Your calculation for the solar panel output seems too simple. It leaves out the angle between the sun and the solar panel. If your panel isn't pointing at the sun directly, it will catch less sunlight. (looking from the sun, the projected surface of your panel becomes smaller if it doesn't point at the sun)

Second, the rating for a solar panel usually assumes that the panel is kept at its Maximum Power Point ("MPP") by an internal or external controller. If the panel isn't operating at or near its MPP, its efficiency will plummet, and it will put out much less power than rated. The rating is also based on full sunlight (around 1100 W/m2, IIRC).

You also assume that all of the energy that the solar panels put into the batteries will come out. Unfortunately that is not the case. The electronics monitoring the charge use some, and the charge/discharge efficiency can vary between 50-80%. Additionally, the batteries will loose between 3-20% of their charge each month.

Roland --- I thought about putting in a lot of caveats, but I wanted to keep it simple. For someone who's planning on getting all of their energy from a solar system, they'd need to know all of the things that will lessen the actual amount of energy they'll get, but if you've already got panels and are just sizing up your batteries, it seems to me that using maximums like this should work. None of these factors are going to cause there to be more energy than can "fit" in the batteries, so worst case scenario, you've just slightly over-purchased on batteries.

(Or at least that's what I'm assuming. Please tell me if I'm off base there!)

Just giving you the heads-up that the complete setup might be a lot less useful than you might think.

Both the solar input and power usage will vary considerably. And I'm guessing that you will use more power in the winter because you have more time, but in that period you'll also have the least input.

That's why I would suggest you get a good charge controller that has at least a battery voltage display and preferable an indication of the charge level for each of the batteries. So you can monitor the situation for a year. Only after that will it be possible to draw real conclusions about the viability and effectiveness of solar power for you.

While I can appreciate the curiosity aspect of experimenting and the satisfaction aspect of being self-sufficient, the cost of "free electricity" via PVs is about 3x the cost of grid power, not to mention the unreliability for those of us who don't live in the SunBelt. Don't forget that the capital put out for the hardware has to be spent again every 20 yrs or so and that the capital could have been earning interest for those 20 yrs....Germany is re-thinking its green energy policy now that they see the havoc PV power is recking on the European grid and the economic hardship it's placing on consummers & industry...Living with all green energy is a nice pipe-dream. I wish I could fly by flapping my arms, too. Neither is gunna happen in the real world.

Doc --- I think the real environmentally friendly energy solution is cutting back, not changing gears to another technology. Which isn't to say that solar or some other "green" power source might not prove its worth in the future. (That's my mildly researched opinion, subject to change. )

However, at the household scale, having a small system like this to protect us during those 10 day grid outages makes sense, even if the solar power is three times as expensive and not as green as many like to think.

Since I'm living in Europe, I think I can have a say about the electricitry grid... Which is world-class, AFAICT. Here in the Netherlands, power outages tend to make the headlines, because they are few and far between. That's why I've never even bothered to put a UPS on my computer.

But I agree that the focus on PV is not optimal for households here in northern Europe. We tend to use more energy for heating out homes than for our electrical appliances. E.g. I use around 18-20 MJ of electricity per day (used for appliances), averaged over several years. In the same period the average amount of energy used for heating my home and producing hot water (in the form of natural gas) was 70-100 MJ per day. So if I were to look for renewable energy, I'd look for a renewable source of methane before electricity.

Roland --- I'm glad you chimed in!

Over here, we measure our energy consumption in kilowatt-hours. Could I trouble you to convert from MJ? I'm always curious to hear how much energy other people use, but seem to be only able to think in terms of kilowatts.

So, your total household energy bill for an average month is for 353 to 432 kilowatt-hours, or are there other categories beyond appliances and heat? (Is light in with heat or appliances?) That's pretty good for someone using electric heat!

Mark hasn't weaned himself off his air conditioning in the summer and electric heat when it's only moderately cold, and I haven't even tried to wean myself off our electric stove and freezer. That's where I'd cut corners if I was going to make an effort to reduce our energy use, which has averaged 646 KWH (179 MJ) per month over the last year.

