The battery is usually the most expensive part of a solar or inverter system, and the part people get wrong most often. Buy too little and your lights go out at 1 a.m. Buy too much and you have spent money on storage you will never use.
This guide is a battery-only calculator. It shows you how to work out the right capacity for your inverter system, how many batteries that means, how long they will last on your load, and how lithium and tubular batteries differ in the maths. If you want the wider picture first, including inverter and panel sizing, read our solar load calculator guide.
At PA DigiTech Solar, we size batteries from your real appliances and backup needs. If you would rather have us check your numbers, call 07068853073.
The 60-Second Answer
Required battery capacity (Wh) = Energy you need from the battery ÷ Depth of discharge ÷ Efficiency
Battery Ah = Required Wh ÷ System voltage (12V, 24V or 48V)
Everything else in this article explains where those numbers come from and how to avoid the mistakes people make with them.
Step 1: Understand Watt-Hours, Not Just Amp-Hours
Batteries are sold in amp-hours (Ah), but amp-hours alone do not tell you how much energy a battery holds. You need the voltage too.
Energy (Wh) = Voltage (V) × Capacity (Ah)
| Battery | Calculation | Energy |
|---|---|---|
| 12V 200Ah | 12 × 200 | 2,400 Wh |
| 24V 200Ah | 24 × 200 | 4,800 Wh |
| 48V 100Ah | 48 × 100 | 4,800 Wh |
| 48V 200Ah | 48 × 200 | 9,600 Wh |
This is why a “200Ah battery” can mean very different things. A 12V 200Ah battery holds one quarter of the energy of a 48V 200Ah bank. Always compare batteries in Wh or kWh.
Step 2: Find the Energy You Need From the Battery
Your battery only needs to supply energy when the panels are not producing enough, mainly in the evening, at night and on cloudy days.
- List the appliances you want to run after sunset and overnight.
- Multiply each one’s watts by the hours it will run in that period.
- Add them up. This is your night energy (Wh).
If you have not listed your appliances yet, the first steps of our load calculator guide walk you through it.
Grid-backup systems (where the grid recharges the batteries) work the same way. Use the hours of outage you want to cover instead of night hours.
Step 3: Apply Depth of Discharge (DoD)
Depth of discharge is how much of a battery’s rated capacity you can safely use. Draining a battery too deeply, too often, shortens its life.
| Battery type | Typical usable share (DoD) |
|---|---|
| Lithium (LiFePO4) | Often around 80 to 90 percent |
| Tubular lead-acid | Usually around 50 percent for long life |
These are working figures. Always follow the manufacturer’s specification for the batteries you choose, because products differ.
So a 4,800 Wh lithium bank gives you roughly 3,840 to 4,320 Wh of usable energy, while a 4,800 Wh tubular bank gives you about 2,400 Wh. That gap is the main reason lithium and tubular are sized so differently.
Step 4: Allow for System Losses
Some energy is lost when the inverter converts DC to AC, and in wiring and heat. A working figure of about 90 percent efficiency is common. Using 100 percent makes your battery look bigger than it is in practice.
Step 5: Put It Together
Required rated capacity (Wh) = Night energy ÷ DoD ÷ 0.9
Then convert to amp-hours at your system voltage:
Ah = Required Wh ÷ System voltage
Which System Voltage Does Your Inverter Use?
Your inverter decides the battery voltage. You cannot choose it freely. As a rough guide:
| Inverter size (approximate) | Common battery voltage |
|---|---|
| Up to about 1.5 kVA | 12V |
| About 2 to 3 kVA | 24V |
| 5 kVA and above | 48V |
Always follow your specific inverter’s specification. Some models differ.
Quick-Lookup Table: Battery Needed for a 48V System
This table does the maths for you using 90% DoD for lithium, 50% DoD for tubular and 90% efficiency. It is illustrative, so adjust for your actual batteries.
| Night energy you need | Lithium needed (Wh) | Lithium (Ah at 48V) | Tubular needed (Wh) | Tubular (Ah at 48V) |
|---|---|---|---|---|
| 2,000 Wh | about 2,470 | about 51 Ah | about 4,440 | about 93 Ah |
| 4,000 Wh | about 4,940 | about 103 Ah | about 8,890 | about 185 Ah |
| 6,000 Wh | about 7,410 | about 154 Ah | about 13,330 | about 278 Ah |
| 8,000 Wh | about 9,880 | about 206 Ah | about 17,780 | about 370 Ah |
| 10,000 Wh | about 12,350 | about 257 Ah | about 22,220 | about 463 Ah |
Round up to the next practical battery size. For a 24V system, double the Ah figures. For a 12V system, multiply them by four.
Worked Examples
These examples are illustrations of the method, not recommendations for your home.
Example A: Small home, essentials only
Night energy: 3,000 Wh (lights, fans, TV, router, share of fridge and freezer)
- Lithium: 3,000 ÷ 0.9 ÷ 0.9 = about 3,700 Wh, which is about 77 Ah at 48V. A single 48V 100Ah lithium battery would cover it.
