Battery Quantity Calculator

Nobody knows how many batteries their house eats in a year. You buy them four at a time at the checkout, so the running total never shows up anywhere. This works it out.

The count:

Batteries per year = Devices × Batteries per device × Changes per year

Three numbers, and the third is the one people get wrong. Count changes, not devices. A wall clock takes one AA and eats it once a year. A child’s toy takes four Cs and goes flat every six weeks. Those two entries look similar on a shelf and differ by a factor of thirty in the annual count.

If your devices vary a lot, run the calculator twice: once for the heavy users and once for the clocks and remotes, then add the answers.

Where rechargeables come out ahead

The comparison below is deliberately simple, because the real answer is simple. Rechargeables cost more once and nothing after that, so the question is only how long the payback takes.

Break-even (years) = Rechargeable setup cost ÷ Annual disposable cost

Setup cost is one rechargeable for every battery slot you own, not one for every change. That is the number the calculator uses. Buy a second set if you want a charged spare in the drawer, and double the setup figure.

After the break-even point the whole annual disposable cost becomes a saving, minus a few cents of electricity. A good NiMH cell handles several hundred charges, which for most household use means five to ten years before capacity drops enough to matter.

Where they do not. Smoke alarms, TV remotes, wall clocks, anything that runs for a year or more on one cell. NiMH rechargeables self-discharge, and they sit at 1.2 V against alkaline’s 1.5 V, which some devices read as a low battery from day one. Leave those on disposables and put the rechargeables in the game controllers, the flash, and the kids’ toys.

Common battery capacities:

Capacity is also why a smoke alarm goes a year on one 9V while a wireless mouse does not go a year on two AAs despite having twenty times the stored energy. The alarm draws about 10 microamps sitting there. 500 mAh divided by 0.01 mA is 50,000 hours, close to six years, which is why the chirp always comes as a surprise.

Stocking for an outage: plan on 72 hours of running your flashlights and radio, work out how many changes that is, and keep that many spare in a cool dry drawer. Alkalines lose roughly 2% of capacity a year in storage, so a pack bought today is still fine in five years.