RV Charger Size Calculator

Related Calculators

Table of contents

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

Battery bank size (Ah): The usable rating you enter for the 12V house battery bank. Ah means amp-hours, a measure of stored electric charge.

Battery type: Used only in Illustrative chemistry assumptions to select a planning percentage. It is not a substitute for the actual battery instructions.

12V loads used while charging (amps): The DC current used by RV items that may be on while the converter is also charging the battery.

Battery level now (percent): The estimated state of charge, meaning how full the battery is before charging starts.

Target battery level (percent): The battery level you want to reach, such as 100 percent for full or 80 percent for a shorter stop.

Extra headroom (percent): Added room for small 12V loads that turn on after you sized the charger. It is applied to loads, not to the battery charge limit.

Battery charge current limit used: The manufacturer limit you supplied, or an explicitly illustrative percentage-based assumption. This number does not cause a physical charger to limit current.

Charging amps left after 12V loads: The rating or verified total-output setting minus the entered DC loads, with a minimum of zero. The loads-off result checks how much current could instead reach the battery.

Estimated 120V AC draw: The rough shore power or generator current used by the converter charger only. Shore power supplies 120V AC that an RV converter changes to 12V DC for the batteries [1].

Mistake: Entering one battery's rating in Battery bank size (Ah) when several 12V batteries are wired in parallel.
Fix: Add the Ah ratings of parallel batteries and enter the total Battery bank size (Ah).

Mistake: Adding microwave, air conditioner, or outlet loads to 12V loads used while charging (amps).
Fix: Enter only 12V DC loads, such as lights, pump, fans, and control boards, in 12V loads used while charging (amps).

Mistake: Setting Wanted recharge time (hours) very low and assuming a bigger converter will always make charging faster.
Fix: Check Battery charge current limit used and Target time check before shopping for the Recommended converter charger size.

Mistake: Treating a chemistry planning percentage or a BMS protection cutoff as an actual charger-current setting. Fix: Use Battery manufacturer instructions for the bank limit. Select adjustable control only when the converter actually regulates total DC output to Verified total DC output setting (amps).

Mistake: Ignoring Current wiring or converter limit (amps, optional) when replacing a smaller factory unit.
Fix: Enter the known wiring, fuse, panel, or existing converter limit so Recommended converter charger size does not exceed it.

Mistake: Comparing Estimated 120V AC draw to the full RV power draw.
Fix: Treat Estimated 120V AC draw as the converter charger draw only, not the microwave, water heater, or air conditioner.

• The recharge time is a simple bulk-charge estimate. Real chargers often slow down near full charge, so the last part can take longer than shown.
• Illustrative chemistry assumptions are retained for hypothetical comparisons only. They do not verify the current, voltage, temperature or profile suitable for a particular battery.
• The calculator does not verify wire gauge, fuse size, converter compartment cooling, battery temperature limits, or charger profile settings.
• Current wiring or converter limit (amps, optional) is treated as a hard cap when entered. Do not upgrade past the rated wiring, fuses, and panel without checking the RV electrical design.
• Estimated 120V AC draw assumes 120V input and power factor 1, using Charging voltage (V), Converter efficiency (percent) and the modeled total DC output. Real RMS input current can be higher with a lower power factor; use the charger nameplate for AC circuit planning.
• Standard converter sizes are shopping buckets, not a complete product catalog. If the raw need is above 150 amps, use multiple charging sources or get a custom RV electrical design.
• The calculator assumes a 12V house battery bank. It is not meant for 24V or 48V RV electrical systems.

Calculation steps

The calculator first finds how many amp-hours need to be replaced between Battery level now (percent) and Target battery level (percent).

needed_ah = battery_ah * (target_soc_pct - current_soc_pct) / 100

The default mode uses Battery charge limit from manual (amps). The separate illustrative mode uses 0.13, 0.20 or 0.50 times bank Ah for flooded, AGM or LiFePO4 respectively, unless its optional percentage is entered. Those planning assumptions do not establish product suitability.

max_charge_amps = entered_manual_amps OR explicitly_selected_planning_amps

The charger current requested by your time goal is the amp-hours to replace divided by Wanted recharge time (hours).

requested_charge_amps = needed_ah / recharge_hours

The time-goal current is bounded by the supplied battery limit for capacity planning. The later equipment/current-control checks determine the available current; no automatic battery-side clamp is assumed.

sizing_charge_amps = min(requested_charge_amps, max_charge_amps)

Next, the calculator adds the entered 12V loads and applies Extra headroom (percent) to those loads only.

raw_converter_amps = dc_load_amps * (1 + safety_buffer_pct / 100) + sizing_charge_amps

The raw need is rounded up to the first standard size in this list: 15, 20, 30, 35, 45, 55, 60, 70, 75, 80, 100, 120, and 150 amps.

unconstrained_converter_amps = first standard_size >= raw_converter_amps

The entered wiring/converter capacity limits the standard size. Without adjustable control, the converter rating must also stay at or below the battery charge limit, because all DC loads can turn off. Adjustable mode requires a real total-output setting no higher than the battery limit and supported by the selected converter capacity.

fixed: rating <= battery_limit and wiring_limit; adjustable: verified_output_setting <= rating and battery_limit

Available total output is the fixed converter rating or the entered, verified adjustable setting. Subtract the actual DC loads, not the headroom allowance. When those loads turn off, all available output could reach the battery; that amount is checked against the same limit.

charge_current_after_loads = max(available_total_dc_output - dc_load_amps, 0)

Estimated recharge time divides the needed amp-hours by the charging amps left. If no charging current is left after loads, the result is N/A instead of a divide-by-zero value.

estimated_recharge_hours = needed_ah / charge_current_after_loads

Estimated 120V AC draw uses the modeled total DC output, Charging voltage (V), 120V input, efficiency and an assumed power factor of 1. It is an idealized converter-only estimate, not a branch-circuit rating.

ac_input_amps_at_pf_1 = available_total_dc_output * charge_voltage_v / (120 * converter_efficiency_pct / 100)

Worked example

Suppose a 200 Ah bank needs 100 Ah replaced, its manufacturer permits 100 A charging in the stated conditions, DC loads use 10 A, headroom is 10%, and the time goal is 3 hours. The raw need is 10 x 1.10 + 100 / 3 = 44.3 A, rounded to a 45 A converter. With no adjustable output limit, 35 A remains after loads and the bulk estimate is 100 / 35 = 2.86 hours. If loads turn off, the full 45 A could charge the battery, still below the supplied 100 A limit. At 13.6V, 85% efficiency and power factor 1, estimated AC draw is 6.0 A.

How to read the result

Read Recommended converter charger size together with Total DC output used in this estimate and Battery current available if 12V loads turn off. A larger adjustable converter is conditional on the verified setting; the rating alone does not provide that limit. Target time check is a bulk-charge estimate, and the unconstrained size may not be suitable under your battery or wiring limits.