- Tubular: 3,000 ÷ 0.5 ÷ 0.9 = about 6,670 Wh, which is about 139 Ah at 48V. Four 12V batteries of 150Ah or more in series would cover it.
Example B: Medium home with a pump and one air conditioner at night
Night energy: 6,000 Wh
- Lithium: about 7,410 Wh, or about 154 Ah at 48V. Two 48V 100Ah units in parallel give comfortable margin.
- Tubular: about 13,330 Wh, or about 278 Ah at 48V. That points to roughly 300Ah at 48V, such as two parallel strings of four 12V 150Ah batteries.
Example C: Large home, several night loads
Night energy: 10,000 Wh
- Lithium: about 12,350 Wh, or about 257 Ah at 48V, which points to about three 48V 100Ah units.
- Tubular: about 22,220 Wh, or about 463 Ah at 48V. This is a very large bank, and the bank size, weight and floor space become real practical issues.
Notice how quickly tubular banks grow. That is why total cost, lifespan and space are compared alongside the purchase price.
Series and Parallel: How Batteries Are Connected
Two terms you will hear from every installer:
- Series connection adds voltage. Four 12V batteries in series make 48V. The Ah stays the same as one battery.
- Parallel connection adds amp-hours. Two 48V strings in parallel double the Ah at the same voltage.
| Bank | Voltage | Capacity | Energy |
|---|---|---|---|
| 4 × 12V 200Ah in series | 48V | 200 Ah | 9,600 Wh |
| 8 × 12V 200Ah (two strings of four, in parallel) | 48V | 400 Ah | 19,200 Wh |
Bear in mind that connecting too many lead-acid strings in parallel can cause uneven charging and ageing, so many installers prefer to limit the number of parallel strings, or to move to fewer, larger batteries. Ask your installer how they plan to wire the bank.
How Long Will My Batteries Last? (Backup Time Formula)
If you already have a battery bank, or want to test a proposal, calculate runtime like this:
Backup hours = Battery Wh × DoD × 0.9 ÷ Load in watts
Here are the results for a 48V 100Ah lithium battery (4,800 Wh) and a 48V 200Ah tubular bank (9,600 Wh), using the same assumptions as above:
| Constant load | Lithium 48V 100Ah | Tubular 48V 200Ah |
|---|---|---|
| 500 W | about 7.8 hours | about 8.6 hours |
| 1,000 W | about 3.9 hours | about 4.3 hours |
| 1,500 W | about 2.6 hours | about 2.9 hours |
| 2,000 W | about 1.9 hours | about 2.2 hours |
Notice that the tubular bank has double the rated energy yet delivers only slightly more usable runtime. That is the effect of the lower depth of discharge.
These are estimates. In real use, heavy loads can reduce lead-acid capacity further, because batteries deliver less when drained quickly.
Don’t Forget the Discharge Rate
Capacity is only half the story. A battery must also be able to deliver power fast enough for your inverter.
A 5kVA inverter running a heavy load at 48V can pull well over 80 amps from the battery. A battery bank that is too small, or not rated for that continuous current, will struggle, overheat or trip its protection. This is why you should never pair a large inverter with a tiny battery just to save money.
When comparing batteries, check:
- Maximum continuous discharge current (does it match your inverter’s demand?)
- Maximum charge current (how fast can it be recharged?)
- Battery management system (BMS) on lithium units, and its compatibility with your inverter
Can Your Panels Recharge the Battery?
A bigger battery needs enough panel capacity to refill it each day. As a quick check:
Energy to replace (Wh) ÷ Peak sun hours ≈ Extra panel watts needed (before losses)
If you size a large battery bank but install too few panels, the batteries will rarely reach full charge and will age faster. Battery and panel capacity must be designed together. Our load calculator guide covers panel sizing in detail.
Lithium vs Tubular: How the Sizing Differs
| Factor | Lithium (LiFePO4) | Tubular lead-acid |
|---|---|---|
| Usable share of capacity | Higher | Lower, around half |
| Rated capacity needed for the same usable energy | Less | Much more |
| Weight and floor space | Generally lighter and more compact | Heavier and bulkier |
| Maintenance | Generally low | Needs periodic checks, including electrolyte level for flooded types |
| Lifespan | Generally longer, check manufacturer cycle ratings | Generally shorter |
| Upfront cost | Often higher per rated kWh | Often lower per rated kWh |
| Cost per usable kWh over the system’s life | Can be competitive, depending on usage | Depends on how often it is replaced |
The right choice depends on your budget, backup hours, available space, how often you cycle the batteries and how long you want them to last. Compare cost per usable kWh over the battery’s life, not only the sticker price.
What Changes Your Battery Size
- Air conditioners. One unit running at night can use more energy than all your other loads combined.
- Backup hours. Every extra hour adds to the battery requirement.
- Rainy season. Cloudy days reduce panel output, so some homes add battery or panel margin, or keep a grid or generator fallback.
- Heat. High temperatures, especially in cities like Kano, can shorten battery life. Place batteries in a cool, ventilated, shaded, dry location.
- Future growth. If you plan to add appliances, mention it now so the bank can be expanded.
- Critical loads. Medical equipment, security and business refrigeration may justify extra reserve.
