Battery chemistry & storage

C rate discharge calculator

Enter your battery's capacity and voltage, then your continuous and peak loads. Get the discharge current, C-rate, and runtime, checked against your battery's rating.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your battery
Total usable energy capacity, from the spec sheet or nameplate.
Common residential packs run 48V or 51.2V. Check the nameplate for yours.
Check your datasheet. Leave blank to use a 1.0C default, typical for residential LFP.
3 Your load
The steady wattage you expect to draw for minutes or hours at a time.
Startup draw for motors, pumps, or compressors. Leave blank if not applicable.

How it works

Three steps to check your load against your battery

Step 1

Find your battery's capacity and voltage

Pull the usable kWh capacity and nominal voltage from your battery's spec sheet or nameplate. If you're still choosing a voltage, the bank voltage selection tool covers the tradeoffs.

Step 2

Add up your continuous and peak loads

List the steady wattage you'll run for minutes or hours, then separately note any motor or compressor startup surge, since the two draw very different currents from the battery.

Step 3

Compare your C-rate against the rating

Check the calculated C-rate against your battery's continuous rating. If you're consistently close to the limit, the voltage sag calculator shows how much delivered voltage drops under that load.

Understanding your C-rate result

What C-rate actually measures

C-rate describes discharge current relative to battery capacity: a 1C rate empties a fully charged battery in one hour, a 0.5C rate takes two hours, and a 2C rate takes 30 minutes. Most residential LFP batteries carry a continuous rating between 0.5C and 1C, meaning a 10 kWh battery can typically sustain 5 to 10 kW of continuous output without exceeding its design limit, a rating derived from the same discharge testing NREL's energy storage research uses to characterize battery performance across chemistries.

Why continuous load and peak load are different questions

A continuous rating covers what the battery can sustain for minutes or hours, while a surge or peak rating covers a brief spike, often just a second or two, from motor startup on a well pump, air conditioner compressor, or refrigerator. The C-rate vs inverter output tool checks whether your inverter's own surge rating becomes the limiting factor before the battery does, and if a well pump specifically is behind your surge number, the inverter sizing for well pump startup tool walks through that startup current in more detail.

Why exceeding your C-rate rating matters

Pushing past the rated continuous C-rate raises internal resistance losses and heat, which increases voltage sag under load and can trigger the battery management system to reduce or cut output. Staying within the rating over the long run also protects cycle life, since the internal resistance impact calculator shows how heat buildup compounds over repeated high-current cycles, exactly the kind of sustained discharge stress the UL 9540A test method evaluates as part of its safety testing.

What to check next

Once your C-rate checks out, confirm your battery bank's amp-hour math with the parallel configuration calculator if you're combining multiple units, or check state of charge to see how far your battery can run this same load before needing a recharge. How long a battery lasts depends on how you treat it. The roof penetration maintenance cost tool in our Maintenance & Lifespan section can help you plan for capacity loss over time.

Questions

Frequently asked

Most sized-correctly home battery systems run between 0.1C and 0.3C for everyday loads. A 10 kWh, 48-volt battery running a 3,000-watt continuous load draws about 62.5 amps, which works out to a 0.3C discharge rate.

Exceeding the rated continuous C-rate increases internal heat and voltage sag, and most battery management systems will cut power or shut down to protect the cells. Regularly drawing above a 1C rating on a battery rated for 1C continuous can also reduce deliverable capacity and shorten cycle life over time.

Yes. Most batteries and their inverters allow a higher C-rate for a few seconds to handle motor startup or appliance surges, separate from the continuous rating meant to be sustained for minutes or hours. Check both figures separately: a well pump might draw 3 times its running wattage for under a second at startup.

Divide your load's current draw in amps by the battery's capacity in amp-hours. For example, a 3,000-watt load on a 48-volt, 208 amp-hour battery draws about 62.5 amps, which divided by 208 amp-hours gives a 0.3C discharge rate.

Related tools

Keep checking your system

C rate vs inverter output Internal resistance impact Voltage sag under load Fast charge capability comparison Battery state of charge estimator Battery bank voltage selection Parallel configuration calculator All battery chemistry tools