Off-Grid Solar Battery Calculator

Use our solar battery calculator to easily calculate the battery bank size needed for your off-grid solar system.

Solar Battery Calculator

What Size Solar Battery Do I Need?

Here are the main steps involved in sizing a solar battery bank:

1. Calculate Your Energy Consumption
2. Pick a Battery Type
3. Pick a Battery Voltage
4. Pick a Depth of Discharge
5. Pick a Number of Backup Days
6. Calculate Your Solar Battery Size

Let’s run through each.

1. Calculate Your Energy Consumption

Before you can size your solar batteries, you need to know how much energy your system consumes.

1. Use our off-grid solar load calculator to calculate your system’s energy consumption. The number it returns is listed in units of kWh/day.

PHOTO – result from load calc

2. Convert kilowatt hours to watt hours by multiplying by 1,000. For instance, based on the value above, you’d do the following calculation:

Wh/day = kWh/day × 1,000

Wh/day = 2.76 kWh/day × 1,000

Wh/day = 2,760

3. Save this number for the final step. You’ll need it to size your battery bank.

2. Pick a Battery Type

The 2 main types of solar batteries are LiFePO4 (lithium iron phosphate) batteries and lead acid batteries. Lead acid batteries include sealed (SLA), flooded, gel, and AGM batteries.

1. Consider the differences between LiFePO4 and lead acid batteries. LiFePO4 batteries last longer, charge and discharge more efficiently, and have 100% usable capacity. Lead acid batteries have much shorter lifespans, charge and discharge less efficiently, and typically only have 50% usable capacity.

LiFePO4 batteries used to be much more expensive. But, in recent years, lithium battery prices have plummeted to the point that budget LiFePO4 batteries are now cheaper than comparable lead acid batteries. Nowadays, I almost always recommend lithium batteries.

2. Decide on a battery type.

3. Pick a Battery Voltage

The most common voltages for solar batteries are 12V, 24V, and 48V.

Picking a battery voltage (aka system voltage) has lots of downstream effects on the size of your charge controller, solar array, and wiring. Give this step the time it deserves.

1. Watch this video from Explorist Life. Although it’s targeted toward campervan electrical systems (and quite technical for beginners), it’s the best resource on the topic that I’ve found.

2. Decide on a battery voltage and save this number for later. LiFePO4 nominal voltages are actually slightly higher than the standard whole-number nominal voltages. So, if you’re using LiFePO4 batteries, also keep in mind the alternate nominal voltage, which I’ve listed below:

4. Pick a Depth of Discharge

Now that you know what type of battery you want, you’ll want to decide on your target depth of discharge (DoD). Essentially, this number captures how much of a battery’s total capacity you use.

1. Consider the standard depths of discharge based on battery type. For lead acid batteries, the standard DoD is 50%. For LiFePO4 batteries, most people use a value of 100%. If you want, you can just use these standard values. I almost always do.

2. Consider if you want to extend your battery bank lifespan by reducing your target depth of discharge. A battery’s average depth of discharge affects its lifespan, and the estimated cycles you get from various depths of discharge are usually laid out in a battery’s datasheet. For instance, some solar DIYers who use LiFePO4 batteries will aim for an 80% depth of discharge, since doing so can greatly increase the battery’s lifespan.

3. Decide on a target depth of discharge and save this number for later.

5. Pick a Number of Backup Days

Battery backup days, also called days of autonomy, refer to how many days your battery bank can last without being recharged by your solar panels. They’re meant as a hedge to prevent your batteries from dying during stretches of bad weather, when solar panel output can be greatly reduced.

When considering how many backup days I want for my battery bank, I consider 3 main factors.

1. Consider how much sun your location gets. Use our peak sun hours calculator, or consult the map below, to find your location’s average sunlight hours. I consider 3 or fewer peak sun hours to be low, around 4 to be medium, and 5 or more to be high.

Tip: If your solar system will be mounted on a vehicle, such as a van or RV, consider the peak sun hours of your planned destinations at the time of year you plan on visiting them.

2. Consider how important it is that your battery bank not die. How critical are the loads you’ll be powering with the battery? For instance, maybe you’ll be boondocking in your RV and your solar system will power important appliances like your fridge. The more critical the loads, the more backup days you’ll want.

3. Consider how much variation you have in your energy consumption. The more variation, the more likely it is that your average energy consumption calculated in Step 1 doesn’t paint the whole picture, and the more backup days you’ll want to account for days with much higher than average energy consumption.

4. Decide on a number of backup days. There’s no right or wrong answer here, it’s more what you’re comfortable with given your specific situation.

If your area has a low number of peak sun hours, your solar system will power critical loads, and your energy consumption varies a lot day to day, then consider 5 backup days.