Common Battery Sizing Mistakes
- Comparing Ah without voltage. A 12V 200Ah battery is not equal to a 48V 200Ah bank.
- Ignoring depth of discharge. A 100Ah battery does not give you 100Ah of usable energy.
- Skipping efficiency losses. The inverter and wiring waste some energy.
- Buying batteries before choosing the inverter. The inverter sets the system voltage.
- Pairing a big inverter with a small battery. The battery cannot deliver the current.
- Mixing old and new batteries. Different ages and capacities charge and discharge unevenly and shorten life.
- Mixing battery types or brands in one bank. Keep the bank uniform unless your installer confirms compatibility.
- Too many parallel lead-acid strings. Uneven charging is a real risk.
- Poor placement. Heat, damp and poor ventilation shorten battery life.
- Choosing on price alone. Compare usable capacity, lifespan, warranty and discharge ratings.
Your Battery Calculation Template
Fill this in for your own system:
| Step | Your figure |
|---|---|
| 1. Night energy needed (Wh) | |
| 2. Battery DoD (lithium about 0.9, tubular about 0.5, check the spec) | |
| 3. Efficiency (about 0.9) | |
| 4. Required rated capacity (Wh) = Step 1 ÷ Step 2 ÷ Step 3 | |
| 5. System voltage (12V, 24V or 48V, per your inverter) | |
| 6. Required Ah = Step 4 ÷ Step 5 | |
| 7. Number of batteries needed (round up) |
Can a Calculator Replace a Professional Assessment?
A calculator gets you to a very good estimate and helps you ask the right questions. But a proper design also checks:
- Your inverter’s real voltage, surge and discharge limits
- The battery’s specified depth of discharge, cycle life and warranty terms
- Whether the battery management system works with your inverter
- Cable sizes, breakers and fuses for battery current
- Placement, ventilation and safety
- How the bank can be expanded later
Treat your result as a strong starting point, then have an installer verify it.
Get Your Battery Size Checked by PA DigiTech Solar
At PA DigiTech Solar, we design and install solar panels, inverters and battery systems for homes and businesses across Nigeria. We size your battery bank from your actual appliances, backup hours and budget, and make sure it matches your inverter and panels.
Send us your appliance list, or your completed template, and we will help you work out the right battery capacity. Call PA DigiTech Solar on 07068853073 to discuss your requirements.
Frequently Asked Questions
How do I calculate the battery size for my inverter?
Work out the energy you need from the battery at night, divide by the battery’s depth of discharge and by about 0.9 for efficiency, then divide by your system voltage to get amp-hours.
How many batteries do I need for a 5kVA inverter?
It depends on your backup hours and battery type, not on the inverter alone. A 5kVA inverter usually runs at 48V, and the number of batteries comes from your night energy use. For example, 6,000 Wh of night use would point to roughly 154Ah of lithium or 278Ah of tubular at 48V.
How many batteries do I need for a 3.5kVA or 3kVA inverter?
These often run at 24V. The same method applies: calculate your night energy, apply depth of discharge and efficiency, then divide by 24V.
How long will a 200Ah battery last?
It depends on the voltage and your load. A 12V 200Ah battery holds about 2,400 Wh, and with tubular depth of discharge and losses, that is roughly 1,000 Wh usable. On a 500W load, that is only about two hours.
What is the difference between lithium and tubular battery sizing?
Lithium allows a deeper usable discharge, so you need less rated capacity for the same usable energy. Tubular batteries are usually limited to around half their rated capacity for long life, so you need roughly double the rating.
How do I convert kWh to Ah?
Multiply kWh by 1,000 to get Wh, then divide by the system voltage. For example, 5 kWh at 48V is about 104Ah.
Can I connect batteries of different sizes or ages?
It is not recommended. Mismatched batteries charge and discharge unevenly and can shorten the life of the whole bank. Ask your installer before mixing.
What is depth of discharge?
It is the share of a battery’s rated capacity that you use before recharging. A higher DoD means more usable energy but, depending on the battery type, can affect lifespan.
Does heat affect battery life?
Yes. High temperatures can shorten battery life. Keep batteries in a cool, dry, ventilated place out of direct sun.
Why does my battery not last as long as the calculation says?
Common causes include loads higher than estimated, an old or degraded battery, an incompletely charged bank, high heat, voltage drops in thin cables and discharge rates higher than the battery handles well.
Can I add more batteries later?
Often yes, if the system was designed for it. Your inverter, battery management system and wiring must support expansion, so mention your plans at the start.
How do I get an accurate battery quotation?
List your appliances and the backup hours you want, then call PA DigiTech Solar on 07068853073.
Conclusion
Battery sizing comes down to a short chain of steps: find your night energy, adjust for depth of discharge and losses, convert to amp-hours at your inverter’s voltage, then check that the battery can deliver the current your inverter demands. Compare batteries in watt-hours, not amp-hours alone, and weigh lifespan and usable capacity alongside price.
Get those steps right and your system will carry you through the night as promised.
Ready to size your battery bank properly? Call PA DigiTech Solar on 07068853073.