If your solar battery drains before sunrise, you’re not alone. Many solar users struggle with limited battery backup, forcing the inverter to switch to grid power—defeating the purpose of going solar.
This blog explains:
Why this happens
How to avoid it
What battery and inverter combo actually gives full-night backup
How to maximize electricity savings
🚨 Why the Battery Doesn’t Last All Night
Most users face these issues:
Undersized or wrong battery type
No control over solar vs grid charging
Inverter not optimized for battery performance
Heavy night loads running on a small battery
The solution? ✅ Right battery + ✅ Smart inverter + ✅ Smart load usage
🔋 Battery Types Comparison
Choosing the right battery is the first step to ensure lasting backup.
The below table gives a comparison for different types of battery, all of 150Ah, 12V and running a load of 300W.
📊 Detailed Battery Comparison Table (150Ah, 12V Battery @ 300W Load)
| Feature | 🔧 Lead-Acid (Flooded/AGM) | 🌿 🔧 Lead-Acid (Sealed/VRLA) | ⚡ Lithium-Ion (LiFePO4) |
|---|---|---|---|
| Depth of Discharge (DoD) | 50% | 65% | 90% |
| Usable Energy (Wh) | 900 Wh | 1170 Wh | 1620 Wh |
| Backup Time (@ 300W Load) | 3 hours | 3.9 hours | 5.4 hours |
| Cycle Life | 500–1000 cycles | 1000–1500 cycles | 3000–6000+ cycles |
| Lifespan (years) | 3–5 years | 4–6 years | 10–15 years |
| Maintenance | High (Flooded) | Low | None |
| Weight & Size | Heavy | Heavy | Light & Compact |
| Cost (Initial) | Low 💰 | Medium 💰💰 | High 💰💰💰 |
| Efficiency | ~80% | ~85% | ~95% |
| Temperature Tolerance | Medium | Good | Excellent |
| Inverter Compatibility | Widely supported | Widely supported | Needs compatible BMS |
| Space Required | High | Medium | Low |
📘 Understanding Battery Terms & How They Affect You
It’s essential to understand key battery-related terms. These directly impact how long your battery lasts, how much power you can use, and how much you save. Let’s simplify them for you.
🔋 Depth of Discharge (DoD)
What is DoD (Depth of Discharge)?
DoD refers to the percentage of a battery’s total capacity that has been used.
Formula:
DoD (%) = (Energy Drawn from Battery ÷ Total Battery Capacity) × 100%
Let’s understand its impact on different types of battery from the table given below:
📊 Battery Types vs Depth of Discharge (DoD)
| 🔋 Battery Type | ⚡ Typical DoD (%) | 📦 Usable Capacity from 150Ah, 12 V Battery | Backup Time |
|---|---|---|---|
| Flooded Lead-Acid | 50% | 150 × 12 × 0.5 = 900Wh | 900 ÷ 300 = 3 hrs |
| Sealed Lead-Acid (AGM/VRLA) | 65% | 150 × 12 × 0.65 = 1170Wh | 1170 ÷ 300 = 3.9 hrs |
| Lithium-Ion (LFP – LiFePO₄) | 90% | 150 × 12 × 0.9 = 1620Wh | 1620 ÷ 300 = 5.4 hrs |
💡 Why It Matters:
Higher DoD = More usable energy
LFP batteries offer nearly double the usable capacity of lead-acid in the same size
This helps you get longer backup with fewer batteries and also reduces long-term costs.
🔁 Round-Trip Efficiency
Round-trip efficiency accounts for both charging and discharging losses.
It shows how much energy you can actually use compared to how much you put in.
Formula:
Round-Trip Efficiency (%) = (Energy Output ÷ Energy Input) × 100
The table given below explains the impact of battery roundtrip efficiency on different typs of battery, taking an example of 1000 wh input.
🔋 Battery Round-Trip Efficiency Comparison
| ⚡ Battery Type | 🔁 Round-Trip Efficiency (%) | 🔌 Energy Input (Wh) | 📤 Usable Output (Wh) | ❌ Energy Loss (Wh) |
|---|---|---|---|---|
| Flooded Lead-Acid | 75% | 1000 | 750 | 250 |
| Sealed Lead-Acid (AGM/VRLA) | 80% | 1000 | 800 | 200 |
| Lithium-Ion (LFP) | 95% | 1000 | 950 | 50 |
💡 Key Takeaways:
Lithium (LFP) gives the highest usable output with minimal energy loss
Lower efficiency means more solar energy wasted during charging/discharging
Over time, LFP batteries offer better value and longer backup
🔋 C Rating in Batteries:
What is C Rating?
