Check a DC load budget for your target runtime using this page’s inputs.
Battery & Power
Estimate 12V battery runtime
Enter the 12V-class battery bank, selected charge range and average DC load to estimate operating time. Optional checks cover current headroom and lead-acid rate effects.
Calculation breakdown
Planning estimate only. Nominal current is not a wiring, fuse or startup-current value. Confirm battery discharge data, cutoff behavior, BMS limits, cable losses and protection requirements before relying on the result.
Find a simple-energy DC load budget for a target duration
Use the original nominal voltage, Ah, parallel count, charge range, condition, delivery efficiency, overhead and optional whole-bank current limit. Enter a duration below. This reverse worksheet uses the simple energy model only; it is unavailable while the optional Peukert mode is selected. The original Peukert calculator remains available.
Estimate runtime or reverse a simple DC energy budget
This tool is for one nominal 12V-class DC system, including matching batteries in parallel. The original form estimates runtime at an entered average load and optionally applies a lead-acid Peukert estimate. The separate reverse worksheet finds a simple-energy load ceiling for a chosen duration. It intentionally refuses to run while Peukert mode is active, so a linear inverse is not presented as a Peukert-adjusted result.
Use the selected charge range and complete battery-side demand
Total Ah = Ah per battery × parallel count. Selected Wh = nominal V × total Ah × condition fraction × (starting % − planned minimum %) ÷ 100. Minimum charge cannot exceed starting charge. Equal values leave no selected energy above that planning boundary.
When entering load amps, load W = nominal voltage × entered amps at that same voltage. For a device operating at another voltage, convert its demand to watts first. Battery-side demand W = external load W ÷ delivery efficiency fraction + separate controller or standby overhead W. Simple runtime hours = selected Wh ÷ battery-side demand W.
If a complete battery-terminal measurement already includes conversion losses and standby draw, use the full measured voltage × current as watts with 100% efficiency and zero separate overhead. Mixing that measurement with another loss deduction would count losses twice.
Keep optional Peukert assumptions visible
For the original optional lead-acid model, let C be condition-adjusted bank Ah, H the capacity rating period, I the nominal battery current, k the entered exponent and f the selected charge fraction. Its estimate is H × [C ÷ (I × H)]k × f. Use only for the supported lead-acid selection with an appropriate rating period and exponent; the setting is not a general correction for every chemistry.
Victron’s capacity and Peukert explanation describes why rated capacity depends on discharge rate. This approximation does not reproduce varying-current discharge histories, temperature changes, voltage sag or a real BMS cutoff. Nominal-current and model assumptions remain relevant even when the displayed number has several decimal places.
Find a load budget for a target duration
With Peukert intentionally off, the reverse worksheet calculates battery-side power from energy as selected Wh ÷ target hours. If a whole-bank continuous-current limit is entered, its nominal power ceiling is bank V × limit A. Allowed battery-side power is the smaller known ceiling. Blank current limit means no current ceiling was applied; it does not prove unrestricted output.
External average load budget = max(0, allowed battery-side W − overhead W) × delivery efficiency fraction. Overhead is subtracted before applying efficiency because it is already battery-side. If overhead alone exceeds allowed power, the target cannot be met even with zero external load under these assumptions.
For 12 V, 100 Ah, 100% condition and a 100%–50% range, selected energy is 600 Wh. A ten-hour target allows 60 battery-side watts. With 10 W overhead and 95% delivery efficiency, the external average load ceiling is 47.5 W. A separate 4 A bank-current limit caps battery-side power at 48 W and reduces the ceiling to 36.1 W. These are conditional model outputs, not device or equipment recommendations.
Limits and interpretation
The reverse check retains the 10–15 V nominal range, up to 100 matching parallel batteries, positive per-battery capacity up to 10,000 Ah, percentages within 0–100, overhead up to 1,000 W and optional current limit up to 2,000 A. Target hours must be positive and no greater than 1,000,000. These software bounds are not hardware ratings. Inputs accept plain decimals up to 18 places; exact comparisons precede six-place display rounding.
A computed average-watt budget does not verify startup current, low-load automatic shutoff, voltage regulation, wiring, temperature or whether a device can operate at that draw. A zero-energy window is not a usable operating-power recommendation. Original Peukert and load-input modes remain available, and source or target edits clear the reverse result. No battery records are saved.
Use Inverter Battery Backup Time for AC conversion and W/VA/surge planning, or Battery C Rate for component current-limit comparisons.