Free RV Electrical Calculators — Ohm's Law, Battery, Voltage Drop

RV Electrical Calculators & Reference Tools

Ohm's Law · Battery · Voltage Drop · State of Charge · Fuses · Load Reference

Understanding Your RV's Electrical System

Most RVs run two separate electrical systems. The 12V DC system — powered by your battery bank — runs lights, fans, the furnace blower, water pump, slides, and leveling jacks. The 120V AC system — powered by shore power, a generator, or an inverter — runs the air conditioner, microwave, wall outlets, and the water heater's electric element. These two systems are connected through your converter, which charges the 12V batteries from 120V shore power.

Understanding basic electrical math and knowing how to read your battery's resting voltage are two of the most practical skills an RV owner can have. The tools on this page won't make you an electrician — but they will help you plan your power use, catch problems early, and ask better questions when something doesn't add up.

All six tools are free, browser-based, and do not require a sign-up.

E = I × RP = E × II = P ÷ ER = E ÷ I

Planning tools only: These calculators are for education and planning. Always verify results against your device's data plate, owner's manual, and the specifications for your specific RV, battery, and wiring. Results are estimates — real-world conditions vary.

Ohm's Law Calculator

Calculate Voltage, Current, Resistance, Or Wattage

Choose the result you need, then enter the known values. This one tool handles the same amp-draw-from-wattage calculation without needing a separate calculator.

E = I × R | P = E × I | I = P ÷ E | R = E ÷ I

How to use it:

To calculate voltage / E: choose Voltage / E, enter amperage and resistance. Example: finding voltage drop across a known resistance.
To calculate amperage / I from wattage: choose Amperage / I, enter voltage and wattage, leave resistance blank. Example: 1,500 watts ÷ 120 volts = 12.5 amps.
To calculate amperage / I from resistance: choose Amperage / I, enter voltage and resistance, leave wattage blank.
To calculate resistance / R: choose Resistance / R, enter voltage and amperage.
To calculate wattage / P: choose Wattage / P, enter voltage and amperage. Example: 120 volts × 10 amps = 1,200 watts.

Calculator

Calculate

Round

Result—

Choose a calculation and enter the known values.

Battery Runtime Calculator

Estimate how long a battery bank can run a load. Use amp draw for 12V DC loads or wattage for 120V inverter loads.

Runtime = Usable Battery Capacity ÷ Load

How to use it:

Battery Bank Capacity: enter the total amp-hours of the battery bank. Example: two 100Ah batteries = 200Ah.
Battery Voltage: choose the battery bank voltage — usually 12V for most RV battery systems.
Usable Capacity: the percentage you plan to use. A conservative lead-acid or AGM estimate is 50%; check your battery manual for lithium.
Load in amps: use this for 12V DC loads such as lights, fans, furnace blowers, and water pumps.
Load in watts: use this for 120V AC loads powered through an inverter. Enter inverter efficiency so the calculator can account for conversion loss.

Real-world runtime may be lower because of inverter losses, battery age, temperature, wiring resistance, and startup surges.

Estimate Runtime

Battery Bank Capacity

Battery Voltage

Usable Capacity

Load Type

Load

Result—

Enter battery capacity and load.

Voltage Drop Calculator

Estimate voltage drop for copper wire using one-way circuit length, amperage, system voltage, and wire gauge.

Voltage Drop = Amps × Round-Trip Feet × Wire Resistance

How to use it:

System Voltage: choose the circuit voltage. Voltage drop matters more on low-voltage DC circuits such as 12V systems.
Load Current: enter the amps the load draws. If you only know watts, use the Ohm's Law calculator first: choose Amperage / I and enter wattage plus voltage.
One-Way Wire Length: enter the distance in feet from the power source to the load. The calculator automatically doubles it for the round-trip circuit path.
Copper Wire Gauge: choose the wire size. A lower AWG number is a larger wire and usually has less voltage drop.
Result: lower is better. A common planning target is about 3% voltage drop — but always follow applicable codes, manufacturer instructions, and safe wiring practices.

Estimate Voltage Drop

System Voltage

Load Current

One-Way Wire Length

Copper Wire Gauge

Result—

Enter amperage, length, and wire gauge.

Battery State-Of-Charge Reference

Estimate Battery Charge From Resting Voltage

Use this as a rough reference for resting battery voltage. Battery voltage should be checked with the battery at rest — not while charging or under a heavy load.

