Battery Capacity Explained
Battery capacity describes stored energy, but useful runtime also depends on voltage, inverter losses, reserve settings, temperature and load behavior.
Watt-hours are the most practical capacity unit for comparing portable power stations because they combine voltage and amp-hours into stored energy.
Capacity should never be confused with inverter output: watt-hours answer how long, while watts answer how much power at once.
Use Watt-Hours for Runtime and Watts for Output
A 1,000Wh battery theoretically supplies 100W for ten hours, but real AC runtime is lower after conversion losses and reserve. Amp-hours can only be compared after accounting for voltage.
Core Sizing Rule
Estimate load energy in watt-hours, reduce battery nameplate capacity to a realistic usable amount, then verify the inverter separately for continuous and surge power.
Watts, Watt-Hours and Amp-Hours
| Unit | Measures | Example |
|---|---|---|
| Watt (W) | Instantaneous power | A refrigerator drawing 150W |
| Watt-hour (Wh) | Energy over time | 150W used for 2 hours = 300Wh |
| Kilowatt-hour (kWh) | 1,000Wh | A 10kWh home battery |
| Amp-hour (Ah) | Charge at a stated voltage | 100Ah at 12V is about 1,200Wh nominal |
| Volt (V) | Electrical potential | 12V battery or 120V AC load |
Core Capacity Formulas
Watt-hours equal volts multiplied by amp-hours. Runtime in hours is approximately usable watt-hours divided by average load watts.
These are energy estimates, not guarantees. AC inverters, DC converters, standby electronics and battery protection consume part of the stored energy.
Nameplate Capacity vs Usable Capacity
Battery Reserve
The system may keep energy unavailable to protect cells.
Inverter Loss
DC-to-AC conversion is not perfectly efficient.
Idle Consumption
The inverter and display consume energy even at low load.
Temperature
Cold or heat can reduce available performance.
High Load
Some batteries deliver less total energy under severe demand.
Aging
Capacity gradually declines with time and cycling.
Runtime Examples
| Battery | Average Load | Ideal Runtime | More Realistic Range |
|---|---|---|---|
| 500Wh | 50W | 10 hours | About 8–9 hours |
| 1,000Wh | 100W | 10 hours | About 8–9 hours |
| 1,000Wh | 500W | 2 hours | About 1.6–1.8 hours |
| 2,000Wh | 150W | 13.3 hours | About 11–12 hours |
| 5,000Wh | 1,000W | 5 hours | About 4–4.5 hours |
Series and Parallel Battery Connections
Series connections increase voltage while amp-hour capacity remains the same. Parallel connections keep voltage the same while amp-hour capacity increases. In both cases, total watt-hours add when matched batteries are combined correctly.
Do not build or modify battery banks without proper electrical design, protection and manufacturer-approved compatibility.
How Aging and Temperature Affect Capacity
Lithium batteries gradually lose capacity through calendar aging and cycling. Cold can temporarily reduce available energy and charging capability, while persistent heat accelerates long-term degradation.
Capacity warranties often define a minimum retained percentage after a time or energy-throughput limit.
Common Capacity Mistakes
Comparing Ah Without Voltage
100Ah at 12V is not equal to 100Ah at 48V.
Ignoring Inverter Output
Large capacity does not guarantee an appliance can start.
Using Nameplate Watts
Power-supply maximum ratings may overstate average consumption.
Forgetting Surge
Motor startup may exceed the continuous load.
Assuming Full Capacity
Reserve and conversion losses reduce delivered energy.
Ignoring Recharge
Daily solar or generator input changes outage endurance.
Frequently Asked Questions
What does 1,000Wh mean?
It means the battery stores a nominal 1,000 watt-hours of energy, theoretically enough for 100W for ten hours before losses.
Is Ah or Wh better for comparing batteries?
Wh is better when batteries use different voltages because it represents total energy.
How do I convert Ah to Wh?
Multiply amp-hours by nominal voltage.
Why is actual runtime lower than the calculation?
Inverter losses, standby use, reserves, temperature, load behavior and battery aging reduce delivered energy.
Does a bigger battery provide more watts?
Not necessarily. Capacity and inverter output are separate ratings.
What is usable capacity?
The energy the system allows loads to consume after protection reserves and operating limits.
How many Wh does a refrigerator use?
It varies by model, temperature, compressor cycling and environment. Measure actual daily energy for reliable sizing.
Does solar input increase battery capacity?
No. It adds energy during operation but does not change the battery's rated storage capacity.