Two spec sheet numbers — C-rate and depth of discharge — quietly determine how much usable power and energy you actually get from a battery.
C-rate: how fast a battery can safely deliver current
C-rate expresses charge or discharge current relative to a battery's capacity. A 1C rating means a current equal to the full Ah rating, theoretically emptying or filling the battery in one hour. Most LiFePO4 batteries handle 0.5-1C continuously; some lead-acid types are limited to 0.2C or less. Exceeding the rated C can shorten lifespan or trigger the battery's protection circuit.
This matters most for high-surge loads like well pumps or power tools — check your battery's rated C against the actual current needed with the C-rate calculator before assuming any battery can handle any load.
Depth of discharge: why two batteries with the same rating aren't equal
Depth of discharge (DoD) is how much of a battery's total capacity you can actually use before recharging. LiFePO4 batteries tolerate deep, near-full discharge with minimal lifespan penalty — often 90% or more. Lead-acid batteries degrade rapidly if regularly discharged past 50%, which is why a 200Ah lithium battery often delivers as much practical daily capacity as a 300-400Ah lead-acid bank.
Always compare usable capacity, not rated capacity, when shopping across chemistries — see the depth of discharge calculator for the real number behind any battery's spec sheet.
Why this changes how you size a system
A battery bank sized purely on Ah or kWh rating without accounting for DoD can leave you with far less usable energy than expected — especially when comparing across chemistries. Run your actual usable capacity through the battery size calculator alongside your daily load, rather than assuming the rated number is what you'll actually get to use.
Frequently asked questions
Why do lead-acid batteries need such shallow discharge limits?
Lead-acid chemistry suffers accelerated plate degradation when regularly discharged deeply, so limiting to 50% (or even 20-30% for longest life) is standard practice to get a reasonable cycle life out of the battery.
Does a higher C-rate battery cost more?
Generally yes — batteries built for high sustained discharge current (needed for power tools or fast EV-style charging) use different cell construction than standard batteries, which typically adds cost.