Indian homes have lived with batteries for decades: the tubular battery behind the inverter that keeps fans running during load-shedding. Solar adds new questions. How many units can the battery really give? How long will it last if it is cycled every day? Why does a 150 Ah lithium battery cost so much more than a 150 Ah lead-acid one? This lesson gives you the vocabulary to answer them.
Voltage, amp-hours and kilowatt-hours
- Voltage (V): set by the chemistry and the number of cells in series. A lead-acid cell is 2 V nominal, so a "12 V battery" has 6 cells. A lithium iron phosphate (LFP) cell is 3.2 V nominal, so a 16-cell LFP pack is 51.2 V (often sold as a "48 V" battery).
- Amp-hours (Ah): how much charge the battery holds. On its own it tells you very little, because a 150 Ah battery at 12 V and a 150 Ah battery at 48 V store very different amounts of energy.
- Kilowatt-hours (kWh): the energy stored, which is what the customer actually uses.
kWh = V × Ah ÷ 1,000
| Battery | Calculation | Nominal energy |
|---|---|---|
| 12 V, 150 Ah tubular | 12 × 150 ÷ 1,000 | 1.8 kWh |
| Two 12 V, 150 Ah in series (24 V) | 24 × 150 ÷ 1,000 | 3.6 kWh |
| 51.2 V, 100 Ah LFP | 51.2 × 100 ÷ 1,000 | 5.12 kWh |
Always compare batteries in kWh, never in Ah alone.
Series and parallel. Batteries in series add voltage (Ah stays the same). Batteries in parallel add Ah (voltage stays the same). Either way, total kWh is the sum.
Depth of discharge and usable energy
Depth of discharge (DoD) is how much of the stored energy you take out. Taking 3 kWh from a 5 kWh battery is 60% DoD. State of charge (SoC) is the other side: 40% SoC left.
You cannot use the full nameplate energy without shortening the battery's life:
- Lead-acid: plan on about 50% DoD for daily cycling. Going to 80% occasionally (a long outage) is fine, but doing it every day shortens life sharply.
- LFP: typically 80–90% DoD daily, managed by the battery's BMS.
Usable kWh = nominal kWh × DoD
Cycle life
One cycle is one discharge and recharge. Cycle life is the number of cycles before the battery's capacity falls to a set end-of-life level, usually 80% of its original capacity for LFP (lead-acid datasheets use various end points).
Cycle life always comes with conditions. Read the small print for: - The DoD it was tested at. A battery rated 6,000 cycles at 50% DoD may give far fewer at 90%. - Temperature. Most ratings are at 25 °C. A common rule of thumb for lead-acid is that life roughly halves for every 10 °C rise in average battery temperature. A battery in a hot store room in Nagpur at 35–40 °C will not reach its brochure life. - Charge and discharge current.
Typical datasheet figures (check the actual product): - Flooded tubular lead-acid: roughly 1,200–1,500 cycles at 80% DoD; more at shallower DoD. - LFP: roughly 4,000–6,000 cycles at 80–90% DoD.
Calendar life matters too: even an unused battery ages. Expect roughly 4–6 years from tubular lead-acid in daily solar use, and 10 years or more from a good LFP pack in a cool place.
C-rate: how fast you charge and discharge
C-rate is the current relative to capacity. For a 100 Ah battery, 1C is 100 A, 0.5C is 50 A, and C/10 is 10 A.
Lead-acid batteries in India are usually rated at C10: the capacity you get when discharging over 10 hours. A 150 Ah C10 battery gives 15 A for 10 hours. Discharge it faster, say over 2 hours, and you get noticeably fewer Ah out of it. That is why a "150 Ah" inverter battery runs a 1 kW load for much less than 1.8 hours.
LFP batteries keep their capacity much better at higher currents. Typical home LFP packs allow 0.5C to 1C continuous: a 100 Ah pack can supply 50–100 A, which at 51.2 V is about 2.5–5 kW.
Why it matters: the inverter's maximum battery current must match what the battery can safely deliver. A 5 kW hybrid inverter on a 51.2 V battery draws about 5,000 ÷ 51.2 ÷ 0.95 ≈ 103 A at full load. A 100 Ah pack rated at 0.5C (50 A) cannot supply that; you need a 200 Ah pack or a 1C-rated one.
Round-trip efficiency
Not all the energy put in comes back out. Round-trip efficiency is energy out ÷ energy in: - Lead-acid: about 75–85% - LFP: about 92–96%
On a daily-cycled solar battery this difference adds up to hundreds of units a year.
Worked example: cost per kWh delivered
Prices move quickly, so the figures below are illustrative. Use current quotes from your suppliers.
Option A: LFP, 51.2 V 100 Ah (5.12 kWh), ₹1,10,000 - Usable: 5.12 × 0.9 = 4.6 kWh per cycle - One cycle a day for 10 years: 3,650 cycles, well within its cycle rating - Lifetime energy: 4.6 × 3,650 = 16,790 kWh - Cost per kWh delivered: 1,10,000 ÷ 16,790 = ₹6.55
Option B: two 12 V 150 Ah tubular (3.6 kWh), ₹30,000 - Usable: 3.6 × 0.5 = 1.8 kWh per cycle - Realistic life in daily cycling: about 1,500 cycles (around 4 years) - Lifetime energy: 1.8 × 1,500 = 2,700 kWh - Cost per kWh delivered: 30,000 ÷ 2,700 = ₹11.1
The cheaper battery costs more per unit delivered, before you even count water topping, ventilation and the extra 15% lost in round-trip efficiency. For a home that only needs a few outages a year, the picture changes: there the lead-acid battery mainly sits on float, and its lower purchase price may be the right choice. Match the battery to the duty.
Key points
- Compare batteries in kWh (V × Ah ÷ 1,000), never in Ah alone.
- Usable energy = nominal energy × DoD: about 50% for daily-cycled lead-acid, 80–90% for LFP.
- Cycle life depends on DoD, temperature and current; heat is the main enemy in India.
- C-rate links battery size to the inverter's battery current; check the battery can supply it.
- Cost per kWh delivered over the battery's life is a fairer comparison than purchase price.
Check yourself
- How much energy is in four 12 V 200 Ah batteries connected in series?
- An LFP pack is 51.2 V, 200 Ah, 90% DoD. What is the usable energy?
- Why does a 150 Ah C10 battery run a 1 kW load for less time than the label suggests?
Answers
- 48 V × 200 Ah ÷ 1,000 = 9.6 kWh nominal.
- 51.2 × 200 ÷ 1,000 × 0.9 = 9.2 kWh.
- A 1 kW load at 12 V draws about 90 A including inverter losses, far above the 15 A C10 current. At high discharge rates lead-acid delivers fewer Ah, and you should also not take it below about 50% DoD.