If you are designing a solar electricity system and don't have access to the grid, you are going to have to deal with batteries. When you start looking at solar batteries you are going to encounter a little math. Fortunately,  SolarTown is here to guide you through the calculations!

The general goal when designing an off-grid battery bank is to get a system that is big enough to supply all your needs for a few cloudy days, but is also small enough to be charged by your panels. Before you get started here you may want to check out this article. It serves as a good introduction to the solar batteries I'll be talking about:


Alright let's get started! Here are the steps to sizing your off-grid system.

Step 1: Calculating Your Amp-hour Needs

1.  Inverter size
To determine the inverter size we must find the peak load or maximum wattage of your home. This is found by adding up the wattage of the appliances and devices that could be run at the same time. Include everything from microwaves and lights to computers and clocks. The sum will tell you which inverter size you need.

Example: A room has two 60 watt light bulb and a 300 watt desktop computer. The inverter size is 60 x 2 + 300 = 420 watts

2.  Daily energy use
Next find the energy used in a day. Figure out how long each electronic device will be run in hours during a day. Multiply the wattage of each device by its run-time to get the energy in watt-hours per day. Add up all the watt-hour values to get a total for your home. This estimate is likely too low as there will be efficiency loses. To get a very rough idea of the real value with system loses, multiply by 1.5. This will help account for decreasing performance when temperature increases.

Example: Light bulbs run for 5 hours a day. Computer runs for 2 hours a day. 120 x 5 + 300 x 2 = 1200 watt-hours. 1200 x 1.5 = 1800 watt-hours

3.  Days of autonomy
Now decide how many days worth of energy you want to store in your battery bank. Generally this is anywhere from two to five.

4.  Battery bank capacity
Finally we can calculate the minimum battery AH capacity. Take the watt-hours per day and multiply them by the number you decided upon in 3. This should represent a 50% depth of discharge on your batteries. Therefore multiply by 2 and convert the kwh result into amp hours (AH). This is done by dividing by the battery voltage.

