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Pump runtime from battery

How long a load will run on a battery bank. Accounts for battery chemistry, depth of discharge, and inverter losses.

Pump runtime from battery battery → hours
Ah × voltage. A 100Ah 12V battery is 1200 Wh; a 100Ah 24V battery is 2400 Wh.
Sets the safe depth of discharge. Going deeper damages the battery and shortens its life.
100% if you're starting from a full charge.
Total continuous wattage of everything drawing from the battery.
AC loads lose ~10% to inverter conversion. DC loads use the battery directly.
85-92% is typical. Pure sine wave inverters at the higher end; modified sine wave at the lower end.
Runtime -- hours
In context --
Usable energy -- Wh delivered
Battery state --
Real-world runtime is typically 5-10% shorter than this math suggests because voltage sags as the battery drains and the inverter pulls more current to maintain power. For critical loads, plan for the lower end of the range.

What this answers

You've got a battery bank, the sun went down or got cloudy, and you need to know how long your pump (or heater, or grow light) will keep running before the bank's empty. This is the answer.

Why it isn't just "Wh divided by W"

Three things bite the simple math:

  • Depth of discharge limits. You can't safely use 100% of a battery's nameplate capacity. Pulling a flooded lead-acid below 50% destroys it. AGM tolerates 60%. LiFePO4 will go to 80% (some chemistries publish 90%, but margin is a friend). The runtime calc starts from your current state of charge and stops at your chemistry's safe cutoff.
  • Inverter losses. AC inverters typically run 85-92% efficient. A 100W AC pump pulls more like 110W from the battery. DC pumps avoid this loss entirely but most aquaponics gear is AC.
  • Voltage sag. As batteries drain, voltage drops, and current rises to maintain power. This isn't modeled here (the calc assumes constant-power loads) but it's the reason real-world runtime is usually 5-10% shorter than the math says.

A few sanity-check numbers

For gut-checking the output:

  • 30W submersible pump on a 100Ah / 12V LiFePO4 battery (1.2 kWh nameplate, ~960 Wh usable at 80% DoD), 90% inverter efficiency. Runtime: about 29 hours. Roughly a day of riding out cloud cover.
  • Same battery, 200W heater. Runtime: 4.3 hours. Tank heaters drain batteries fast.
  • Same battery, 5W air pump. Runtime: 173 hours. Air pumps are tiny loads.

If the answer feels wrong, the most common cause is mismatching battery capacity in Ah vs Wh. Ah × voltage = Wh. A 100Ah battery at 12V is 1200 Wh; at 24V it's 2400 Wh. The calc takes Wh directly to avoid this trap.

The relationship to autonomy

The battery sizing calculator asks "how much battery to ride out N cloudy days at this load?". This one asks the reverse: "given the battery and load on hand, how long before it dies?". Same physics, different question.

Further reading