Picture of How to Size Your Off-Grid Solar Batteries
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.
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Step 1: Calculating Your Amp-hour Needs

Picture of 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.

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billsmile6 months ago

how calculate watts for the solar panel and charger controller.

You have to add your load, work out what you maximum use time is. That gives you demand in Wh (Watt hours)

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.
So you need to get all your Watthours from first paragraph in that 4 hours.
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).

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.

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.

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)

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.

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 !
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).

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.
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.
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.

The kettle and microwave only really see about 30 minutes use a day, but they can survive nicely on a cheaper "modified sine wave" 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)

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.

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.

tylerpittson9 months ago

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

The 60A charge controller will buy you some expansion space.
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).

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 "fringe events". so anything over 24-25A would be plenty. Remember you can parallel up the charge controllers in the future if needed,

mist42nz1 year ago

400W x 24 hrs = 9.6kW. 9600 * 2days = 19200kW.19200 /50% = 38400kW.
38400/24 v = 1600Ah. 130Ah SCS225 = 28 batterys? sounds pretty expensive/brutal for a basic computer....

go for liquid battery

SCS225 (which is a flooded battery designed for deep cycle in solar)
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.
And flooded batteries dry out in hot weather....

Great help, I have installed a 2 x 235 watt Solar panels and they are up and running

It was easy to follow and calculate all the energy needed.

keep it up. Pretoria South africa

kmyers64 months ago

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

Can buy a lot of fuel for small generator for this price of batteries

Starting to sound like a yacht price of sails mast rigging "need replacing every ten years max if lucky" etc ropes,pulleys etc. ends up better off to buy fuel get there lot easier.

Please guide me
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

Thanks! I could never figure this out. This helps me a lot!

dontno11 months ago

Hey Jon

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.

mganpate2 years ago
great job ... .. we are interested thanks
MacOSJoey2 years ago
Thanks! Great article! I'm preparing to buy another battery and I found your instructable verey informative and decision-making.
sopranosp2 years ago
Great article, I am very interested in matters of Solar and wind energy
nanosec122 years ago
A lot of great information, but did you really need to PLUG your website in 3 of the 4 steps?
Schmidty162 years ago
looks good