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How many batteries do I need for my solar panels?

Solar panelMost people start sizing their solar setup by figuring out how much energy they use in their home, then choosing enough panels and batteries to provide that much juice.  We're coming at the issue from a completely different direction.

We plan to buy a few solar panels now and then as we can afford them, to hook up the appropriate number of batteries, and to slowly work various parts of our household off the grid.  For example, our first panels might be just enough to power our laptops and a couple of lights, so we'll wire those gadgets directly into the solar setup, sticking to DC if possible.  Since it's a bad idea to hook old and new batteries together, we'll probably keep each little system separate, which will cost a little more but will provide backups.

Which is all a long way of saying that we have two 45 watt solar panel kits (each of which consists of three 15 watt panels) and we're trying to figure out how many batteries we need to support those panels.  My first step is to determine how much energy we're likely to get from the panels on an average day. 

Sun hoursYou can either size your system based on the average peak sun hours (which will give you an over-estimate for the winter and an under-estimate for the summer) or based on the winter peak sun hours (a worst case scenario).  If you wanted to ensure you had enough power even during the shortest days of the year, you'd want to use the winter peak sun hours in your calculations, but I think it makes more sense in our system to use the average peak sun hours, which is roughly 4.2 for our area.

The amount of energy your solar panels will produce per day can be calculated using this simple formula:


Solar panel output = Solar panel rating (watts) X Sun hours

Solar panel output = 90 watts X 4.2 hours

Solar panel output = 378 watt-hours


Most solar systems recommend that you multiply your solar panel output by by 3 (or 4 or 5) when sizing your battery bank so that you'll still have juice after several days of cloudy weather.  Since we're not planning on going off-grid anytime soon, I think I'll stick to the bare minimum figure above, though, and just plan on plugging our appliances back into on-grid power when our batteries get low.

Next, you can calculate how many watts the battery of your choice will hold.  Batteries are generally rated by volts and amp-hours, which allows you to calculate watt-hours as follows.  (This example is a typical golf cart battery: 6 volts and 200 amp-hours.)

Battery watt-hours = Volts X Amp-hours

Battery watt-hours = 6 volts X 200 amp-hours

Battery watt-hours = 1,200


That sounds great, right?  I'd just need one battery for three days!  Wrong.  Deep cycle batteries lose a lot of life if you discharge them below 30 to 40%, so the amount of usable energy in the battery is more like:

Usable watt-hours = 0.6 X Battery watt-hours

Usable watt-hours = 720 watt-hours


Batteries in seriesIt's still looking like one golf cart battery would be enough for nearly two days, but there's one more factor to consider.  Since we want to plug 12 volt DC appliances directly into the system rather than losing efficiency by converting from DC to AC, we need our battery system to be 12 volts, not 6 volts.  That means we need two batteries wired together in series to boost the voltage.

As a side note, we got an advertisement from Harbor Freight as I was researching, and the company wants to sell me a "solar battery" for $75.  Was it a good deal?  The battery is rated at 12 volts and 35 amps, so it would hold 420 watts, or 252 usable watts.  The benefit of the battery is that it is already 12 volts, but I'd still need two of them since the battery isn't even enough to soak up the energy from our solar panels for one day.  If I'm doing my math right, the Harbor Frieght battery would cost 30 cents per usable watt, versus 21 cents for the golf cart battery (using a rough estimate of $150 for the cost of the golf cart battery).  Of course, I'd need to factor in longevity to really get an idea for which set of batteries would be a better deal, and there's no information about Harbor Freight's battery life span on the internet.  Since our nearest golf cart battery supplier is closer than our nearest Harbor Freight, we'll probably settle on the former.

Our chicken waterer is perfect for chicken tractors since it never spills on uneven ground and is easy to fill without crawling into the tractor.


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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.

Comment by Roland_Smith Sat May 26 11:28:08 2012

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!)

Comment by anna Sat May 26 11:46:39 2012
Next to the max 40% discharging that you've already calculated in, the efficiency of the batteries will mean that you get much less out of the batteries (which essentially is what the system is about) than the solar panel put in.
Comment by Roland_Smith Sat May 26 12:20:49 2012
Roland --- But there should be less going into the batteries than I calculate too since the solar panels won't be 100% efficient, etc. What I'm trying to do is to match the batteries to the panels, not to figure out exactly how much is going to come out of the panels or out of the batteries. Am I just being dense, or are we talking about two different things?
Comment by anna Sat May 26 15:01:13 2012

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.

Comment by Roland_Smith Sun May 27 03:18:28 2012
Roland --- Got it! Luckily, I have absolutely no expectations for this system. It's more of a learning opportunity, figuring out the parts and how they hook together. Any electricity we get will make my day! :-)
Comment by anna Sun May 27 11:21:55 2012

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.

Comment by doc Sun May 27 20:44:29 2012

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.

Comment by anna Mon May 28 07:21:14 2012

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.

Comment by Roland_Smith Mon May 28 08:33:28 2012

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. :-)

Comment by anna Mon May 28 11:51:54 2012
1 kW = 1000 J/s and 1 h = 3600 s. So 1 kWh = 3600000 J = 3.6 MJ
Comment by Roland_Smith Mon May 28 12:22:11 2012

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.

Comment by anna Mon May 28 16:19:44 2012

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.

Comment by Jane Mon May 28 18:20:00 2012

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. :-)

Comment by anna Mon May 28 19:16:30 2012
Conservation is, of course, the key, particularly if recyclable energy sources rather than non-renewables can be employed. If it's back-up power you're after, why not consider an old engine/alternator/battery system run on wood gas? You can run it on anything that burns.
Comment by doc Mon May 28 19:29:31 2012

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....

Comment by anna Mon May 28 20:21:44 2012

@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. :-)

Comment by Roland_Smith Tue May 29 15:18:45 2012

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....

Comment by anna Tue May 29 15:58:58 2012
Hi Anna. Without getting into too many technicalities, your calculations are correct. For 45W-90W of solar power I would just get a 12V and approx. 100Ah battery to keep your life simple. This leaves you room for some expansion. Be careful when charging laptops, some of them consume A LOT of energy. If you still questions regarding your solar panel setup please do not hesitate to give us a call 510 400 814. There are engineers in staff with a lot of experience in solar off grid systems.
Comment by Linda Fri Nov 2 13:01:43 2012

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. :-)

Comment by Jim Wed Jan 16 22:32:58 2013

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

Comment by Jeremy Whittaker Thu Jan 17 19:27:58 2013

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.

Comment by LED House Tue Jan 29 17:51:41 2013

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.

Comment by jon kirby Mon Feb 25 05:22:36 2013

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?

Comment by James Sat Mar 16 00:50:44 2013