Solar Investigation : How many solar panels do you need?
- Filed under: Solar Power and Off-Grid Living
- Date: Apr 11,2008
I’ve already written a few articles about solar power, this one is going to be ONLY about figuring out how many solar panels you need to buy to meet your electrical needs.
If you’re currently living with “regular” power, and are wanting to switch it all over to solar, you’re in for a pretty large investment. (Well over $10000) However, if you just need some power for a weekend cottage, an RV, a garage, etc ; solar power may be a great option !
The first step is figuring out how many WATTS of power you use every day. Basically, Watts = Volts * Amps. I do all my calculations in WATTS. If you don’t know what a WATT is, just think of it as a unit of power. That’s all you REALLY need to know to understand this article.
Most household appliances run on either 120 volts AC (The Americas), or 220 VOLTS. (Most other places) You should atleast know what you’re running before going any further. If you don’t, i’m sorry, I can’t help you. Maybe you can send me an email telling me where you live … 
Make yourself a chart listing everything you NEED to run on solar electricity, how many hours per day you want to run it, and how many AMPS it takes. (Look on the unit, under it or behind it, there should be a sticker that says INPUT 120 volts XXXX amps or something along those lines) Multiply the volts by the amps and you will get how many WATTS that thing takes per hour of run time. Add that to your chart for each item.
Here is a small chart of the approx. watts per hour certain things take (List from EVSOLAR) :
| Appliances | Watts | Appliances | Watts |
| Blender | 300 | Refrigerator (20 cf.) | 150 |
| CB radio | 5 | Satellite dish | 30 |
| CD player | 35 | Sewing machine | 100-500 |
| Ceiling fan | 10-75 | Table fan | 1-25 |
| Clock radio | 1 | Toaster | 800-1500 |
| Coffee maker | 800-1200 | TV: 25″ color | 150 |
| Computer | 80-150 | TV: 19″ color | 70 |
| Dishwasher | 1200-1500 | TV: 12″ B&W | 20 |
| Dryer (elec.) | 4000 | Vacuum cleaner | 200-1200 |
| Dryer (gas) | 300-400 | VCR | 40 |
| Elec. blanket | 200 | Waffle iron | 1200 |
| Elec. clock | 3 | Washing machine | 500 |
| Evap. cooler | 300-1000 | ½” Drill | 900 |
| Frying pan | 1200 | 71/4″ Circ. saw | 750 |
| Furnace blower | 300-1000 | ||
| Garage door opener | 350 | Lights: | |
| Garbage disposal | 450 | 100W Incandescent | 100 |
| Hair dryer | 900-1500 | 60W Incandescent | 60 |
| Iron | 900-1200 | 25W Incandescent | 25 |
| Laptop | 20-50 | 25W Comp. flourescent | 28 |
| Microwave | 600-1500 | 15W Comp. flourescent | 18 |
| Printer: laser | 300-475 | 42W Halogen | 42 |
| Printer: ink jet | 60-75 |
Running a standard fridge on solar power is tricky. I wouldn’t recommend it. You have to get a fancy 12 volt fridge, or a propane fridge. Otherwise you’re looking into a LARGE investment to get the proper inverters to support the start up load and enough solar panels to keep up. That’s not what this article is about though.
So now you should have a chart with all the electrical things you want to run, how many hours per day you will run it, and how many watts per hour it takes while running. Multiply the WATTS by the HOURS for each item, and then add up all your readings. This will tell you how many watts per day you will be using.
At this point in time, you need to decide how often you will be using this amount of power. If it’s for a weekend cottage, you don’t need to recharge the batteries in one day, you have all week to charge the batteries to last a weekend. If you’re using it for an off-grid house however, you need to replenish your battery bank on a daily basis. For this article, we will pretend you need to replenish the battery bank every day.
