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Battery Math

How we compute these numbers

What we do

We read the spec sheets that makers and retailers publish. Then we do math on them so the numbers line up and can be compared side by side. We have never had any of these products in our hands. We have not tested them, measured them, or run them for a day. Nothing on this site is a review, and nothing here is a hands-on report. Every figure is a calculation from a published spec, and we say so in this page and on the product pages.

Where the data comes from

Each product file is built from the pages we actually read. That is usually the maker's own spec page, plus retailer listings. Every one of those pages is listed at the bottom of its product page, with the date we read it. If a page is gone or the number changed, we can tell which page to re-check and when we last looked.

When we could not find a spec, we do not guess. The value shows as a dash (–) instead. A dash means we do not have the number, not that the number is zero and not that the feature is bad.

Every derived number, one by one

The specs we copy straight from a page are simple: capacity, price, weight, output watts. The numbers we compute are below. Each example uses the Anker SOLIX C1000 Gen 2, a real product in our database, and the math is run by the same code that builds the product pages.

Usable watt-hours

Watt-hours is the energy a battery stores. Nameplate (the printed) watt-hours are stored inside the pack. What you actually get out of an AC outlet is a little less, because the inverter (the power converter inside the unit) loses some on the way out. So: usable watt-hours is nameplate watt-hours times inverter efficiency.

The Anker SOLIX C1000 Gen 2 is rated at 1,024 Wh. Times 85%inverter efficiency, that is 870.4 Wh.

Good for: judging what you can really pull through an AC outlet. It is also what our runtime math uses, so a 100 W lamp runs this unit about8.7 hours, not the full nameplate number.

Watt-hours per dollar

Nameplate watt-hours divided by price. If a street (going) price is listed, we use it. Otherwise we use the maker's list price, the MSRP (manufacturer's suggested price).

The Anker SOLIX C1000 Gen 2: 1,024 Wh divided by $530 is1.93 Wh per dollar.

Good for: the most battery for the money. It is our main ranking number, and the highest one wins the front page.

Cost per kilowatt-hour cycled

Kilowatt-hour (kWh) is a unit of energy. One is 1,000 watt-hours. Makers rate a battery for a number of cycles, the full charge-and-empty rounds it survives before capacity fades to 80%. We multiply cycles by nameplate watt-hours, apply a 90% average for that fade, and convert to kilowatt-hours. That is the total energy the pack is rated to deliver over its life. Then the price divided by that total is the cost per kilowatt-hour.

The Anker SOLIX C1000 Gen 2 is rated for 4,000 cycles.4,000 times 1,024 Wh, times90%, is 3,686.4 kWh over the pack's life. $530 divided by that is$0.14 per kWh.

Good for: the true price of energy, spread over the whole life of the pack. This is the number that separates LiFePO4 (a battery chemistry with very long cycle life) from NMC (the shorter-life chemistry), and it is the one nobody else publishes. When a maker does not publish a cycle rating, we show a dash.

Watt-hours per kilogram

Nameplate watt-hours divided by weight.

The Anker SOLIX C1000 Gen 2: 1,024 Wh divided by 11.3 kg is 90.6 Wh per kilogram.

Good for: how much you have to carry per unit of energy. The higher the number, the lighter the unit is for what it holds.

Watts per kilogram

Continuous AC output watts divided by weight.

The Anker SOLIX C1000 Gen 2: 2,000 W divided by 11.3 kg is 177.0 W per kilogram.

Good for: how much appliance you can run per kilogram you carry. A heavy unit and a light one may hold the same energy, but only one of them can run the space heater.

Charge rate

Maximum wall charging watts divided by nameplate watt-hours. The result is a ratio to pack size. A 1.0 means the wall input could fill the pack in an hour if it held that rate the whole way.

The Anker SOLIX C1000 Gen 2 takes in 1,600 W from the wall and holds 1,024 Wh, so its rate is 1.56.

Good for: how fast a wall outlet refills the unit, without doing the division yourself. Real wall times are longer, because charging slows down near the end.

Solar refill hours

Nameplate watt-hours divided by the maximum solar input watts. That is the hours of full sun, from empty, at the panel limit.

The Anker SOLIX C1000 Gen 2: 1,024 Wh divided by 600 W of solar input is about 1.7 hours of full sun.

Good for: planning a refill from panels. It is an optimistic number on purpose. It uses the full nameplate, not the 85% usable figure, and full sun is the best the panels will ever see. Clouds and angles make the real time longer.

The assumptions

Two of our numbers rest on numbers we chose, not read. Here they are, rendered from the constants in our code, so this page updates if the code does.

  • Inverter efficiency: 85%.Real units measure between 82% and 90%, and the exact figure depends on the load. We use 85% for every product. A single shared number is fairer than a different guess per product, because one guessed number for one unit could quietly push it up or down the ranking. We would rather be the same for everyone.
  • Discharge floor: 100%. Makers rate capacity down to 0%, so we take the full nameplate figure. Nobody drains a battery to zero, so this leans optimistic.
  • Average capacity over the fade: 90%.A pack rated for, say, 4,000 cycles to 80% does not give full capacity for all 4,000. It fades from 100% down to 80%. If the fade is even, the average cycle delivers (100% + 80%) / 2, which is 90% of nameplate. We use that average for lifetime totals.

What we do not know

  • Real inverter efficiency changes with the load. Our85% is one number for all units and all loads.
  • Cycle ratings are the maker's own claim, measured under laboratory conditions. Our lifetime numbers inherit that claim.
  • Prices move. Each product page says the date we last checked its price, and the ranking shifts when we update them.
  • Surge (the short burst of extra watts at startup) ratings are not standardized between brands. One maker's surge test may not match another's, so we show the numbers without ranking on them.
  • A spec sheet is not a measurement. Every input to our math is a claim somebody published.

How we make money

Some product pages carry buy links. When you buy through one, we may earn a commission from the retailer. It costs you nothing, and the price you pay is the same as if you had gone to the retailer directly.

We may earn a commission when you buy through our links. It costs you nothing and does not change our rankings, which are computed from published specs by the same code for every product.

The rankings on this site are computed from published specs by code that is the same for every product. Same formulas, same constants, same rules, for every unit from every brand. For that to be a lie, we would have to rank by something other than the published numbers, or apply different math to different products. Both are checkable. The formulas are on this page, and the arithmetic for each product is shown on its product page, so you can re-run it yourself.