Inverter Idle Consumption Calculator

Related Calculators

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Idle draw: The power an inverter uses while it is on but running little or no load.

Watts (W): A rate of power use. In this calculator, watts show how fast the inverter is using energy.

Amps from battery (A): The no-load current pulled from the battery bank. The calculator converts it to watts using battery voltage.

Watt-hours (Wh): Energy used over time. A 20 W idle draw for 24 hours uses 480 Wh.

Kilowatt-hour (kWh): 1,000 Wh. Electric bills and cost estimates commonly use kWh.

Amp-hours (Ah): Battery charge capacity. Battery used per day converts idle energy into Ah at the selected Battery bank voltage.

Usable battery before recharge: The percent of rated battery capacity you plan to use before charging again.

Mistake: Entering amps in Idle draw amount while Idle draw unit is set to Watts.
Fix: Set Idle draw unit to Amps from battery, or convert the value before entering Idle draw amount.

Mistake: Using one battery's Battery capacity when several batteries are wired in parallel.
Fix: Enter the total Battery capacity for the bank at the selected Battery bank voltage.

Mistake: Setting Idle time per day to 24 when you normally switch the inverter off for part of the day.
Fix: Enter the real Idle time per day so Idle energy used per day matches your routine.

Mistake: Treating Days until usable battery is spent as full RV, boat, or cabin runtime.
Fix: Remember that Days until usable battery is spent only includes inverter idle draw, not lights, fridge, pumps, chargers, or other loads.

Mistake: Entering a Lower idle draw to compare in a different unit than Idle draw unit.
Fix: Use the same unit for Lower idle draw to compare as the selected Idle draw unit.

• The calculator treats idle draw as steady during the Idle time per day you enter. Real inverters may use more or less power as temperature, battery voltage, or search mode changes.
• Battery bank voltage is nominal, such as 12 V, 24 V, or 48 V. Actual battery voltage changes while charging and discharging, so measured amp-hours can differ.
• Runtime results include inverter idle draw only. Other loads, charging losses, battery age, cold temperature, voltage sag, and battery cutoff settings can shorten real runtime.
• Battery capacity should match the full bank at the selected voltage. Parallel batteries add amp-hours; series batteries raise voltage but do not add amp-hours.
• 30-day idle cost uses 30 days, and Yearly idle cost uses 365 days. Your billing month or seasonal use may be different.
• Lower idle draw to compare must use the same unit as Idle draw unit and should describe the same number of inverters unless you also change Number of inverters.
• Electricity rate is an energy replacement estimate. It does not include generator fuel details, solar equipment cost, demand charges, taxes, or utility fixed fees.

Core calculation

The calculator first converts the selected idle draw into total idle power for all identical inverters. If Idle draw unit is Watts, the entered value is already power. If it is Amps from battery, the calculator uses the basic electric power relation power equals current times voltage.

If unit = watts: idle_power_w = idle_draw_amount * inverter_count

If unit = amps: idle_power_w = idle_draw_amount * battery_voltage_v * inverter_count

Daily idle energy uses the same practical method used for appliance energy estimates: power in watts times hours used [2]. Inverter standby power can drain a battery even when no useful load is running [3].

daily_idle_wh = idle_power_w * idle_hours_per_day

daily_idle_ah = daily_idle_wh / battery_voltage_v

Battery runtime

The usable battery energy is based on rated amp-hours, nominal bank voltage, and the percent you plan to use before recharging.

usable_battery_wh = battery_capacity_ah * battery_voltage_v * (usable_battery_percent / 100)

continuous_idle_hours = usable_battery_wh / idle_power_w

idle_days_at_daily_schedule = usable_battery_wh / daily_idle_wh

If idle power is 0, the calculator shows no idle drain instead of dividing by zero.

Energy cost

Watt-hours are converted to kilowatt-hours by dividing by 1,000. The monthly estimate uses 30 days, and the yearly estimate uses 365 days.

daily_kwh = daily_idle_wh / 1000

monthly_kwh = daily_kwh * 30

yearly_kwh = daily_kwh * 365

monthly_cost = monthly_kwh * electricity_rate_per_kwh

yearly_cost = yearly_kwh * electricity_rate_per_kwh

Comparison draw

When Lower idle draw to compare is entered, it is converted with the same unit rule as the main idle draw. Positive savings mean the comparison draw uses less energy than the current draw.

comparison_yearly_kwh_saved = ((idle_power_w - compare_power_w) * idle_hours_per_day * 365) / 1000

comparison_yearly_savings_dollars = comparison_yearly_kwh_saved * electricity_rate_per_kwh

comparison_extra_idle_hours = (usable_battery_wh / compare_power_w) - (usable_battery_wh / idle_power_w)

If the comparison draw is 0, yearly energy and dollar savings can still be calculated, but extra continuous idle time is treated as not limited by idle draw.

Mini example

For a 20 W inverter left on 24 hours per day with a 12 V, 100 Ah battery bank and 80 percent usable battery, daily idle energy is 20 * 24 = 480 Wh/day. Battery used per day is 480 / 12 = 40 Ah/day. Usable battery energy is 100 * 12 * 0.80 = 960 Wh, so continuous idle time is 960 / 20 = 48 hours, or 2 days at that daily schedule. At $0.18 per kWh, yearly idle cost is (480 / 1000) * 365 * 0.18 = $31.54.