Hi, I just found your blog and had to jump in here. We live off the grid and I can assure you that you can read all these statistics and equations, and then throw most of them out of the window. I am always shocked at some of the information out there. It might look good on paper, but there is just very little truth in it when it comes to practical application. First let me tell you that you will be making power from sun up to sun down. These 'peak' hours do not mean that much. We even generate power on cloudy days and we live in an area that is second only to Seattle for worst area for solar. New PV panels are very efficient 90-95% and at times we can get 110% efficiency out of them under the right conditions. Now there are many,many aspects that go into how efficient your system runs, but for your experimental purposes it wont really matter and can bog you down. We also made power with the panels just laying on the ground for weeks while we built the panel tracker. I think it is wonderful you are going to play around with this little system because I think that far too many people 'read' about solar rather than have any experience with it and thus misinformation makes its way out there.

Now regarding this: "None of these factors are going to cause there to be more energy than can "fit" in the batteries, so worst case scenario, you've just slightly over-purchased on batteries." Yes you CAN fit too much in the batteries and actually boil them and will release hydrogen gas which can be flamable. You might be surprised what your little panels will produce. That is why you really will want a good charge controller that will prevent overcharging of your batteries. If nothing else you are going to want to get an inexpensive DC volt meter to monitor what you are putting into the batteries and for how long. You do not want to over charge your batteries or you will be making that purchase again. Batteries are expensive.

I wish you lots of luck on your experiment with solar.

Jane --- Thanks for chiming in! It's good to hear from someone else who can appreciate the idea of starting to experiment even if you don't have all of the answers yet.

We'll definitely have a charge controller --- I think it actually came with the solar panel kit from Harbor Freight. I just didn't want to size the batteries so small that I'd be wasting too much juice if the batteries ever became full and the panels were still chugging along, if that makes sense... What I haven't researched yet is if the components of the kit are enough or if we need to buy something additional, but now that I've pretty much settled on the batteries, I can work my way outward from there.

My theory with backups is that they should be used all the time, or they won't work when you really need them (and generally won't be at all cost effective). That's why we're working on building a system that we'll use all the time for a few items that we'll really want during power outages.

We do have a gasoline generator, but it always seems to need a tuneup and is dicey when needed....

@anna: I'm not using electric heating! That would be a huge waste of energy, in my opinion. More on that later. My heating and hot water is made by a boiler fed with natural gas. These were my actual daily average numbers for 2011:

2011-01-02 - 2011-12-31 (363 days); 5.15 kWh (19 MJ), 1.90 m³ (60 MJ) /day

The first two numbers are electricity, the second two numbers are natural gas.

So my average monthly energy usage was (19+60)*365/12 = 2403 MJ or the equivalent of 667.5 kWh.

What is not taken into account in this is generation and transport losses: An electric power plant is generally around 40% efficient, so for every kWh of electric energy it puts out, it uses 2.5 kWh worth of coal, oil, gas or whatever the heat source is. The efficiency of my water heater on the other hand is much better; >90%. That is why I don't want to use electric heating. This decision is simplified because we have very good infrastructure here. Almost every house is connected to both the electricity and natural gas grid.

BTW, Have you switched the numbers or units in your "Monthly energy usage"? Because they don't make sense. And of course you haven't calculated in the energy of the wood you burn.

Roland --- You're right, I should have multiplied instead of divided. Oops. We use 646 KWH or 2326 MJ of electricity.

I didn't even try to factor in firewood used. That gets really complex since we burn wood from many different species of trees....

I searched for this because I am doing exactly the same thing you are. My various parts should be getting here this week and then I will have everything except the batteries. I am thinking I will buy 4 six volt batteries from Sams Club. These golf cart batteries have 220 AH and the cost is not bad, around $80.00. I've bought 2- 100 watt panels to play with and the charge controller will handle up to 30 amps so I have room to expand.

There is still a lot to learn, but I think this will be fun and not a total waste of money and time. Living a simple lifestyle is the key. I enjoyed your posts here and just had to put my two cents in.