The C rating of a battery describes how quickly it can be charged or discharged relative to its total capacity.
Definition:
C (Capacity) Rating = The rate of charge or discharge relative to battery capacity.
Formula:
C Rating = Discharge Current (A) ÷ Battery Capacity (Ah)
Or, to calculate discharge current based on C Rating:
Discharge Current (A) = C Rating × Battery Capacity (Ah)
For Example:
for 100 AH Battery:
- 0.5 C = 50 A (0.5C X 100AH) → full charge/discharge in 2 hours (0.5C X 100AH), Since it is 100 Ah battery, it means, it will give current of 50 amp x 2 hrs.
- 1C = 100 A (1C X 100AH)→ full charge/discharge in 1 hour
- 2C = 200 A → full charge/discharge in 0.5 hours (30 min)
Case: You have a 150Ah Lithium battery with 1C rating then
Max Discharge Current = 1 × 150Ah = 150 Amps.
You can safely run up to 1800W – 2000W load (at 12V or 24V system) without damaging the battery.
The table given below makes a comparison of C rating of different batteries.
📊 C Rating Comparison Table by Battery Type
| 🔋 Battery Type | ⚡ Battery Capacity (Ah) | 🔁 Typical C Rating | 🔌 Max Discharge Current (A) | 🕒 Discharge Time at Max Rate | ⚠️ Impact |
|---|---|---|---|---|---|
| Flooded Lead-Acid | 200 Ah | 0.2C | 40 A | 5 hours | Slower output; best for steady loads |
| Sealed Lead-Acid (VRLA/AGM) | 200 Ah | 0.3C | 60 A | ~3.3 hours | Medium output; still not ideal for high-load spikes |
| Lithium-Ion (LFP) | 200 Ah | 1C – 2C | 200 – 400 A | 1 hour or less | High power output; suitable for heavy appliances and longer backup |
💡 Why C Rating Matters:
| ✔️ Higher C Rating Benefits | ❌ Lower C Rating Limitations |
|---|---|
| Can handle large load spikes | May overheat or degrade under high loads |
| Better for inverters with high power output | Limited to light or steady usage |
| Supports fast charging/discharging | Not suitable for ACs, pumps, heaters |
✅ Recommendation:
For homes with fans, lights, fridge, TV, a 0.5C–1C battery is sufficient.
For air conditioners, motors, pumps, go with LFP battery rated 1C or higher.
Peukert’s Law – Why Some Batteries Run Out Faster Under High Load
Peukert’s Law explains how the effective capacity of a battery decreases when the discharge rate increases. It mostly affects lead-acid batteries, while lithium batteries are nearly unaffected.
It is quantified by the Peukert exponent (n)—lower is better.
Formula:
t = H × C / (I)^k
Effective Capacity = t X I
Where:
t = time the battery will last (in hours)
H = rated time (usually 20 hours for lead-acid)
C = rated capacity (Ah)
I = actual discharge current (A)
k = Peukert exponent (depends on battery type)
The comparison table below gives the Expected Runtime in hrs. and Effective Capacity in Ah considering the Peukert Effect of a 100 AH battery, with C value of 0.05C (20 hr. rate) of different kinds being discharged at 20 Ampere.
🔄 Calculated Runtime & Usable Capacity:
| Battery Type | Peukert Exponent (k) | Runtime @ 20A (hrs) | Effective Capacity (Ah) |
|---|---|---|---|
| Flooded Lead-Acid | 1.3 | 100/(20)^1.3 ≃ 2.04 Hrs. | 2.04×20=40.8 Ah |
| Sealed Lead-Acid | 1.2 | 100/(20)^1.2 ≃ 2.58 Hrs. | 2.58×20=51.6 Ah |
| Lithium-Ion | 1.05 | 100/(20)^1.05 ≃ 4.24 Hrs. | 4.24×20=84.8 Ah |
Battery Capacity: 100Ah @ 20-hour rate (5A)
Discharge Current: 20A
Peukert Exponents:
Flooded Lead-Acid: 1.30
Sealed Lead-Acid (AGM/Gel): 1.20
Lithium-Ion: 1.05
📊 🧾 Summary Table – Peukert Effect
| Feature | Flooded Lead Acid | Sealed Lead Acid | Lithium-Ion |
|---|---|---|---|
| Peukert Exponent (k) | 1.3 | 1.2 | 1.05 |
| Runtime @ 20A (approx.) | 2.04 hrs | 2.58 hrs | 4.24 hrs |
| Effective Capacity (Ah) | 40.8 Ah | 51.6 Ah | 84.8 Ah |
| Capacity Loss (%) | 59.20% | 48.40% | 15.20% |
💡 Why It Matters:
At high loads (ACs, pumps, etc.), lead-acid batteries drain much faster due to Peukert loss.
