How to Calculate Battery Capacity in kWh for Off-Grid Solar
Correctly sizing your battery bank is one of the most important steps in designing an off-grid solar system. Undersized batteries lead to frequent deep discharges and short lifespan, while oversized banks waste money. This guide walks you through the calculation step by step.
Understanding Battery Capacity Units
Battery capacity is expressed in two ways:
- Amp-hours (Ah): How much current a battery can deliver over time at its nominal voltage
- Kilowatt-hours (kWh): The total energy stored, calculated as Ah x Voltage / 1,000
For system design, kWh is more useful because it directly relates to how many watts you can run and for how long.
The Conversion Formula
kWh = Ah x Nominal Voltage / 1,000
Example: A 200Ah battery at 48V stores 200 x 48 / 1,000 = 9.6 kWh of total energy.
Step 1: Calculate Your Daily Energy Consumption
List every appliance, its wattage, and daily hours of use:
| Appliance | Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| LED lights (x6) | 60 | 6 | 360 |
| Refrigerator | 150 | 12 | 1,800 |
| TV | 80 | 4 | 320 |
| Fans (x3) | 225 | 8 | 1,800 |
| Phone chargers (x4) | 40 | 3 | 120 |
| Washing machine | 500 | 1 | 500 |
| Total | 4,900 Wh |
This household consumes approximately 4.9 kWh per day.
Step 2: Account for Days of Autonomy
Days of autonomy is how many cloudy or low-production days your battery bank should cover without any solar charging. For the Middle East, 1.5-2 days is typical. For sub-Saharan Africa during rainy season, 2-3 days is recommended.
Required storage = Daily consumption x Days of autonomy
4.9 kWh x 2 days = 9.8 kWh
Step 3: Apply Depth of Discharge (DoD)
You should never discharge batteries to 0%. The maximum recommended depth of discharge depends on battery type:
| Battery Type | Max DoD | Effective Capacity |
|---|---|---|
| Flooded lead-acid | 50% | Half of rated capacity |
| Gel (AGM/GEL) | 50-60% | About half of rated capacity |
| LiFePO4 (Felicity) | 80-90% | 80-90% of rated capacity |
Total battery bank capacity = Required storage / DoD
With gel batteries at 50% DoD: 9.8 kWh / 0.50 = 19.6 kWh
With LiFePO4 at 80% DoD: 9.8 kWh / 0.80 = 12.25 kWh
Step 4: Add Inverter Efficiency Loss
Inverters are not 100% efficient. The Felicity IVPS series achieves 93-95% efficiency. Divide by efficiency:
With gel: 19.6 kWh / 0.93 = 21.1 kWh total bank
With LiFePO4: 12.25 kWh / 0.93 = 13.2 kWh total bank
Step 5: Convert to Ah at Your System Voltage
Ah = kWh x 1,000 / System Voltage
For a 48V system with gel: 21.1 x 1,000 / 48 = 440 Ah at 48V
For a 48V system with LiFePO4: 13.2 x 1,000 / 48 = 275 Ah at 48V
Quick Reference Calculator
| Daily Use (kWh) | Gel (50% DoD, 2 days) | LiFePO4 (80% DoD, 2 days) |
|---|---|---|
| 3 kWh | 12.9 kWh (269 Ah @ 48V) | 8.1 kWh (169 Ah @ 48V) |
| 5 kWh | 21.5 kWh (448 Ah @ 48V) | 13.4 kWh (280 Ah @ 48V) |
| 8 kWh | 34.4 kWh (717 Ah @ 48V) | 21.5 kWh (448 Ah @ 48V) |
| 10 kWh | 43.0 kWh (896 Ah @ 48V) | 26.9 kWh (560 Ah @ 48V) |
Tips for Hot Climates
- High temperatures reduce battery capacity by 10-20% — oversize accordingly
- LiFePO4 batteries handle heat better than lead-acid and maintain capacity up to 45 degrees C
- Install batteries in shaded, ventilated areas to maximise lifespan
- The Felicity IVPS inverter includes temperature compensation for lead-acid charging
Frequently Asked Questions
How many kWh does a 200Ah 48V battery store?
A 200Ah battery at 48V stores 9.6 kWh of total energy (200 x 48 / 1,000). However, usable energy depends on depth of discharge: about 4.8 kWh for gel batteries at 50% DoD, or about 7.7 kWh for LiFePO4 at 80% DoD.
How many batteries do I need for a 5kWh daily consumption?
For 5 kWh daily use with 2 days of autonomy and gel batteries at 50% DoD, you need about 21.5 kWh of battery capacity. With LiFePO4, you need about 13.4 kWh. The exact number of battery units depends on each battery's capacity and your system voltage.
Does temperature affect battery capacity?
Yes, significantly. Lead-acid batteries lose 10-20% capacity in temperatures above 40 degrees C, which is common in the Middle East. LiFePO4 batteries are more resilient, maintaining rated capacity up to about 45 degrees C. Always add 15-20% extra capacity as a heat margin.
