Here is the warning I wish someone had given me at the battery rack: mAh is not a measure of energy. It is a count of charge, and it only becomes energy once you multiply in voltage. Two replacement batteries can both say 600 on the wrapper while holding very different amounts of usable power, and buying on mAh alone is exactly how people end up with winter runtimes that make no sense on paper.
Solar light battery capacity is printed in the least useful unit, on the smallest possible label, often inside a waterproof compartment you need a tiny screwdriver to reach. Reading it properly takes two minutes of arithmetic and one new habit: convert to watt-hours before comparing anything.
The Two-Minute Math Behind Solar Light Battery Capacity
Watt-hours equal voltage times amp-hours. Take the AA cell inside a typical set of solar garden lights: a 600 mAh NiMH at 1.2 volts holds 0.72 watt-hours. If the lamp circuit draws somewhere around 60 mA, which is a fair figure for a small path light, the arithmetic suggests about ten hours of light. Reality then takes its cut: sensor standby, cold nights, and cell aging all shave the number down, so plan on noticeably less. The point is not precision. The point is that you can now rank two batteries against each other with numbers that mean the same thing. It also explains the mystery of the high-capacity upgrade that changed nothing: if voltage and draw are unchanged, runtime barely moves.
Here is the comparison that makes the habit worth it. A 600 mAh cell at 1.2 volts and a 400 mAh cell at 3.2 volts look like a mismatch on the shelf, 400 is smaller than 600, until you convert: 0.72 watt-hours versus 1.28. The smaller-looking lithium cell holds roughly three quarters more energy. Flip the voltages and the winner flips too, which is the entire reason to stop ranking batteries by the number printed largest on the wrapper.
Why Voltage Turns 600 Into Three Different Batteries
The same 600 mAh means three different energies at three different voltages. At 1.2 volts, that is 0.72 watt-hours. At 3.2 volts, the LiFePO4 chemistry in some newer fixtures, it is 1.92 watt-hours. At 3.7 volts, 2.22. This is why a lithium fixture can run longer on a cell that looks smaller on the label, and why cross-comparing mAh between chemistries is comparing apples to fish. If you want the full breakdown of NiMH, Li-ion, and LiFePO4 and which fixtures use which, the battery types explainer covers it. For label purposes, the rule is short: match the voltage class exactly, then compare capacity.
The Number Nobody Prints: Cycle Life
A cell’s rated capacity is its day-one capacity. Cycle life is how fast that number decays, and it is almost never on the wrapper. A cheap cell can hold its rating when new and sag badly after a season of daily charge cycles, which reads outdoors as an October that mysteriously underperforms September even though the weather did not change. When two replacements cost the same and one is sold as low self-discharge or high-drain, that is usually the one built for daily outdoor cycling, and it is the better buy for a fixture that lives outside year-round.
A practical habit: write the install month on each battery with a paint pen when it goes in. Six cells dated three seasons apart tells you at a glance which one is the weak link when a run starts fading unevenly, and it turns replacement from guesswork into triage.
Can You Stuff a Bigger Cell Into the Same Fixture?
Tempting, and sometimes fine, with cautions. The charging circuit is sized for a certain cell, and a modest step up in mAh at the same voltage and chemistry generally works; it just takes longer to fill. A big jump can mean the panel never fully tops the cell off, leaving you worse off than before the upgrade. Never change voltage class to chase capacity, because the circuit is not built for it. And physical fit matters more than it sounds like it should: a slightly fatter cell can hold the battery door ajar, and that door seal is what keeps rain off the board.
Ask one more question before upgrading: can the panel actually fill the bigger cell? Capacity only matters if the fixture collects enough sun to load it, and a small panel that barely topped the original cell will fall further behind a larger one, especially from October onward. If the fixture spends its life in partial shade, a bigger cell mostly buys you a heavier battery.
Next time a light fades earlier than its neighbors, pull the cell and do the arithmetic before ordering. Write the watt-hours on the replacement’s wrapper with a marker. It feels fussy for about a week, and then it becomes the fastest way you know to tell a tired battery from a bad one.