On the other hand, if your area gets a lot of sun, the consequences of your battery bank dying aren’t too high, and your daily energy consumption is pretty constant, you may be able to get away with, say, 3 battery backup days.

Note: You may be thinking that you can start with a small solar battery bank and expand its capacity later if needed. This is essentially true — you can buy an identical battery and wire it in parallel to increase capacity without much fuss. However, due to how batteries age, it’s best to size your battery bank correctly from the start. And, if it ends up being smaller than you need, it’s best to add more batteries as quickly as possible. Some brands recommend within 12 months, while others recommend within 3-6 months.

6. Calculate Your Solar Battery Size

You should now have the following numbers:

• Daily energy consumption (Wh/day)
• Battery type
• Battery bank voltage
• Depth of discharge
• Battery backup days

Now you (finally!) have all the info you need calculate your solar battery size.

For reference, here’s the formula we’ll be using:

Battery bank nameplate Ah = (Daily energy consumption * Battery backup days * Inefficiency factor) / (Battery DoD% * Battery bank voltage)

Let’s work through it step by step.

1. Multiply your daily energy consumption (in watt hours per day) by your battery backup days. This gives you how much energy your battery bank should be able to supply without any solar charging. Since battery backup days are also called days of autonomy, I’ll refer to this as your autonomous energy consumption.

Autonomous energy consumption = Daily energy consumption * Battery backup days

Autonomous energy consumption = 2,760 Wh/day * 3 backup days

Autonomous energy consumption = 8,280 Wh

2. Multiply your autonomous energy consumption by your battery type’s inefficiency factor to get your battery bank’s usable watt-hour capacity. Batteries don’t charge or discharge with perfect efficiency, and this factor captures that. I recommend a factor of 1.05 for LiFePO4 batteries and a factor of 1.2 for lead acid batteries. Let’s assume you’re using a LiFePO4 battery.

Battery bank usable Wh = Autonomous energy consumption * Inefficiency factor

Battery bank usable Wh = 8,280 Wh * 1.05

Battery bank usable Wh = 8,694 Wh

3. Divide your battery bank’s usable watt-hour capacity by your target depth of discharge to get your battery bank’s nameplate watt-hour capacity. Let’s say you want a target depth of discharge of 80% for your LiFePO4 battery bank.

Battery bank nameplate Wh = Battery bank usable Wh / Depth of discharge

Battery bank nameplate Wh = 8,694 Wh / 80% DoD

Battery bank nameplate Wh = 8,694 Wh / 0.8

Battery bank nameplate Wh = 10,867.5 Wh

At this point, you have your solar battery size in watt hours, which may be all you need to pick your batteries. However, many solar battery brands express capacity in amp hours rather than watt hours. So, as a final step we’ll calculate the battery’s capacity in amp hours.

4. Divide your battery bank’s nameplate watt-hour capacity by your battery bank voltage to get your battery bank’s nameplate amp-hour capacity. Recall that LiFePO4 batteries have slightly higher nominal voltages. So if you have 12V LiFePO4 battery bank you’d use a voltage of 12.8V.

Battery bank nameplate Ah = Battery bank nameplate Wh / Battery bank voltage

Battery bank nameplate Ah = 10,867.5 Wh / 12.8 V

Battery bank nameplate Ah = 849.02 Ah

So you need a battery bank with an amp hour capacity of at least 849Ah.

Solar batteries are most often sold in increments of 100Ah (e.g. 100Ah, 200Ah, 300Ah, etc.) so in this case you’d round your battery bank size up to 900Ah.

You’ve figured out your solar battery bank size…awesome!

5. If needed, decide on how your battery bank will be wired together. For small solar battery banks, you might only need to buy a single battery. However, for larger battery banks, such as greater than 400Ah, you’ll probably need to buy multiple batteries and wire them together in series and/or parallel.

So, for this example, you could buy three 12V 300Ah LiFePO4 batteries and the appropriate battery cables. Then you’d wire the three batteries in parallel to get a 12V 900Ah LiFePO4 battery bank.

Note: Pay attention to your battery’s max wiring configuration and be sure not to exceed it. For instance, many budget LiFePO4 batteries can only be wired up to a “4S4P” configuration, meaning a maximum of 4 batteries in series and 4 in parallel. So, if that were the case for this example, you wouldn’t be able to buy nine 12V 100Ah LiFePO4 batteries and wire them all together in parallel since that would exceed their max parallel length.

3 More Off-Grid Solar Calculators

1. Solar Charge Controller Calculator: Find out what size charge controller you need.
2. Solar Panel Charge Time Calculator: Find out how fast your solar panel will charge your battery bank.
3. Solar Panel Angle Calculator: Find the best solar panel angle for your location.

References

1. Global Horizontal Irradiation Map by the Global Solar Atlas is licensed under CC BY 4.0.