Sizing, Payback Examples and Checklists
Home solar battery in India: lithium (LFP) vs lead-acid, sizing and real cost per unit
Compare lithium-ion (LFP) and tubular lead-acid batteries for home solar and backup in India: usable capacity, cycle life, cost per stored unit, safety, GST and disposal.
For daily solar cycling, a lithium iron phosphate (LFP) battery usually costs more upfront but less per unit of energy stored over its life than a lead-acid battery, because you can use more of its capacity and it survives many more charge cycles. For occasional backup during short power cuts, a tubular lead-acid battery with an ordinary home inverter remains the cheaper way in. Neither makes a rooftop system pay back faster: under net metering the grid already stores your surplus for free, so a battery is worth buying for backup, not for savings. This guide explains the chemistry choices in plain terms, how to size a battery in kWh, the cost-per-cycle arithmetic, and the safety and disposal rules.
The main options
| Type | Usable depth of discharge (typical) | Cycle life (illustrative range) | Maintenance | Good for |
|---|---|---|---|---|
| Flooded tubular lead-acid | ~50% | Several hundred to ~1,500 cycles | Top up distilled water; ventilated space | Backup with a home inverter; tight budgets |
| Sealed lead-acid (VRLA/SMF, gel) | ~50% | Similar or fewer than tubular | Low | Small UPS loads, indoor spaces |
| Lithium iron phosphate (LFP) | ~80–90% | Thousands of cycles | Low; needs a battery management system | Daily solar cycling in hybrid systems |
| Other lithium chemistries (e.g. NMC) | ~80–90% | Varies | Low | More common in vehicles and electronics |
Cycle-life figures depend heavily on temperature, depth of each discharge and charging quality. Use the manufacturer's datasheet and warranty, not brochure headlines.
Sizing in kWh, not "Ah"
Battery capacity is energy: kWh = volts × amp-hours ÷ 1,000. A 12 V, 150 Ah battery holds 1.8 kWh; two in series (24 V) hold 3.6 kWh.
- List the loads you want in a cut: for example, four fans (75 W each), six LED lights (10 W each), a router (15 W) and a fridge averaging 100 W. Total ≈ 475 W.
- Decide the hours: say 4 hours. Energy needed = 0.475 kW × 4 h = 1.9 kWh.
- Add losses: inverter and wiring losses of, say, 10–15%. Need ≈ 2.2 kWh.
- Divide by usable depth of discharge: lead-acid at 50% → 4.4 kWh rated; LFP at 90% → about 2.4 kWh rated.
So the same backup needs almost twice the rated capacity in lead-acid.
Cost per stored unit (illustrative)
The fair comparison is the cost of each unit (kWh) the battery delivers over its life.
| Lead-acid tubular bank | LFP battery | |
|---|---|---|
| Rated capacity | 3.6 kWh (2 × 12 V 150 Ah) | 5 kWh |
| Illustrative price incl. GST | ₹30,000 | ₹1,20,000 |
| Usable per cycle | 1.8 kWh (50%) | 4.5 kWh (90%) |
| Illustrative cycle life | 1,000 | 3,000 |
| Lifetime energy | 1,800 kWh | 13,500 kWh |
| Cost per delivered unit | ≈ ₹16.7 | ≈ ₹8.9 |
Both are more than most domestic tariffs. That is the arithmetic behind the advice above: storing solar in a battery to avoid buying grid units at ₹5–8 does not pay; the battery's value is keeping the lights on. If you cycle a lead-acid battery only during occasional cuts, it may age by time before it reaches its cycle life, and its low upfront cost wins.
Compatibility
- A lithium battery needs an inverter or hybrid inverter programmed for its chemistry and able to talk to its battery management system (BMS). Using a lead-acid inverter with a lithium battery without proper settings can damage it or void the warranty.
- Check the battery's nominal voltage (12, 24, 48 V or high-voltage) matches the inverter.
- Ask whether the warranty is in years, cycles or energy throughput, and what capacity retention is guaranteed.
Safety and installation
- Install batteries in a ventilated, shaded, dry place away from the kitchen and children. Flooded lead-acid batteries can release hydrogen during charging.
- Keep lithium batteries within their temperature range; avoid hot terraces and direct sun.
- Use correctly rated cables, fuses and isolators between battery and inverter.
- Insist on BIS registration for lithium-ion batteries; see DCR modules, ALMM and BIS.
GST on batteries
Batteries are not in the 5% renewable energy entry that covers solar modules and generators. In the GST rate changes effective 22 September 2025, electric accumulators other than lithium-ion (which includes lead-acid) moved from 28% to 18%; lithium-ion batteries were already at 18% and stay there. So a battery is taxed at 18% against 5% for the solar modules. Check the rate on your invoice. See GST on rooftop solar.
Disposal and buy-back
The Battery Waste Management Rules, 2022, made under the Environment (Protection) Act, put extended producer responsibility on battery producers to collect and recycle used batteries. When you replace a battery, return the old one through the dealer or the producer's collection channel rather than selling it to an informal scrap buyer. Ask about buy-back of the old battery in your quotation.
Subsidy and batteries
The PM Surya Ghar residential CFA is calculated on the solar capacity of a grid-connected system. A battery is at your own cost. See PM Surya Ghar subsidy explained.
Back to the pillar guide: how many kW of solar you need. Related: on-grid vs hybrid vs off-grid, solar warranty claims.
Frequently asked questions
Can I put a lithium battery on my old home inverter?
Only if the inverter supports lithium charging profiles, or the battery is sold as a drop-in replacement for that inverter. Ask both manufacturers.
How long does a home battery last?
It depends on chemistry, temperature and use. Lead-acid batteries in daily cycling often need replacing within a few years; LFP batteries are designed for many more cycles. Read the warranty terms.
Will a battery let me go off the grid?
Technically possible, but for most connected homes it costs far more than net metering and gives up the grid as a backup.
Is a bigger battery always better?
No. Unused capacity costs money and still ages. Size for your real outage pattern.
Official sources
- Bureau of Indian Standards — registration search for batteries
- Ministry of Environment, Forest and Climate Change — Battery Waste Management Rules, 2022
- Central Pollution Control Board — EPR for batteries
- GST Council — 56th meeting rate changes