Lets say you NEED 1500 watts of power per day to run all of your electrical needs, EVERY DAY. This means 
you need your solar panels to add atleast 1500 watts of power into your batteries. (off grid solar systems ALWAYS runs on batteries. Panels charge a bank of batteries, batteries powers inverter, and inverter supplies regular 120 volt electricity) I like to not be too optimistic when sizing systems, so I pretend there’s only 4 hours of useable sun per day. So we need to create 1500 watts total in a day, and we have 4 hours to get it in. 1500 watts divided by 4 hours means we need to make 375 WATTS of power per hour.
Solar Panels are rated in this format as well. (Watts per HOUR). If you research solar panels, and one is advertised as being 100 Watts, it means it creates 100 watts of power per hour of sunlight. We’ve decided above that we need 375 Watts of panels in order to meet our minimum power requirements. In this case, I would buy 4 100 WATT panels to make 400watts per hour of sunlight. (Times 4 hours means 1600 Watts into the batteries every day, enough for all your power needs and to make up for Inverter losses)
THAT’S how I calculate how many solar panels are needed for your solar array.
If you’re looking for LOTS of information on solar power and wind power systems, you might consider buying these sets of e-books that are filled with tips and tricks for solar power.
Stay tuned at TechieNATION.com for my future article about figuring out how many batteries you need for your solar installation!
20 Responses for "Solar Investigation : How many solar panels do you need?"
i am in the process of setting up a solar system and i have the article very informative
i live in jamaica and we have sunlight 12 to 14 hrs all year round yet i find the vast majority of the inhabitants are still pay the utility company
thousands monthly and we have free energy we can tap into
ignore the typographical errors i did not proof read
I would kill to be in a place that got so much sunlight. It will really maximize the power you get from your solar panels !
Good luck with your project, feel free to contact me at digitalmind (AT) techienation (DOT) com if you have any questions or need some help.
(You can also chat with me when i’m online, near the top left sidebar on this site it says LIVE SUPPORT ONLINE or OFFLINE, just click on it to chat when i’m online)
Let us know how your solar install goes, and take pictures if you can !
i plan on using a 2500w inverter , (2) 110w panels (initially ) along with (2) deep cycle batteries rated at 12v and 115amps
the inverter and charge controller would be mounted on a piece of 5/8
ply backboard and the batteries on a stand I think that is a better option than in the pan)
i however am very keen on running everthing in the house ultimately on solar power but i was a bit put off to read that to run a fridge requires a significant outlay
what are your thoughts
i plan on getting some of the items in the coming week
Will you be hooking up the batteries in series to have a 24 volt 115 amp hour system or in parallel to be a 12 volts 230 amp hour system ?
Have you calculated how many watts per day you will be using ?
You seem to have more panels than you do batteries. 220 watts of panels, * 10 hours per day of sun = 2200 Watts Generated per day.
Your battery bank (lets say you hook it up as 12 volts 230 amp hours) can only hold 12 * 230 amps which is 2760 Watts, but you should never use more than half your battery bank so you only have 1380 watts in your batteries to play with. You will be generating more power with your panels in a day than what your batteries can take in. (Which is ok if there’s stuff taking a good chunk of power DURING the day) If you do this, DON’T FORGET TO GET A GOOD CHARGE CONTROLLER ! The charge controller is what will stop your panels from overcharging your batteries.
Since you’re lucky to get so much sun, I wouldn’t be surprised if you can pull of running a fridge on solar power. It will have to be a very energy efficient fridge, and that still depends on what other things you will want to be running on solar. ALSO, (and this is the hardest part) you need a really good inverter that will support the startup of the compressor for your fridge. What brand is your 2500 Watt inverter?