Example:  You want the battery bank to last three days without recharging and that you use 1.8 kwh per day. As 1.8 x 3 x 2 = 10.8kwh, this is the energy we need from the batteries. Converting this to AH we have to divide by the voltage of your system. This can be 12, 24 or 48 for commercial application. If we choose to use 48V, the minimum AH capacity is then 10 800/48 = 225 AH. Now if you divide by your battery's rating you find the number of batteries you must use.
<p>You are not done yet.</p><p>Where is the charge and discharge calculation and rates.</p><p>Here is what you said:</p><p>''Example: A solar array is producing 1 kw and charging a battery bank of 24V. The controller size is then 1000/24 = 41.67 amps''</p><p>Batteries have a specific charge and discharge rates and most manufacturers will recommend a charge rate for their specific batteries. </p><p>I will tell you right now that 40 some amps will kill most 225 ahr. batteries very soon since many manufacturer recommend a C10 or 10% for lead acid batteries of the 225 ahr which would be 22.5 amps recommended charging and discharging.</p><p>Now we have to parallel two 225 ahr batteries which makes 450 ahr to met the requirements of the manufacturers charge/discharge rate just to keep those batteries alive. A little more batteries would be even better.</p>
<p>Where is the solar array calculation mentioned in step 2 ?</p>
<p>''Example: A solar array is producing 1 kw and charging a battery bank of 24V. The controller size is then 1000/24 = 41.67 amps.''</p><p>You can NOT use the nominal voltage of the battery or battery bank to calculate the amps the controller has to handle. </p><p>Besides the controller size got to be calculated from all the panels voltage and amps given on the panel data sticker and not from the actual output a solar panel produces.</p>
<p>Wait a sec ... is the example above correct? Fine, we need 1800wh per day of &quot;inverted&quot; electricity, but the inverter draws amps off the 12v batteries, not watts. I think we need a Step 2.5 to convert the 110v watts to 12v draw off the batteries as follows:</p><p>1800w/110v = 16.4AH draw on batteries by inverter</p><p>then,</p><p>Step 4: 16.4AH x 3 x 2 = 98.4 AH</p><p>But then, no one sensible would use old 60w incandescents over modern 13w CFL's in this application, so realistic requirement is closer to 98.4AH x (13w/60w) = 21.3AH battery bank capacity.</p><p>Hope this is correct, else going green is not economical. </p><p>Comments?</p>
<p>And why use an desktop computer that draws 300 watts per hour, when a laptop computer can draw as little as 13 watts.</p>
<p>I think you are confused. Watts are Watts. 110v or 12v it doesn't matter. More voltage means less amperage to produce the wattage. 1800W/110V = 16.4A, 1800W/12V = 150A. </p>
<p>Sorry, think final calc should be:</p><p>49.2 (laptop) + 49.2 x (13/60) (lights) = 59.9AH</p>
<p>Another reality check: my HP laptop only uses 65w rather than 300w, so total requirement for above is now back to 21.3AH.</p>
<p>what about the factor, efficiency of battery, 75-85%. it's need more capacity for our own need</p>
<p>Maybe the author accounted for that with the &quot;system losses&quot; multiplier of 1.5?</p>
<p>Thank you! This article has really helped me. I'm in the grass roots stage of converting to solar and I find myself getting overwhelmed when I starting looking at all the numbers. There is a lot to consider. Sometimes I feel like I'm drowning in opinions when I start to read forums and blogs on the topic..</p><p> <a href="http://www.solarpiper.com" rel="nofollow"> www.solarpiper.com </a> set me on the right track, providing me with ton of unbiased educational and instructional articles. I've been starting to piece together a way to transition my energy consumption from an electric company to that of solar energy but have no idea where to start or what you would even need, but thankfully, <a href="http://www.solarpiper.com/" rel="nofollow">http://www.solarpiper.com/</a> created a calculator to assist in making decisions about how to add solar power to your property. It can be used to find out how much solar energy can be collected on your property, what your needs and requirements are, and how much it will cost you. <br> I can't wait to be leading the way to the future and supporting clean energy techniques!<br></p>
<p>Thanks for the information</p>
<p>If you want to maintain you solar batteries in top condition or recover weak ones check this site out http://www.recovermybatteries.com/ex02-12-120-800.html </p>
<p>thank you very much, good explanations. but I found some tips as valuable here <a href="https://www.patriotdirect.org/choosing-the-right-batteries/" rel="nofollow">https://www.patriotdirect.org/choosing-the-right-batteries/</a></p>
<p>regarding the inplix site... thanks for sending me and other to a site with a trojan imbedded in it.</p><p>grrr.</p>
<p>i have one 12 volt batterie &amp; 20 watt solar panel how much time taken full batterie</p>
<p>This irks me that folks come here to create a instructable just to promote their business and to make a sale.</p><p>Shame on you.</p>
<p>Why pay a huge amount like $1000's for utilization of solar or wind power when you can have the opportunity to build your own home made solar system for less than $200. You can Learn more on http://inplix.com</p>
<p>good place to learn about it , but I've chosen offer from inplix. It was good and easy way to build my own photovoltaic panel system :)</p>
<p>How you can DO IT YOURSELF :</p><p>1. Go to http://inplix.com </p><p>2. Search for your solution </p><p>3. Think a little bit </p><p>4. Prepare your screwdriver :)</p><p>4. Build your own tool</p><p>5. Enjoy Free energy for rest of your life</p>
<p>how calculate watts for the solar panel and charger controller.</p>
