Off-Grid Battery Autonomy Calculator

Size a nominal battery-energy reserve from measured daily critical AC load and no-recharge autonomy days. The reserve stack explicitly derates for inverter efficiency, usable depth of discharge, temperature, and end-of-life capacity, then converts nominal kWh to system-voltage Ah and whole parallel-equivalent modules. It is an energy screen, not a safe electrical, power, solar-resource, generator, or code design.

Battery systems can cause fire, explosion, toxic exposure, arc flash, shock, and lethal fault current. Chemistry, listed equipment, battery-management limits, series/parallel configuration, conductor and overcurrent protection, disconnects, grounding/bonding, ventilation, separation, temperature control, fire detection/suppression, permits, and emergency access require qualified design and current adopted codes. Do not construct a battery bank based only on this estimate.

How the sample reserve stack is calculated

The sample critical loads use 18 kWh of AC energy per day and require three days without solar or generator recharge, so the load-side target is 54 kWh. Delivering that through a 92% efficient inverter would require more DC energy. Only 80% depth of discharge is permitted, cold/temperature capacity is represented at 90%, and the bank must still work at 85% retained end-of-life capacity.

Dividing 54 by 0.92 x 0.80 x 0.90 x 0.85 gives 95.91 nominal kWh. At 48 nominal volts, that is about 1,998 Ah. A 100-Ah parallel-equivalent module at 48 V has 4.8 nominal kWh, so 19.98 modules are rounded upward to 20. The installed bank is 96.00 nominal kWh.

Applying the same four factors to 96 kWh produces about 54.05 kWh of deliverable AC energy at the stated end-of-life and temperature assumptions, only slightly more than the target. Dividing by 18 kWh/day gives 3.00 days. The combined delivery factor is 56.3%, demonstrating why nominal label energy is not the same as dependable AC autonomy.

Daily load must come from a critical-load profile

A monthly utility bill divided by days can hide seasonal peaks, occupancy changes, well-pump cycles, electric heat, refrigeration defrost, water heating, workshop tools, and cloudy-period behavior. Off-grid design needs interval energy and power. Build a circuit-by-circuit critical-load list, measure representative operation, and create winter and summer day profiles.

Separate continuous, cycling, discretionary, and deferrable loads. Reduce waste before buying storage. Heating, cooling, cooking, water pumping, water treatment, communications, medical equipment, refrigeration, and freeze protection can be critical, but each has different power and reliability consequences. Do not quietly assume occupants will curtail essential loads during every poor-weather period.

Autonomy days do not replace solar-resource modeling

Three days without recharge is an arbitrary reliability criterion unless linked to site weather, array size, seasonal load, generator strategy, fuel access, and acceptable loss-of-load probability. A large battery cannot compensate indefinitely for an undersized winter array. Consecutive storms, snow cover, smoke, shading, high temperature, soiling, and equipment outages can reduce production.

Use long-term site-specific hourly solar resource and load simulation, with PV temperature, orientation, shading, inverter clipping, wiring, charge efficiency, battery limits, degradation, and generator dispatch. Test multiple weather years and critical contingencies. Document what event the autonomy requirement is intended to survive.

Energy capacity and power capacity are separate

A 96-kWh bank may have ample energy but insufficient current for a well pump, compressor, induction range, or simultaneous load. Inverter continuous and surge rating, motor starting, voltage sag, battery current limit, BMS trip, ambient temperature, conductor drop, and protection determine power. The calculator does not accept watts or amps and cannot validate starting performance.

Create a coincident load schedule and the worst credible surge. Use manufacturer time-current and surge data, not a single nameplate sum. Consider neutral and unbalanced loads, power factor, harmonic loads, transformer/inverter topology, and split-phase requirements. Qualified electrical design coordinates source, inverter, distribution, grounding, and generator transfer.

Depth of discharge is chemistry and warranty specific

Usable depth of discharge should come from the selected battery’s operating window, warranty, cycle-life objective, and BMS. Lead-acid and lithium chemistries behave differently; even products with similar chemistry have different voltage limits and state-of-charge calibration. A nominal 100-Ah label does not guarantee 80 Ah at every temperature, current, age, and cutoff.

Battery capacity can depend on discharge rate. Lead-acid Ah ratings commonly specify a time rate, and high current can reduce available capacity. Lithium BMS current and low-temperature rules can abruptly limit use. Do not mix ages, capacities, chemistries, or incompatible battery modules unless the manufacturer and engineered system explicitly allow it.

Temperature affects both capacity and charging safety

The sample 90% temperature factor is visible but generic. Obtain capacity curves at the battery’s minimum operating temperature and actual discharge rate. Cold can reduce available energy; heat can accelerate aging. Some lithium chemistries must not be charged below specified temperatures without approved heating and controls.

Design the enclosure for the manufacturer’s ambient range, heat generation, ventilation, condensation, flooding, pests, salt/corrosion, and service access. Never improvise heaters or insulation around batteries. Temperature sensors, control logic, spacing, and fire provisions belong to the listed system and approved design.

End-of-life reserve is a service criterion

The 85% factor means the bank must deliver the autonomy target after nominal capacity has fallen to 85% under the modeled temperature and DoD. It is not a prediction of when that occurs. Calendar aging, cycles, temperature, average state of charge, current, balancing, firmware, and maintenance affect degradation.

Warranty “retained capacity” may use defined test conditions and throughput limits. Build a monitoring and replacement plan, including compatible future modules and disposal/recycling. A system that barely passes at installation but cannot meet its documented end-of-life objective is underspecified for that objective.

Series and parallel design cannot be inferred from Ah alone

The module field means one approved module or series string already at the system voltage. It returns a parallel-equivalent count. Real batteries may have a different nominal voltage, so modules must be configured in series, parallel, or both within BMS and inverter limits. Series changes voltage; parallel changes Ah/current capability.

Recharge losses are outside the autonomy stack

The 92% input represents battery-to-AC delivery through the inverter. It does not include PV-to-battery charging, generator efficiency, battery coulombic/energy efficiency, wiring, standby, heater, BMS, or charge-controller losses. Those matter when sizing the array and estimating fuel/time to restore state of charge.

After an autonomy event, loads continue while charging. A 54-kWh deficit cannot be restored with exactly 54 kWh of gross PV. Model available surplus power, charging limits, absorption/taper behavior, weather, generator minimum loading, fuel consumption, noise, maintenance, and emissions. Preserve emergency reserve if the operating plan requires it.

Reliability needs layered controls

Commission state-of-charge calibration, charge limits, low-voltage cutoff, inverter modes, transfer, generator start, load shedding, alarms, communications, and shutdown. Keep an emergency manual and inspect to manufacturer/code requirements. For a separate U.S.

Frequently asked questions

For a simpler single-load runtime check, compare the autonomy model with the battery duration calculator.