Solar Electricity
Solar Autonomy Calculator
Calculate autonomy duration from usable battery energy and daily load. The default values illustrate the workflow; replace them with measurements or ratings from one documented condition.
Run the Solar Autonomy scenario
Replace the examples with values from the same case.
Understanding the equation
Start with the equation days = usable energy ÷ daily load. The formula draws on Usable battery energy and Daily load.
Using the loaded examples gives 3.00 days. Your saved case should identify where each entry came from.
Calculate autonomy duration from usable battery energy and daily load. If you also need required nominal storage, continue with Off-Grid Battery Bank Calculator.
Input notes
Base energy estimates on the limiting season rather than an annual average alone. Nameplate and meter values are not interchangeable unless they describe the same condition.
At least one field has a positive lower bound because it appears in a denominator. A prefix error can move the result by several orders of magnitude. A connected part of the work can be checked with Amp-Hour Watt-Hour Calculator.
- Usable battery energy
- Default example: 18 kWh. Enter usable battery energy in kWh.
- Daily load
- Default example: 6 kWh. Enter daily load in kWh.
Reading Autonomy duration
Read Autonomy duration as the outcome of this equation, not as automatic equipment approval. Compare it with module, inverter, controller, and storage ratings.
Keep the input set beside the output so another reader can reproduce it. Continue with Off-Grid Solar Array Calculator to evaluate required array size.
Practical limits
Solar input during the outage is excluded.
The equation does not include temperature effects, shading patterns, wiring loss, and equipment availability. Resolve material omissions before selecting a standard size or rating.
Use site-specific irradiance, temperature, shading, and equipment data.
A second scenario
Change Usable battery energy from 18 kWh to 21.599999999999998 kWh and leave all other entries untouched. The displayed value changes from 3.00 days to 3.60 days.
The comparison demonstrates sensitivity, not a guaranteed field response.
Documenting the calculation
Document usable battery energy and daily load with autonomy duration. Keep enough context to reproduce the result later.
Transfer autonomy duration unchanged when passing it to the next worksheet. Apply standard sizes only after all checks are complete.
Before relying on the answer
Why does the field result disagree with this page?
First confirm that the measurements match the equation. Then review weather variability, mismatch, clipping, soiling, degradation, and curtailment.
How should I compare two Solar Autonomy cases?
Change one uncertain entry at a time and preserve the baseline. Base energy estimates on the limiting season rather than an annual average alone.
Does the answer include a design margin?
The calculator does not approve equipment or supply an unexplained margin. Compare against cold-voltage, hot-voltage, current, energy-yield, and controller limits.
Why does Autonomy duration show several decimals?
Do not round between dependent calculations. Match the final display to the precision of the source data.
Which input can create an undefined result?
Daily load is used in a denominator and must be greater than zero.