How to Calculate Solar Battery Storage in Kilowatt-Hours (kWh)
Battery manufacturers rate deep-cycle energy storage in Ampere-hours (Ah) and Nominal Voltage (V). Because utility power and residential consumption are measured in kilowatt-hours (kWh), sizing an off-grid or hybrid solar storage system requires converting Ah into kWh using the universal electrical formula:
For example, a single standard 12V 200Ah battery stores:
Usable Storage: Lithium (LiFePO4) vs. Lead-Acid (Tubular / AGM)
The most critical specification in energy storage is Depth of Discharge (DoD). Discharging a battery beyond its recommended limit causes accelerated chemical degradation, plate sulfation, or complete cell failure.
| Battery Chemistry | Safe Depth of Discharge | Nominal 12V 200Ah (kWh) | Real Usable Energy (kWh) | Expected Lifespan |
|---|---|---|---|---|
| Lithium Iron Phosphate (LiFePO4) | 80% – 90% | 2.40 kWh | 1.92 – 2.16 kWh | 4,000 – 6,000 cycles (10+ yrs) |
| Tubular Deep Cycle (Lead-Acid) | 50% | 2.40 kWh | 1.20 kWh | 1,000 – 1,500 cycles (3–5 yrs) |
| AGM / Gel Sealed Lead-Acid | 50% | 2.40 kWh | 1.20 kWh | 600 – 800 cycles (2–3 yrs) |
*Note: To extract 5 kWh of usable storage overnight, you need either 5.5 kWh of LiFePO4 batteries or 10.0 kWh of lead-acid tubular batteries.
Worked Example: Sizing a Battery Bank for 8 Hours of Overnight Outages
Suppose your household operates the following essential continuous loads during an 8-hour nighttime grid outage:
- 3 Ceiling Fans: 3 × 75W = 225 Watts
- 4 LED Bulbs: 4 × 12W = 48 Watts
- 1 Wi-Fi Router + Fiber ONT: 25 Watts
- 1 Inverter Refrigerator (cycling at 30% duty cycle): average 120 Watts
Frequently Asked Questions: Solar Battery Storage
How many kWh can a solar battery store?
Nominal storage is calculated as (Ah × Volts) ÷ 1,000. For instance, a 12V 200Ah battery holds 2.4 kWh of nominal energy. However, usable capacity is lower due to chemistry Depth-of-Discharge (DoD) limits.
What is the difference between lithium and lead-acid battery usable kWh?
Modern Lithium Iron Phosphate (LiFePO4) batteries allow 80% to 90% usable depth of discharge with minimal degradation. Traditional tubular or AGM lead-acid batteries must not exceed 50% discharge depth; routinely drawing more than 50% permanently damages lead plates.
How many kWh of battery do I need to run essential loads overnight?
Sum the wattage of all night appliances, multiply by required backup hours, and divide by 1,000 to get required usable kWh. For typical households running 3 fans, lights, and a refrigerator (approx. 400W total), an 8-hour outage requires roughly 3.2 kWh of usable energy, necessitating a 3.6 kWh Lithium bank or 6.5 kWh Lead-Acid bank. Check individual load requirements with our AC kWh Calculator and Ceiling Fan Calculator.
Does battery kWh capacity degrade over time?
Yes. Usable capacity degrades based on cycle count, ambient operating temperature, and calendar aging. LiFePO4 cells typically retain 80% capacity after 3,500–6,000 daily cycles (approx. 10–15 years), whereas lead-acid batteries lose significant capacity after 800–1,200 cycles (2–4 years).