Why Watt-hours Are the Only Reliable Comparison in Solar Lighting

In outdoor solar lighting specifications, battery capacity is commonly expressed using ampere-hours (Ah). While this unit is familiar, it is often misunderstood and can lead to incorrect comparisons between different solar lighting systems. From an engineering perspective, watt-hours (Wh) are the only reliable unit for comparing usable energy.

Understanding the difference between Ah and Wh is essential for evaluating runtime, autonomy days, and long-term performance.

Ah describes charge, not energy

Ampere-hours measure electric charge, not energy. Ah indicates how much current a battery can deliver over time, but it does not account for voltage. Since energy is the product of current and voltage, Ah alone cannot represent how much energy a battery actually stores.

Two batteries with the same Ah rating can store very different amounts of energy if their nominal voltages differ.

Wh represents usable energy

Watt-hours combine voltage and current into a single metric:

Wh = Ah × V

This value directly reflects how much energy is available to power the lighting system. From a system-design perspective, all runtime calculations—daily energy consumption, autonomy days, and charging balance—must be based on Wh, not Ah.

When calculating the capacity of large battery packs, the number of series and parallel connections must be considered. The battery type also needs to be taken into account: ternary lithium batteries are typically calculated at 3.7V, while lithium iron phosphate batteries are calculated at 3.2V.

Why Ah is often used in specifications

Ah values are sometimes used because they appear larger and are easier to market. Without voltage context, Ah numbers can create the illusion of higher capacity while hiding the true energy content.

This becomes especially misleading when comparing batteries of different chemistries or configurations.

How buyers should compare batteries correctly

When evaluating solar lighting systems:

• Always convert Ah to Wh

• Compare batteries at the same nominal voltage

• Consider usable capacity, not theoretical maximum

• Account for depth of discharge and aging

Engineering takeaway

Ah is a partial descriptor. Wh is the system-relevant metric.
Any comparison that does not convert to watt-hours is incomplete.