All,

I would like to do a little DIY project to experiement with LED, solar, and battery lighting. They sell these torch flash lights on Ebay for only $40 which claim to be 1600 lumens(link below) which is pretty bright. I would like to take one of these and either put rechargable batteries in it or purchase a battery pack. So my question is if I wanted to use this to light up my backyard for say 12 hours a night can someone help me calculate the size of battery and solar panel required? Or offer any links to how I can wire this together? Look forward to the response. I also realize I may be taking the wrong approach by purchasing a flashlight. I would love to build this from scratch if possible I'm just not sure I would have the required skills.

http://www.ebay.com/itm/BRIGHTEST-1600-LUMENS-6-CREE-Q5-LED-T6-TORCH-FLASHLIGHT-/220566010740?pt=US_Flashlights&hash=item335ac23b74

1- Reading a clean blog without insults is great guy/gals. Sharing knowledge & ideas without attitude is what I've been seeing here and LOVE it!!!

2- LED flashlights: assume you get 1000 lumens from the 1600 lumen LED flashlight, that is still good. Buy the $20 LED Ebay light. Put in the battery and turn it on HI. How long did it stay bright?

Calculate the power consumed from that for sizing a battery supply for the solar system to operate that LED light. Multiply as necessary. CAUTION -- LED small flashlights will get HOT and degrade the LED or the electronic control circuits if left on HI Beam for extended periods of time. Consider the 10w LED square flood light for $15 on Ebay. I have one and I save 90w per outdoor incandescent bulb removed. Good Luck to all. De-Energy is the simpliest energy savings & the savings are immediate. Bye. LED House.

Hi This is Jon In Hawaii, I think it's grand what your doing and the world is full of folks saying it can't be done till someone like you just does it! Since the guy says solar is poo poo Might i suggest another approach. how about conversions of organics to create biodiesal and do a gas/diesal to electric where needed and a gas stove or heater where needed. Just a thought. Right now i am looking into the duckweed and BSF for organic fish food, and grow a moringa tree for forage for the livestock as well as the leaves supply about 18 or the 20 amino acids needed. Ok visit my site http://hawaiianparadisecoop.wordpress.com/ and keep up the good work.. Say no to poison GMO! foods.

I just posted this in a much older thread, but then I found this recent one, so I thought I'd jump into the current conversation.

Could you put the electricity to use as it comes in, instead of trying to sell or store it with a grid tie or battery bank system? Isolate the solar electricity in a closed circuit system, and plug in as many applications that could use as much or as little electricity as the solar panels happen to generate, storing as much as possible?

I'm thinking of a system where you might:

-rotate through a bunch of rechargeable AA batteries, letting them charge when the sun is out, then use them for low-watt LED lighting in your home while another bunch recharge.

-charge up laptops.

-run an ice chest freezer, essentially using it as a "cold battery", possibly with several jugs of water(ice) in the bottom to maintain freezing temps when there's no sun. This one I'm not so sure about, as I don't know how well it would maintain cold, but if it would work it would be a great option. They say that frozen food will keep for a week or so during a power outage, so... maybe if you only open the freezer when the sun is out and it's running.

-charge electric power tool batteries (for a power drill, circular saw, reciprocating saw, etc).

-run a food dehydrator.

Other ideas?

I happened on your site because I was trying to figure out how many batteries I could charge with a 220 watt panel. However the maths involved is way beyond me. I like the look of where you live, I see you are spoilt by having an open sky to capture as much sun as possible. I'm in a wooded place where I get good sun when the sun is high but winter can be difficult. I've been living off grid for thirty years and it is now second nature to turn off the light once you find what you're looking for.

It has long been clear to me that solar power is not for anyone who wants to live a "normal" life but for those who can accept living off grid without feeling hard done by, it is fine. I started my system with 5 45 watt panels with a 30 amp regulator and a 600 watt converter. I make toys and I have a few electric tools which work fine through the day, even for a couple of hours in winter.

What I wanted to mention was just a little thing which you do not seem to have touched on, second hand stuff. I recently bought 3/220 watt panels for €400 which I think is about the same in dollars, which is almost the list price for a similar "new" panel. Mine were actually new, and they came from a fire sale of a bankrupt dealer.

What I want to do is fix a panel onto the roof of a truck and I was curious about how many batteries I could hitch to it.

