuckyGuy The biggest load by far is the electric heater: 1.5kW x 4 hours per day = 6 kWhr /day . 6kWhr divided by 12 volts = 500 amp hours / 42 amp hours per battery = 12 batteries I’d ditch that and go with propane.
Is this uncharged or being charged for 5–6 hours a day by solar panals?

If you live in a cloudy place like Seattle or Buffalo, for the same load you will need 2–4x panels than if you lived in Arizona.
If I had to guess, I would say there is sunlight around 7–8 hours a day between April and October, and easily 5 hours of good sun the rest of the year.

The next step is understanding the location and the site. How much sun does it get? Are there trees?
As the aforementioned post says, lots of sun, and the only tree doesn’t drags its shade on the structure until about two hours before sundown.

That means you need a 2000 watt hours / 4 hours = 500 Watt of power.
I eyed 500 Watt panels but did not know for sure if they would be overkill or lacking, but I knew they would be better than 45w, 100w, or 250w panels.

@Judi Solar is pretty darned close to that now too.
Oh really? If Plug-n-Play was this hard, people would have ditched it long ago.

I tried to cut and paste into your reply. Hopefully this will make sense to you.

The biggest load by far is the electric heater: 1.5kW x 4 hours per day = 6 kWhr /day . 6kWhr divided by 12 volts = 500 amp hours / 42 amp hours per battery = 12 batteries I’d ditch that and go with propane.
Is this uncharged or being charged for 5–6 hours a day by solar panals?
Those 12 batteries are required if you expect to use the system for one day. If you have a 500 Watt panel it will take 4 days to charge those batteries. Then you would only have one day of use.

If you live in a cloudy place like Seattle or Buffalo, for the same load you will need 2–4x panels than if you lived in Arizona.
If I had to guess, I would say there is sunlight around 7–8 hours a day between April and October, and easily 5 hours of good sun the rest of the year.
Remember the sun does not directly face the panel all day . It moves across the sky and changes it elevation. The solar output varies with the sine of the angle of incidence. If the sun is hitting the panel at 45 degrees yo are only going to get 0.7 of the output. That is why I use the rule of thumb 4 hours. The DOE site has the data for your spot. I’ll bet I am not far off.

The next step is understanding the location and the site. How much sun does it get? Are there trees?
As the aforementioned post says, lots of sun, and the only tree doesn’t drags its shade on the structure until about two hours before sundown.
Good. I’ll stick with the 4 hours approximation.

That means you need a 2000 watt hours / 4 hours = 500 Watt of power.
I eyed 500 Watt panels but did not know for sure if they would be overkill or lacking, but I knew they would be better than 45w, 100w, or 250w panels.
500W is not overkill for what you want to do. Panels rarely put out what they are advertise. You only get those numbers if the sun is directly overheat and the panel is facing right at the it and the ambient temperature is at 20C..

Well, 250 watts for 4 hours is 1,000 watt-hours or 1 kwh. And that’s assuming that you’re feeding it right from a 120-volt AC source, but you won’t be; you’ll be using batteries (which are DC, not AC) hooked to an inverter to turn 12v DC into 115v AC.

Inverters are not 100% efficient; their efficiency varies considerably with load, but I generally figure about 80% for an average. To get 1kwh out, you’ll have to put in roughly (1,000 / 0.8) or 1,250 watt-hours. Divide by 12 volts and you get a little over 100 amp-hours. Divide that by 42 amp-hours per battery, round up to the nearest whole battery so that you get at least 4 hours of light, and that’s three batteries worth by itself. And if you only have an average of four hours of usable sunlight for the panels, you’ll need ( 1,250 watt-hours / 4 hours ) or 312.5 watts of panels just to charge those three batteries to use that one light for 4 hours and be able to do that every day.

Alternatively, you could use the same amount of power to charge all of your cordless tools daily instead of weekly, go on a marathon surfing session with your laptop, get a cool breeze from both fans and still have enough left over to keep your fridge cold. That is how much that worklight draws.

Checking around a bit shows me that a 250W Halogen spotlight puts out about the same amount of light (same number of lumens) as a 20W LED. Changing out your worklight would reduce your power draw from the lighting by about 90%. Since that worklight alone is about half of your load (assuming a propane heater), your overall power needs will drop about 45%. For discussion’s sake, we’ll call it half. Take a look at the price of panels and batteries, and you’ll see how spending a few dozen dollars on new worklights can save you hundreds, possibly thousands on your attempt to go off-grid.

Personally, I like to put a little margin in for safety and to account for any erroneous approximations of unknown variables. Even then though, I think a 500W panel should suffice unless you add more loads that you haven’t mentioned.

As for batteries, I’d go a little overboard. I am accustomed to trees taking out lines and being without power for more than four hours at a stretch. If I ever lost power for six days again, I would probably just say, “Screw it!”, and live in the workshop with the propane heater and the working fridge. But there is one other benefit to getting a little overzealous with the batteries. See, certain types of batteries despise deep-cycling. A regular car battery will generally screw the pooch if reduced below 50% capacity. Marine batteries are considerably more tolerant of deep-cycling, but unless you’re using NiCad, draining 8 batteries halfway is a bit better than draining 4 batteries completely.

More importantly, batteries have internal resistance that does increasingly detrimental things as amp draw rises. Simply put, drawing 0.1 amps from each of 10 batteries is better than drawing a full amp from a single battery. Drawing a little too much current won’t do much more than reduce your battery lifespan, but it’s still mildly bad. Drawing enough juice to suck a battery dead in an hour will cut it’s lifespan even more. And as most who have ever run hobby-grade R/C cars knows, sucking a battery dead in under six minutes generates enough waste heat to cause severe injury even if mounted securely to aluminum with plenty of cooling fins in addition to longevity issues.

Different types of batteries have slightly different characteristics there, but without getting too deep, a good rule of thumb is that you want enough Lead-acid batteries to have the amp-hours to last at least five hours. (or 0.2C). If you go NiMH or Lithium, those are more tolerant of loads in that if you have enough amp-hours to last 2 hours or more, you won’t run into issues; the sweet spot is >2 hours for NiMH (0.5C) and >1 hr (1C) for Lithiums.

If you fall short of that threshold, you risk going a little too deep into a discharge cycle, and while the loss of efficiency is tolerable, it does have a detrimental effect on how long the batteries last. I don’t think you want to replace all the batteries every 2–3 years when going just a little overboard now can stretch that replacement time considerably.

Oh, and read this for a little more info.

Remember that the batteries don’t last forever and will have to be replaced at some time equaling added cost.

@kritiper Exactly, but that cost goes down a bit if you extend the time between replacements by treating the batteries properly. Conversely, it goes up considerably if you abuse the batteries. Each type of battery has it’s own requirements for charging and discharging, and those that RTFM generally replace fewer batteries (and thus spend less money) than those who don’t.

@jerv “those that RTFM generally replace fewer batteries (and thus spend less money) than those who don’t.”
Exactly! And that is why my phone batteries last for years!

Also consider how far away your panels are from your batteries. All solar panels are d rated. The further the run the more the d rate.
Here’s a calculator

@Judi That is a fantastic site. It is easy to use and does not get bogged down in the small details. Nice.