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.
The number 3 which indicate 50% depth of discharge is come from where ?<br>
<p>The 3 does not indicate the 50% depth of discharge as you say.<br>It represents your estimated number of days of power use, without the system being recharged by the sun. This example is over a 3 day period of cloudy weather, you may wish to change that to 4 days or 5 days etc. The number 2 is what you would multiply with to represent the 50% discharge. Hope this helps.</p>
<p>If all you are looking for is reducing your energy bill, all you need is a solar panel array and bi directional meter installed. Its super cheap compared to what is being described in this article. What this will do is run the meter in reverse when the sun is shinning and reduce your electricity bill, it can even make you money if you put out more than you use, if permitted by your utilities company. </p>
<p>my electricity supply is 99.9% perfect only ever went off for a hour in the last ten years.</p><p>But my electricity costs are crazy about &pound;12-&pound;15 a week uk residents only get about 150 days of sun a year though so i'm sceptical on this.</p><p>is it possible to be off grid and flip a switch allowing me to go on grid if the batteries were to run dry due to a fortnights bad weather or simply to auto use the grid if not enough power in the batteries </p>
<p>I have an off-grid system using micro hydro and solar. Making the leap from lead acid to l-ion and am looking at these guys to do the job: <a href="http://www.swellenergy.com" rel="nofollow"> www.swellenergy.com</a></p><p>Anyone have experience working with them or know how their batteries stand up?</p>
<p>Did you get set up? We are an American firm that designs and manufactures energy storage. I'd love to talk with you. </p><p>R. Bean- 702.208.9873</p>
<p>I am in the process of designing my small off grid solar system. Help me out plz. Can a 500 watt pure sine wave inverter run a 0.5hp submersible water pump. </p>
sir i have a question on sizing the battery for solar system my battery rating is 12v 100Ah and 12v 55Ah. I follow your tutorial. My inverter is 48v input.you see below my calculation and i divide the nominal voltage of my battery so the answer is 533.82Ah i will divide it again in my available battery 533.82/100ah= 5.33 so i will use 6 battery 100ah.. its coorect?<br>5,445wh/day<br>.85 Battery loss<br>50% DOD<br>5,445x2days= 10,890wh<br>10,8901/.85= 12,811.76<br>12,811.76/.5= 25,623.52<br>25,623.52/48= 533.82 Ah <br>
<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>you say about the kettle using so much power and being not cost efective then consider just stop wasting electricty and using a gas cooker to boil the kettle, I use gas now and it very cost efective. Also gas for water heater and bbq. When I go totally off grid then a wood fired heater for cooking, water heater, etc.</p>
<p>Are you being sarcastic? you use the gas, costs extra to go get the gas, and much of the heater goes around the side of the kettle.<br> submerged elements are highly efficient. What makes the kettle less efficient is people tend to over fill it, and then re-boil the water over and over, and often the water doesn't need to be boiled to 100+ degrees.<br><br>Woodfired can be cheaper if you have ready wood, preferrably deadfall. My parents have that set up. Although it is not available in many areas due to the town/city pollution regulations.</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>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>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>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>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>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.

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