It was a great pleasure to read your opening statement where you said that your plan was to buy your panels and batteries when you had the money. The experts hand you a form on which you list all your appliances etc then they will propose a system to suit, they don't care too much for people like us.

Exactly the info I was looking for. Trying to do the same project, but utilizing much smaller batteries (from a few computer UPSes) and panels (I have 4 x 15W panels). Thank you very much for this post! Also enjoyed reading other people's setups for whole houses (and rooms) trying to figure out my needs for my house which I intend on converting to the off-grid one.

VN

Hi, I found this blog when searching for information about existing solar systems that are actually in use. Great project, and great comments here!

I want to add my own experience with solar: in the last 2 years I have used solar energy for powering the consumer circuits in my campervan(s). My latest project is here: http://microcamper.blogspot.co.uk/ (still work in progress). My approach has actually been slightly different: I did NOT attempt to get the biggest battery so that it could absorb the generated power from the solar panel, but rather tried to get the biggest solar panel so that it could top up the battery. After doing some research, I found that it is important to keep the battery topped-up, as only then it will perform fully. So if the solar panel is not able to recharge the battery completely, even on a cloudy day, then the battery will slowly drain away. Therefore, in my opinion, it is better to oversize the solar panel, even if that means that the solar controller will then not pass on the additional energy into the batteries, once they are full.

Here are a few data from my solar experience: one 150W solar panel, flat mounted on the car roof (therefore no optimal direction towards the sun), in moderate UK climate (probably similar to the US Northwest). The battery is a 12V "leisure battery" which can be drained deeper than regular 12V car battery (still it should not be drained more than 50%) with a capacity of 115 Ah. I am running continuously an electric compressor refrigerator which draws about 3A when it is running. Fortunately the compressor only runs about a quarter of the time, so the refrigerator can be run self-sustained throughout the summer months. In winter, however, the drain on the battery is too much, and the solar panel is not able to keep the battery fully charged every day. Being mounted in a car, the car alternator kicks in when the car is driving, and helps recharging the battery. This is a bit "cheating" regarding the self-sufficient solar aspect, but I have no other choice since my main priority is to keep the battery fully charged.

So my view is that it is better to get more solar power instead of more battery power, so that the solar power is able to charge the battery fully every day. In case there is more energy needed than can be absorbed by the batteries, then one can just use more electric consumers which are then live powered. For a house this could be, for example, an air conditioning system. A/C can easily use a lot of energy, and therefore it would make sense to only use that when surplus energy is available.

Reinhold

I want to use solar panels above my 8'X10" shed to run a 9.500 btu air conditioner for 7 hours a day. I want to connect some batteries to the panel and use them. How many kilowatts should the panel be, and how many batteries are needed? Also what power converter? I am a solar newbie (dummy). Thanks. Arnele

I want to use solar panels above my 8'X10" shed to run a 9.500 btu air conditioner for 7 hours a day. I want to connect some batteries to the panel and use them. How many kilowatts should the panel be, and how many batteries are needed? Also what power converter? I am a solar newbie (dummy). Thanks. Arnele

I live in Southern California and I have a 100 watt solar panel on my patio. I live in a townhouse project and my available hours for direct/ indirect sunlight is about 6 hour a day. I use my system for emergencies only as we have earthquakes here somewhat regularly. I use my system to provide power to my refrigerator and micro wave oven and maybe a light or two. I use 2 deep cycle marine storage batteries rated at 100 amp hour for my system. Most of my friends like gas generators for their spare power but if you go longer that a few days you have run out of gas for the generators.

Solar power is great as it's free and always there for our use.

This is not brain surgery so don't get so wound up about the technical details and go with what works.

Good luck.

Bill

so thankful for info comment by stewart gangwer — Fri Apr 17 01:37:43:15 I have been struggling with my new solar setup and his setup is almost exactly like mine. Thanks for the help!!!

I am from Nigeria, we just moved to a personal house and i'm planning to go off grid.Starting 150w-12volt Ra battery, what size of solar panel and inverter do I go for?

I really enjoy this blog. God bless you all for the contributions