Battery Sizing Reference
How to size a battery bank for any application. Understand Wh vs Ah, nominal vs usable capacity, and the key factors that determine how large your battery needs to be.
Wh vs Ah
Amp-hours (Ah) describe how much charge a battery can deliver at its rated voltage. Watt-hours (Wh) describe how much total energy it stores. The relationship is straightforward: Wh = Ah × Voltage. A 100Ah battery at 12V stores 1,280Wh. The same 100Ah capacity at 48V stores 5,120Wh.
Wh is the more useful metric for comparing batteries across different voltage systems. A 100Ah 12V battery and a 50Ah 24V battery store the same energy (1,280Wh), but the Ah ratings are very different. When sizing battery banks, always work in Wh to avoid confusion between voltage levels.
| Capacity | 12V | 24V | 48V |
|---|---|---|---|
| 50 Ah | 640 Wh | 1,280 Wh | 2,560 Wh |
| 100 Ah | 1,280 Wh | 2,560 Wh | 5,120 Wh |
| 200 Ah | 2,560 Wh | 5,120 Wh | 10,240 Wh |
| 300 Ah | 3,840 Wh | 7,680 Wh | 15,360 Wh |
Wh = Ah × nominal voltage. LiFePO4 nominal cell voltage is 3.2V; lead-acid is 2.0V per cell.
Nominal vs Usable Capacity
Nominal capacity is what the manufacturer prints on the label. Usable capacity is what you can actually extract, accounting for depth of discharge limits, inverter efficiency, and temperature derating. The gap between nominal and usable can be 20–50% depending on chemistry and conditions.
| Factor | Typical Value | Effect on Usable Capacity |
|---|---|---|
| DoD Limit | 50–80% | Reduces usable by 20–50% |
| Inverter Efficiency | 85–95% | AC load requires 5–15% more DC energy |
| Temperature Derating | 0–30% | Reduces capacity in cold environments |
| Battery Aging | 10–20% | Capacity decreases over lifetime |
A 100Ah LiFePO4 battery at 25°C with 80% DoD and 90% inverter efficiency provides roughly 72Ah usable AC capacity (100 × 0.80 × 0.90 = 72Ah). Plan for end-of-life capacity, not just Day 1 performance.
Typical planning ranges only. Actual system sizing depends on load, autonomy, DoD, efficiency, and safety margin.
Typical planning ranges only. Actual system sizing depends on load, autonomy, DoD, efficiency, and safety margin.
Application Sizing Table
The table below shows typical battery configurations for common applications. These are starting points — actual requirements depend on specific loads, autonomy needs, and environmental conditions.
| Application | Typical Voltage | Typical Energy Range | Notes |
|---|---|---|---|
| Small Electronics | 12V | 50–200 Wh | Phones, routers, LED lights |
| RV / Camper | 12V | 1–5 kWh | Weekend camping to full-time boondocking |
| Solar Backup | 48V | 5–30 kWh | 1–3 days autonomy for household loads |
| Marine House Load | 12V / 24V | 2–10 kWh | Navigation, lighting, refrigeration, electronics |
| Home Backup | 48V | 10–40 kWh | Essential loads for 1–3 days |
| Commercial BESS | 48V–800V | 100–10,000+ kWh | Peak shaving, demand response, backup |
Key Sizing Factors
Battery sizing is not just about total energy storage. Several factors must be considered together to determine the right bank size:
Daily Energy Consumption
Total Wh consumed per day from all loads. This is the foundation of sizing. Measure or estimate every load that will run from the battery, multiplied by its daily run time.
Autonomy Days
Number of days the battery must power loads without recharging. Grid-tied backup: 1–2 days. Off-grid solar: 2–5 days. Remote sites: 3–7 days. More autonomy = larger battery.
Depth of Discharge
DoD limits reduce usable capacity. LiFePO4: 80% DoD. Lead-acid: 50% DoD. Size the bank so that the DoD-limited usable capacity meets your energy needs.
Safety Margin
Add 10–20% margin for load estimation errors, capacity degradation over time, temperature effects, and unexpected demand. Undersized banks fail when you need them most.
Common Mistakes
Sizing from Nominal Capacity
A 200Ah lead-acid battery at 50% DoD provides only 100Ah usable. If you need 100Ah of usable capacity, you need 200Ah of lead-acid, not 100Ah. Always size from usable, not nominal.
Forgetting Inverter Efficiency
A 1,000W AC load requires 1,050–1,175W from the battery due to inverter losses. Ignoring this 5–15% overhead means the battery drains faster than calculated.
Not Accounting for Temperature
A battery in a cold environment delivers less capacity. If your battery will be in an unheated space, apply temperature derating or oversize the bank to compensate.
Ignoring Load Growth
If you plan to add loads (new appliances, EV charging, expanded system), size the battery bank for future capacity, not just current needs. Replacing undersized banks is expensive.
Try It
Use the Battery Sizing Calculator to size a battery bank from your specific loads, autonomy requirements, and battery chemistry.
Open Battery Sizing CalculatorFrequently Asked Questions
What is the difference between Wh and Ah?
Amp-hours (Ah) measure charge capacity — how much current a battery can deliver over time. Watt-hours (Wh) measure energy capacity — how much total work the battery can do. Wh = Ah × Voltage. A 100Ah 12V battery stores 1,280Wh. A 100Ah 48V battery stores 5,120Wh. Wh is the more meaningful metric for comparing energy storage.
What is the difference between nominal and usable capacity?
Nominal capacity is the manufacturer's rated capacity under ideal conditions. Usable capacity is the energy you can actually draw, accounting for depth of discharge limits, efficiency losses, and temperature derating. A 100Ah LiFePO4 battery at 80% DoD provides 80Ah usable. A 100Ah lead-acid at 50% DoD provides only 50Ah usable.
How do autonomy days affect battery sizing?
Autonomy days determine how many days the battery must power the load without recharging. More autonomy days require a proportionally larger battery. A system needing 1,000Wh/day with 3 days of autonomy requires a 3,000Wh usable battery (before DoD and efficiency derating). Off-grid systems typically need 2–5 days; grid-tied backup needs 1–2 days.
Why does inverter efficiency matter for battery sizing?
Inverters convert DC battery power to AC for household loads. This conversion is not 100% efficient — typically 85–95%. A 1,000W AC load requires 1,050–1,175W from the battery. Inverter efficiency must be factored into battery sizing, or the battery will drain faster than expected.