Flooded Lead-Acid suffers highest Peukert loss.
Sealed Lead-Acid performs moderately better.
Lithium-ion offers near full capacity even at high discharge rates.
✅ Recommendation:
Use Lithium-Ion (LFP) for:
Consistent backup
High-load appliances
Long-term efficiency and cost savings
🔋Battery Backup Time
Battery backup time refers to how long a battery can power a load before being discharged. It’s a critical factor when choosing a battery system for inverters, solar setups, or backup power.
🧮 Backup Time Formula
Use this simple formula:
Backup Time (hours) = (Battery Capacity × Battery Voltage × Discharge Efficiency) ÷ Load Power
Where:
Battery Capacity = in Ampere-hours (Ah)
Battery Voltage = in Volts (V)
Load Power = in Watts (W)
Discharge Efficiency = varies by battery type ( Accounts for Peukert losses )
⚙️ Example Calculation
For a 100Ah, 12V battery with a 240W load:
Flooded Lead-Acid (40% efficient):
Backup Time = (100 × 12 × 0.40) ÷ 240 = 2.0 hours
Sealed Lead-Acid (50% efficient):
Backup Time = (100 × 12 × 0.50) ÷ 240 = 2.5 hours
Lithium-Ion (85% efficient):
Backup Time = (100 × 12 × 0.85) ÷ 240 = 4.25 hours
📊 Battery Backup Comparison Table
| Battery Type | Efficiency (%) | Backup Time (hrs) |
|---|---|---|
| Flooded Lead-Acid | 40% | 2 |
| Sealed Lead-Acid | 50% | 2.5 |
| Lithium-Ion | 85% | 4.25 |
📝 Conclusion
Lithium-ion batteries provide the longest backup due to minimal internal loss.
Lead-acid batteries degrade quickly under high loads, requiring larger capacity to match lithium performance.
Always consider discharge efficiency while calculating real-world backup time.
⚡ Inverter Selection & Its Impact on Battery Backup
✅ Why Inverter Choice Is Critical
Your inverter is the brain of your power system. It controls how efficiently power is drawn from the battery, how much backup time you get, and what kind of batteries you can use. A poor inverter can waste energy and reduce battery life. A smart one can do the opposite.
🔋 How Inverter Affects Battery Backup
| Key Factor | Impact on Battery Backup |
|---|---|
| Inverter Efficiency | Higher efficiency = more usable battery energy |
| Load Management | Smart load control = better backup allocation |
| Idle Consumption | Low self-power usage = longer battery life |
| Waveform Output | Pure sine wave = less stress on appliances & battery |
| Discharge Algorithm | Controlled depth = improved battery lifespan |
A Smart Hybrid Inverter may give 20–30% more usable backup from the same battery compared to a basic off-grid inverter.
Smart hybrid inverters can auto-detect battery type and adjust charging profiles, ensuring maximum battery life and safety.
The below table gives the comparitive difference between Smart Hybrid and Off-grid Inverter.
✅ Advantages of Smart Hybrid Over Off-Grid with Grid Backup
| Feature | Hybrid Solar Plant | Off-Grid with Grid Backup |
|---|---|---|
| System Efficiency | ✅ High ( 95–98% Efficient) | ⚠️ Moderate ( ~80–85%) |
| Backup Optimization | ✅ Dynamic – adjusts based on load & source | Static – depends only on battery size |
| Battery Support | ✅ Lithium-ion, LFP, Gel, AGM, Lead-Acid | Mainly Lead-Acid |
| Load Management | ✅ Smart – critical vs. non-critical load control | Basic – no prioritization |
| Smart BMS Integration | ✅ Yes | ❌ No |
| Smart Energy Management | ✅ Yes | ❌ No |
| Use of Grid Backup | ✅ Smart and timed | ⚠️ Reactive only |
| Battery Health Management | ✅ Optimized | ❌ Basic |
| Smart Monitoring | ✅ Yes – via mobile app or cloud | ❌ No |
| Grid Export | ✅ Yes | ❌ No |
| Seamless Source Switching | ✅ Smooth and automatic | ⚠️ May be delayed |
| Expandability | ✅ Easily scalable | ❌ Not scalable |
📝 Conclusion
To get full-night backup without touching grid power, you need to:
✅ Choose a high-efficiency battery like Lithium (LFP)
✅ Install a smart hybrid inverter
✅ Size your battery according to your actual night load
✅ Use loads smartly and prioritize solar charging
This combination not only gives you power through the night but also helps you cut electricity bills consistently.