<p>You have to add your load, work out what you maximum use time is. That gives you demand in Wh (Watt hours)<br><br>For fixed solar panels, after a few years they'll drop off to 80% efficiency, so leave room to add a couple more then. But generally you get 4 hours charging time, maybe 5 or 6 if you're in a good spot/climate.<br> So you need to get all your Watthours from first paragraph in that 4 hours.<br> But in the winter you'll probably only get half as much usable light (at best) so if their is going to be needed winter PV, double that panel again (or use a generator to boost in winter, as you might need powerful things like heaters).<br><br> OTOH if you're in a really hot area with mild winters, consider a separate circuit for your airconditioning. Design it so it comes on when the power is there and the day is already sunny, and shut it down in the evening, that way only a small amount of storage is required.<br><br>So Watthours x 6 (to get that 24 hour charging... don't forget that you still draw full power to appliances in the charge time!). x2 again for winter.<br><br>Then if you're expecting no-sun days, either get a generator for those days or multiply the previous result by the number of days you need. ie if you are only getting 4hrs of good light, to charge batteries to last 3 days , you're going to need a LOT of panels... ie 4hrs to cover 72 hours = 18x initial Watthours, then x2 again for winter ( that's thirty six times your _daily_ load)<br><br> For all those panels, you will be able to add the currents together, most panels aren't worth going over pairs (about 60v). So add ALL the currents together, divide by 2 if going to put pairs of panels in series. You'll need at least that much charge control.<br><br>Then batteries. MAXIMUM charge speed is C/10. Thats total Capacity divided by 10. Your charge rate is ALL your panels ADDED together . Multiply that by 10, and you need that many Watthours worth of batteries MINIMUM ! <br> eg a Trojan SCS225 a liquid aka flooded 12v battery with deep cycle useful for solar PV, has a rating of around 1.5kWh (which is 1500 Watt hours).<br><br>What I found helpful is to split off separate circuits in my house. Lighting on one. Computer, screen, router on another. Laptop and NAS on a third. Looking at one for kettle, and another one for microwave. <br> This way I can build my system up step by step. It does cost a little more in the long run, but things like my computer run 24x7, so is really efficient for Inverters and panels etc as I they run at about 90% efficiency.<br> Things like lights are only on for a few hours, so a much smaller charge controller, smaller inverter and a few panels work well there.<br><br> The kettle and microwave only really see about 30 minutes use a day, but they can survive nicely on a cheaper &quot;modified sine wave&quot; inverter but it needs to be BIG, around 3kW for the kettle. but with 30 minutes usage that 1500Watthours a day... one (maybe two because of the big current) battery, and 1600Wh in 4 hours... 2 panels max...and a small 20A charge controller. (my computer has 2kW of panels and 90A worth of charge controllers and uses 7200Wh in a day)<br><br> 10 Led lights, 5 hours, 10 x 14 x 5 = 700Wh, 10Ledx14W = 140W inverter (cheap!), 700Wh/ 4 hours charge time = 175W of panels... one panel. 700Wh meand less than one SCS225 battery.<br><br>One thing to consider is the more run time you have, then the more effective changing to PV will be. A kettle takes a while to pay off that $500 panel, at 1.5kWh a day (about 25cents a day). While something using power continuously is going to be more effective to change over, as it then saves you money continuously.</p>
<p>If I have two 265w panels but their maximum output current is 9A do I still need a 60A charge controller? According to what you are saying it would be 530w/12v = 44A </p>
<p>The 60A charge controller will buy you some expansion space.<br>C/10 is maximum charge rate (rule of thumb) for batteries - you have 530W going in, so you need at least 5300 Wh worth batteries MINIMUM. (not to hard).<br><br>If that two times 9A is truly enough then get yourself a charger that will handle 2x9 = 18A, with some to spare in case of &quot;fringe events&quot;. so anything over 24-25A would be plenty. Remember you can parallel up the charge controllers in the future if needed, </p>
<p>400W x 24 hrs = 9.6kW. 9600 * 2days = 19200kW.19200 /50% = 38400kW.<br>38400/24 v = 1600Ah. 130Ah SCS225 = 28 batterys? sounds pretty expensive/brutal for a basic computer....<br></p>
<p>go for liquid battery </p>
<p>SCS225 (which is a flooded battery designed for deep cycle in solar)<br>approximately $500. a piece. $14,000 worth of batteries?? I can get a LOT of grid power for $14,000. And that was just 400W for my computer.<br>And flooded batteries dry out in hot weather....</p>
<p>Why pay a huge amount like $1000's for utilization of solar or wind power when you can have the opportunity to build your own home made solar system for less than $200. You can Learn more on http://inplix.com</p>
<p>Great help, I have installed a 2 x 235 watt Solar panels and they are up and running</p><p>It was easy to follow and calculate all the energy needed.</p><p>keep it up. Pretoria South africa</p>
<p>200 watts a hour / 24hrs and end up needing, 24 batteries that are 150 amp just to run a small air pump, sorry something wrong in this calculations, these are truck batteries at $100 each don't add up that worth it and 4 solar panel 2x75 and 2x80</p><p>Can buy a lot of fuel for small generator for this price of batteries </p><p>Starting to sound like a yacht price of sails mast rigging &quot;need replacing every ten years max if lucky&quot; etc ropes,pulleys etc. ends up better off to buy fuel get there lot easier.</p>
Please guide me<br>I have 72,000 watts hours a day consumption, how much solar panels should i installed, here we have avg 10 hours solar can charge battreys, also i want to use cells instade of batteries, 48V, please tell me how much solar panels and 48v ah cell capacity required
<p>Thanks! I could never figure this out. This helps me a lot!</p>
<p>Hey Jon</p><p> Thanx for the info for battery sizing. I'm in the planning stages of solar powering my work shed and running a pool pump. This will help greatly help out.</p>
great job ... .. we are interested thanks
Thanks! Great article! I'm preparing to buy another battery and I found your instructable verey informative and decision-making.
Great article, I am very interested in matters of Solar and wind energy
A lot of great information, but did you really need to PLUG your website in 3 of the 4 steps? <br>
looks good <br>

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More by DIY Solar Jon: How to Size Your Off-Grid Solar Batteries DIY Home Solar: Planning a Solar Array (Beginner's Guide)
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