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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsA runtime estimate based only on amp-hours divided by load current can be badly wrong because battery capacity changes with discharge rate, an inverter draws power even when its AC output is lightly loaded, and not all rated capacity may be usable. A useful TypeScript model should make those factors explicit inputs rather than treating a battery as a fixed bucket of amp-hours.
Why amp-hours divided by current can mislead
The simple calculation hours = amp-hours ÷ amps assumes the battery will deliver its full rated capacity at the current draw and that the current stays constant. A battery’s amp-hour rating is tied to a stated discharge rate, however; a capacity quoted at C20 is not guaranteed at a much faster discharge. For power loads, first establish the battery’s nominal voltage and convert watts to current consistently: at a given voltage, current ≈ watts ÷ volts. Real voltage varies during discharge, so this is a modeling approximation, not a complete battery model.
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Victron explains that faster discharge reduces available capacity and that lead-acid batteries are more affected by this effect than lithium batteries. Its illustrative example says a lead-acid battery rated at 100 Ah at C20 may deliver 56 Ah when discharged completely in two hours. That is an example, not a general derating factor for other batteries. Victron’s explanation of battery capacity and Peukert’s law and its Energy Unlimited guide discuss the rate effect.
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Peukert’s law is an empirical way to represent the loss of available capacity as discharge current rises. One common form is:
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Cp = In × t
Here, Cp is a constant for the chosen battery and reference convention, I is discharge current, n is the Peukert exponent, and t is discharge time. With a consistent reference rating and units, the relationship can be rearranged to estimate time at a different current. The exponent is battery-specific; do not infer it from nominal amp-hours alone.
Estimate the exponent from two battery ratings
When the supplier does not specify an exponent, Victron describes calculating one from two capacity ratings at different discharge rates, often C20 and C5. The discharge currents should be substantially different to make the estimate useful. In effect, the ratings give two observations of how time and current relate for that battery. Use the battery supplier’s stated values and discharge conditions; ratings drawn from different products or incompatible test conditions do not define a sound exponent.
Victron’s cited battery-monitor documentation describes an exponent range of 1.00–1.50 and a default of 1.25 for that monitor. Its configuration guidance gives typical fallback values of 1.25 for lead-acid and 1.05 for lithium when a supplier value is unavailable. Those are manufacturer settings and fallback guidance, not universal constants; defer to the battery maker’s data. See Victron’s battery-monitor configuration guidance.
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Know where the approximation weakens
A fixed exponent is a useful correction, not a complete electrochemical simulation. Victron cautions that at very high currents a battery can yield less capacity than the fixed-exponent law predicts. Temperature, battery age, load variation, voltage limits, wiring losses, and battery-management-system cutoffs can also change actual runtime. The manufacturer sources support the model inputs described here, but do not establish one equation as accurate for every battery and operating condition.
Include inverter tare draw, including at low AC load
An inverter’s DC input is not necessarily zero when little or nothing is connected to its AC output. Its own operating electronics consume power while it remains on; that standby or zero-load consumption is commonly called tare loss. Model it as an additional DC-side load for every interval the inverter is on, using the specification for the actual inverter and operating mode.
For example, Victron’s SUN Inverter specification table lists these zero-load figures for two specific models. Its ECO values are the default ECO-mode figures, not the continuous-operation figures:
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|---|---|---|
| 12/250 | 4.2 W | 0.8 W |
| 24/250 | 5.2 W | 1.3 W |
These are model-specific manufacturer specifications, not values to apply to other inverters. ECO operation is not interchangeable with leaving an inverter continuously on. Consult the SUN Inverter specifications for the relevant model and mode.
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When a specification gives idle power in watts, multiply it by the hours the inverter is on to get idle energy in watt-hours: Wh = W × hours. Relate that energy to the battery model using the system voltage and a consistent energy or current calculation. Do not add the inverter’s AC output load and its idle draw as though they were the same measurement point: account for the inverter’s DC-side consumption, including its conversion losses under load, using suitable device data if available. The cited zero-load specifications alone do not provide a general efficiency curve for loaded operation.
Set a battery-appropriate discharge floor
A runtime estimate should stop when the modeled state of charge reaches the configured minimum, rather than assuming every rated amp-hour can be used. Make the discharge floor an explicit, configurable assumption tied to the chemistry and supplier guidance. It may be represented as a minimum state of charge or as a maximum permitted depth of discharge; be clear which convention the code uses.
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Victron’s cited battery-monitor manual uses a 50% discharge floor as a default for lead-acid and says lithium batteries can often be discharged deeper, with a 10–20% setting range unless the battery supplier advises otherwise. These are settings and guidance in that manufacturer’s manual, not universal safety limits for all batteries. Check the battery specification and system requirements before choosing a value. The BMV-710H Smart and SmartShunt manual explains these monitor settings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Make the model inputs explicit in TypeScript
Keep physical assumptions separate from the calculation so a different battery, operating mode, or discharge policy does not require changing hidden constants. A practical input set includes:
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- Battery chemistry and nominal/system voltage.
- Rated capacity in amp-hours together with its discharge-rate basis, such as C20.
- The supplier’s Peukert exponent, or two compatible capacity ratings and their discharge currents from which to estimate one.
- Discharge current or a time-varying load profile, with units stated.
- The allowed minimum state of charge or maximum depth of discharge.
- Inverter model, operating mode, zero-load draw, and the time it remains on.
- Charge efficiency and temperature assumptions if the model accounts for them.
Preserve units in names or types—for example, capacity in Ah, current in A, voltage in V, power in W, and elapsed time in hours. Establish whether your Peukert calculation expects current or a normalized C-rate, then keep that reference convention consistent when deriving the constant and estimating runtime. The model should not silently mix watts, amps, and amp-hours.
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Use a transparent calculation sequence
- Load source data. Record the battery’s capacity rating and its C-rate, chemistry, supplier-provided Peukert data, voltage, and recommended discharge limit.
- Estimate battery-specific rate behavior. Use the supplier exponent where available; otherwise derive an estimate from two substantially different discharge ratings that share compatible test conditions.
- Build the DC-side load. Convert loads to consistent units and add the inverter’s idle consumption during each interval it is on. Use the actual operating-mode specification.
- Estimate delivered capacity or runtime. Apply the Peukert relationship using the same reference convention as the capacity data. For a changing load, calculate interval by interval rather than pretending the current is constant.
- Enforce the discharge floor. End the estimate when modeled state of charge reaches the configured minimum; do not count capacity below that threshold as usable.
- Report assumptions with the result. Include the battery rating basis, exponent source, load profile, inverter mode and idle draw, discharge floor, and any temperature or efficiency assumptions.
This sequence is modeling guidance, not a validated TypeScript implementation or library API. A simple model can be useful for planning, but the result should not be presented with more precision than its inputs justify.
Check estimates against real battery behavior
When practical, compare predicted consumption with a shunt-based battery monitor and an observed load period. Victron describes its monitor as continuously measuring current flowing into and out of the battery and integrating that current over time to estimate state of charge. Its calculation also accounts for Peukert efficiency and charge efficiency, with temperature considered to a lesser extent. A monitor reading is still an estimate: configuration, synchronization, and battery-specific settings matter. See the manufacturer manual.
In particular, the manual warns that a deeply discharged lithium battery can be harmed by even residual current. Avoid treating a model’s remaining-capacity estimate as permission to ignore a battery’s low-voltage protection or supplier instructions. Compare scenarios only when chemistry, capacity-rating basis, discharge profile, configured floor, inverter model and mode, and relevant temperature assumptions are held constant or clearly identified.
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