How to Compare Amp-Hour Ratings Across Different Cordless Battery Voltages

As an Amazon Associate I earn from qualifying purchases. This post may contain affiliate links at no extra cost to you.

A 5.0 amp-hour battery is not automatically more powerful or longer-running than a 4.0 amp-hour battery. Amp-hours (Ah) measure charge capacity at a battery’s rated voltage. To compare batteries from different voltage platforms, account for voltage too: multiply volts by amp-hours to estimate energy in watt-hours (Wh).

Start with watt-hours

The basic comparison is:

Watt-hours = nominal voltage × amp-hours

For example, a 4.0 Ah battery on a 18 V platform stores about 72 Wh. A 5.0 Ah battery on a 20 V platform stores about 100 Wh. The second battery has more energy by this calculation, despite the relatively small difference in Ah.

Use the nominal voltage printed in the battery’s specifications, not necessarily the larger number used in its marketing. Some brands label a platform by its maximum voltage, while others use nominal voltage. A pack marketed as “20 V MAX,” for instance, may have a nominal voltage of about 18 V. Compare the manufacturer’s rated voltage or stated watt-hours when available.

Compare batteries by energy

The 18 V 5.0 Ah and 36 V 2.5 Ah packs have similar nominal energy, but they are not interchangeable. They belong to different electrical platforms, and their tools, connectors, and battery controls must match. The 36 V pack can deliver power at a higher voltage; the 18 V pack has twice the Ah rating. Neither figure alone tells you which tool will cut faster or handle a demanding load better.

What watt-hours can and cannot tell you

For the same tool, under similar conditions, a pack with more watt-hours will usually run longer. A 90 Wh pack has 25% more nominal energy than a 72 Wh pack, so it may offer roughly 25% more runtime in a steady, comparable task. Treat that as an estimate, not a guarantee.

Actual runtime depends on the tool, the work, and the battery. A circular saw cutting thick, wet lumber draws much more power than a drill driving small screws. High current, cold temperatures, an aging pack, and repeated heavy cuts can reduce usable runtime. A battery’s protection system may also shut it down if cells get too hot or voltage falls too low.

Watt-hours are also not a complete measure of performance. A pack’s cells and electronics determine how much current it can supply without excessive voltage sag or overheating. Two batteries with similar Wh ratings can behave differently in a high-demand tool. If you need sustained cutting, grinding, or drilling under load, look for reviews or manufacturer information about that specific tool-and-pack combination, not just the Ah number.

Check size, weight, and fit

More stored energy often means a larger, heavier battery. That trade-off matters more on an overhead drill or a compact impact driver than on a bench setup or a mower. A high-capacity pack can reduce interruptions, but it can make a tool tiring to use and less balanced.

Battery voltage must match the tool’s platform requirements. Do not assume that a pack with more Wh can power a tool designed for a different voltage. Even within one brand, battery families may have different rails, electronics, or tool compatibility. Check the tool and battery compatibility lists before buying.

For occasional household work, a smaller pack is often the sensible choice. If you mainly hang shelves or assemble furniture, a compact battery and a spare may be cheaper and easier to handle than one oversized pack. For longer sessions, consider a compatible spare battery rather than buying a larger pack you rarely need.

Compare charging and replacement costs

Check charger compatibility and charge time as well as battery capacity. A large battery may take longer to recharge, especially on a basic charger. If work cannot stop while a pack charges, two moderate-capacity batteries can be more useful than one large pack, though the pair costs more and takes up more space.

When comparing prices, calculate cost per watt-hour: divide the price by the battery’s Wh rating. A $90 battery rated at 90 Wh costs $1 per Wh; a $72 pack rated at 72 Wh also costs $1 per Wh. This is a useful screening tool, not a reason to ignore weight, warranty, compatibility, or high-load performance. A battery-and-charger bundle for your platform may be better value if you need both, but check that the charger suits the pack.

Choose for the work you do

For short, light tasks, buy the smallest compatible battery that gives acceptable runtime; paying extra for maximum capacity may not help. For repeated heavy work, prioritize a reputable pack with sufficient Wh and a strong charger, then confirm that the tool is rated for that battery family. If you already own batteries in a platform, staying within it can save money, provided the packs are compatible with the new tool.

Before deciding, compare nominal voltage, watt-hours, weight, charger time, and compatibility. Use Ah to compare batteries at the same voltage. Across different voltages, watt-hours provide the fairer estimate of stored energy, while the tool’s design and battery’s current capability determine how well that energy turns into work.