Thanks for the comments, and good luck !
i am getting a little confused .you said in your article that the wattage on an appliance is per hour of run time right?
now my fridge (a 20cf) uses 445w plus maybe a tv and a fan during the day and oh yes the computer making an approximate total of
700w/hr.follow me now if i place that load on the system
the batteries (connected in parallel) should be able to run for awhile until the batteries run low as the panels would only be generating 220w of power per hour so for the 10 hr period i would require 7000 watts am i right or am i totally confused and in the night it gets worse as there would now be maybe two bulbs and two tvs on
so in my mind i need more panels
to answer the other questions i dont know what brand inverter as i plan to do the purchases weekend
by the way what kind of 20cf fridge uses 150watt ( looK at your list)
Yes you are right you would require 7000 Watts for that 10 hour period.
During that 10 hour period if the sun is shining you’ll have added 2300 Watts into the batteries via the solar panels, so technically in that 10 hour period you’re using almost 5000 Watts more than you’re generating. (So you’re taking that power from the batteries)
The way to figure out exactly how many panels and batteries you need is to calculate your daily totals in power consumption. If this place is being used every day, you need enough panels to generate the same amount of power you use in one day. You should have enough batteries to sustain yourself for ATLEAST 24 hours without sun in case of emergency.
So far you’ve told me that you will be USING 7000 WATTS of power in 10 hours time, yet you’re only generating 2300 Watts of power so your power level is ALWAYS going down. Technically in a couple of days you will have NO batteries left and you will only have power WHILE the sun is shining. (And only 230 Watts MAX per hour to use)
Can you give me the TOTAL WATTS you will be using in a day ??
I really, REALLY recommend getting a propane fridge unless you have a big budget for your solar setup.
Here is the chart I made a while back while sizing a “small” solar system for someone :
Item Hours per Day Watts / HR Watts / Day
Small cooler fridge 24 36 864
Ceiling lights (x5) 5 55 275
Lamp 10 11 110
Laptop 10 180 1800
Cell phone charger 1 20 20
Total 3069 Watts
Total Usable Watts with 6 batteries : 3960 Watts
Solar Panels Required to recharge every day : 4 (About 175 Watts Per Panel Per hours of sunlight - 5 hrs calculated per day)
Where did you get the number for the power your fridge takes? (The 445 Watts??) The thing with fridges is that they take LOTS of power to turn on, and then little power to keep going. You should call your fridge manufacturer, and they could probably tell you the average power consumption of your model fridge in a year. Then divide by 365. But again, you need an inverter that will handle that high startup power from the fridge.
I know this is lots of information, hope you’re not going to give up !!! My head was spinning for months while learning this. I met with electricians and a solar specialist so that I can understand it all ..
Good luck, keep me updated !
i think i will keep i simple initially by finding a way to run the other appliances with the solar system because the power generated should be more than sufficient
later i will endeavour to up grade to accommodate the fridge
by the way the label on the fridge is in spanish it reads
limite de consumo de energia(kwh/ano) 484
consumo de energia(kwh/ano) 450
my limited spanish give me the impression that it says that the consumption is 484 or 450 kwh per year
how is your spanish?
i wish as one global nation we would all be more concerned about alternative forms of energy as the price of oil is ridiculous ( and it is being artificially manipulated) maybe you can use your site to start spreading the word that we need to take a more proactive approach to limiting our use of fossil fuel
as we say in jamaica :”we need to do something before something do us” if you understand
2 things :
1 - Great idea to start slowly and migrate your things over to solar power to see how far you can go with what. You can do a lot with 230 Watts of Solar Panels and 10 hours of sun per day !!! (Seriously, i’m so jealous of you right now!)
2 - My spanish is horrible, but it’s obviously saying it takes an average of 450 KiloWattHours per YEAR. That is NOT the same as 450 Watts per hour. Not at all.
That fridge will eat up 450000 Watts in a year. Divided by 365 days = 1232 Watts per day (Average), divided by 24 hours a day means that fridge is using roughly 52 Watts per hour. If you had nothing else running on Solar, you could run this with your 2 panels and 2 batteries assuming your inverter can handle the Start up Current (which is MUCH higher than the average current it’s pulling).
great !!
by next week i will be able to tell you on my progress i will even upload some pictures if possible
That’s great Oral, I look forward to hearing about how it goes !! DON’T FORGET TO INSTALL A CHARGE CONTROLLER !!!!! (It won’t take long for your panels to completely recharge your batteries)
I will send you an email soon so that you can know where to email the pictures too. I’ll write an article about your installation once done !
