Choose the energy boundary
AC mode takes energy already deliverable to the load after discharge losses. DC mode takes energy available at the inverter's DC input and converts it to delivered AC energy using your one-way efficiency. Switching modes clears the energy and efficiency fields to prevent reinterpreting a value at another boundary.
Both modes require remaining energy above your reserve for the current state of charge and relevant conditions. Do not deduct a reserve or discharge limit twice. These inputs stay in the page and are not saved or transmitted.
Method and examples
For AC energy, runtime h = available AC kWh ÷ AC load kW. For DC energy, delivered AC kWh = available DC kWh × one-way efficiency % ÷ 100; runtime = delivered AC kWh ÷ AC load kW.
Example: 8 kWh deliverable at the AC load supplies a constant 2 kW load for 4 hours in this model. Alternatively, 10 kWh available at the inverter DC input with an assumed 80% one-way efficiency provides the same 8 kWh and 4 hours. These are illustrative inputs, not battery specifications.
Zero available energy returns zero runtime. A zero load is not assigned infinite runtime: standby consumption and self-discharge require another model. Arithmetic retains precision; results display three decimals or scientific notation.
Conditions this model does not predict
Battery energy and power capability are different limits. This tool does not test continuous output ratings, starting surges, inverter compatibility or protection requirements. It also does not select battery capacity or an approved backup installation.
Temperature, discharge rate, ageing, cutoff settings and battery management can change available energy. Supply energy data appropriate to the operating conditions. No chemistry curve, Peukert correction, Ah-to-kWh approximation, changing load, recharge input or independent standby-load model is applied.
Common questions
Can I enter round-trip efficiency in DC mode?
No. Round-trip efficiency includes charging and discharging. This model needs only the one-way ratio from energy drawn at the inverter DC input to AC energy delivered to the load. Do not take an assumed square root of a round-trip specification.
Can I enter the battery's Ah rating?
No. Convert the nominal pack voltage and rated Ah to nameplate energy with the battery energy calculator, then establish how much energy is actually available at this tool's selected boundary. The conversion alone does not determine usable energy or runtime.
Why is the available energy cleared when I change modes?
The measurement point changed. Keeping the same number could apply losses twice or omit them. Confirm the energy for the newly selected boundary before recalculating.
Related tools
Sources
BC Hydro, Battery Energy Storage System (BESS) Best Practices Guide, section 3.1, printed page 6: energy capacity versus power and energy/power storage duration. Section 2 defines usable capacity and round-trip efficiency. The two selectable boundaries and explicit one-way conversion are this tool's modelling contract; no generic efficiency value or system rating is imported.
Natural Resources Canada / CIPEC, Energy Savings Toolbox — An Energy Audit Manual and Tool, section 8.2.4, printed page 79: energy efficiency = output energy ÷ input energy. The DC-mode boundary is defined explicitly here; the manual does not supply a battery/inverter efficiency default.
Rolls Battery, How Does Temperature Impact Battery Efficiency and Lifespan? (September 6, 2023): temperature/capacity limitations in its lead-acid context. No temperature factor or chemistry-specific correction is imported into this model.
Sources checked November 9, 2026. Report a problem or suggest a source; inputs are not attached automatically.