DigitalMind
Dear(!) DigitalMind,
I buyed a house with NO electricity (grid), there is just a small solar installation with 4 panels 85 w and 2 car batteries of 180 Amph eatch.
The house in the nord of Spain so we have there 8 hours piek sun / day.
Where i live now i consume 6500 Kw/year.
I want make a installation or this amount of electricity over there.
The solarpanels i can buy are 230 wp.
How much panels and houw much deepload betteries and type, and wich converter(s) i need for this installation, let say with a reserve of 500 kw/year what ives us 7000kw/year.
Can you calculated this for me ?
Sorry for the grammatical foalts, because englisch is not my moher language.
how many volts are your 2 X 180amp/hour batteries ? (I’m wondering if you have a 12 volt 180 amp/hour system, or a 12 volt 360 amp/hour system, or a 24 volt 180 amp/hour system)
I am going camping for the weekend, and will give you my estimates as to how many panels and batteries you need to get your 7000kw per year when I return on Monday.
Speak to you soon !
Hello Digimind,
Good article! I have a few questions concerning a solar project I am doing for my school work. Would you know how many volts or watts a small refrigerator of about 1.7 cf carries, and how much solar panels I would need to power it? How much should I estimate for this project? Kindly give advice on anything else I may need to know concerning this project.
Thanks.
Hi Chike,
fridges are kind of complicated. First of all, look at the back of the fridge and look at how many amps it pulls at what voltage. Volts * Amps = Watts used per hour. For a fridge it’s not so accurate. Fridges use a LOT of power for a short period of time, and then uses very little power for a long time. The very BEST way to get the power consumption of your fridge is to call the customer service people of the manufacturers. They will tell you the total Kilowatt hours per year that your model uses, and will probably be also able to give you the startup current.
Sorry this was probably not the simple answer you were looking for.
I tell people to invest in a 12 volt fridge if they plan to run it on solar. Much more efficient and you don’t have to spend a fortune on an inverter to power the fridge. (And inverter’s have power losses too even when nothing is turned on)
hi digitalmind
i have been quiet for a while becuse in addition to other things i have been doing a whole ton of research on alternative energy and what i have come across is the wind alternative
even though i have in hand a 2.5kwatt inverter and charge controller i have not proceeded further as i am really trying to weigh both alternatives when you bear in mind that where i live in jamaica is not only sunny but also windy
what are your thoughts on both as energy sources
Hi Oral ,
It’s nice to hear from you again ! It’s funny that you mention this now since I just finished writing my first blog post about wind power
. ( http://www.techienation.com/understanding-wind-power-wind-generators-turbines/ )
From what I gather, wind power is cheaper as long as you have the wind and the ability to put it all together. The price per Watt generated by wind turbines is always much lower than the cost of solar panels ESPECIALLY, if you learn how to build you own wind turbine, which is what i’m working on now.
Solar is a lot easier though. (unless you’re buying a pre-made wind turbine, and in that case the only “hard part” is putting up a pole or tower to hold up the turbine in less turbulent winds)
I was intimidated with building my own turbine for a long time, but now I feel confident I can do it.
Keep me updated ! Can’t wait to see what your installation will look like !
i am still in the initial stages of indecision as to the best motor to use i am partial to an alternator or if i can find a suitable permanent magnet motor . i guess that the motor would be best as i think an alternator requires a current to run the field coil
the rest is fairly easy .
the proper motor is my problem right now
From what I understand an alternator is not the best solution because it requires a high RPM. You would need to gear it up in a certain way to make it close to efficient.
I would suggest getting a good AMETEK PM motor (Use Ebay), or if you’re able too, make your own and that will really give you the best bang for your buck.
i am checking ebay
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