How to use it:

Choose battery type: select flooded lead-acid, AGM, or lithium / LiFePO4.
Enter resting voltage: measure at the battery terminals after the battery has rested for several hours. Recent charging or heavy loads will make the reading misleading.
Read the estimate: the tool gives an approximate state of charge range and a plain-language note.
Use manufacturer specs: lithium batteries and some AGM batteries have different voltage curves. Always verify with the battery manual, BMS, or battery monitor app when available.

Lead-acid and AGM batteries are generally considered discharged when resting voltage falls to 50% state of charge (roughly 12.2V). Repeated deep discharge shortens battery life.

Estimate Charge

Battery Type

Resting Voltage

Result—

Enter resting voltage for a 12V battery bank.

Common RV Fuse Color Reference

Electrical Reference

Common Automotive Blade Fuse Colors And Amp Ratings

Most RV 12V circuits are protected by automotive-style blade fuses. Fuses are rated by amperage and color-coded so you can identify the rating at a glance. Use this as a quick visual reference when inspecting your RV's 12V fuse panel.

Important — never upsize a fuse to fix a blowing fuse. A fuse protects the wiring, not the device. The fuse is designed to fail before the wire does. Installing a higher-rated fuse to stop a fuse from blowing can overheat the wire and create a fire hazard. If a fuse keeps blowing on the same circuit, the fuse is doing its job — find and fix the cause before replacing it with anything.

1

Black

1 amp

2

Gray

2 amps

3

Violet

3 amps

4

Pink

4 amps

5

Tan

5 amps

7.5

Brown

7.5 amps

10

Red

10 amps

15

Blue

15 amps

20

Yellow

20 amps

25

Clear

25 amps

30

Green

30 amps

40

Orange

40 amps

Fuse color standards are commonly used across the industry, but always verify the number printed on the fuse body and the rating specified in your RV's owner's manual or circuit label. This chart is a visual owner-reference only.

Common RV Electrical Loads

Reference Guide

Common Wattages For Everyday RV Devices

Use this as a rough planning guide alongside the Battery Runtime Calculator above. Actual wattage varies by brand, size, age, and startup surge. Always check the data plate or owner's manual on your specific device — that number is always more accurate than any reference table.

Device / Load	Typical Running Watts	Planning Notes
LED light fixture	3–10 W	Small draw, but multiple fixtures add up over a long night.
Phone charger	5–20 W	Usually low draw.
Laptop charger	45–100 W	Check the charger label — varies widely.
TV	50–150 W	Larger TVs use more power.
Roof vent fan	20–40 W	Speed setting matters.
RV water pump	60–120 W	Intermittent load, not usually continuous.
Furnace blower	80–150 W	Major overnight battery draw in cold weather.
Propane refrigerator controls	5–20 W	Controls only — not electric cooling mode.
Absorption refrigerator on electric	300–600 W	Can be a heavy continuous AC load.
Residential refrigerator	100–250 W	Startup surge may be higher than running watts.
Microwave	1,000–1,500 W	High draw; inverter and battery bank must be sized correctly.
Coffee maker	600–1,200 W	Short run time, high draw.
Toaster	800–1,500 W	High draw on a 120V circuit.
Hair dryer	1,200–1,875 W	Often near the limit of a 15A circuit.
Electric space heater	750–1,500 W	A 1,500W heater draws about 12.5A at 120V.
Electric water heater element	1,200–1,500 W	Large AC load.
13,500 BTU roof air conditioner	1,200–1,800 W	Startup surge can be significantly higher than running watts.

When To Stop And Call A Professional

Know When To Step Back

These calculators support planning and owner-level inspection. They are not a substitute for a qualified RV technician or licensed electrician. Stop and get professional help if any of the following apply.

A fuse on the same circuit keeps blowing. A fuse that blows repeatedly is protecting you from something — a short circuit or an overloaded circuit. Swapping in the same fuse again without finding the cause is not a fix.
You get a tingling sensation or small shock when touching the RV's exterior. This is called hot skin and can be a sign of a serious AC wiring fault. Stop using shore power and call a tech or campground staff immediately.
You smell burning plastic or see scorched wires, melted insulation, or any sign of arcing near the fuse panel, battery, or wiring.
Any planned work involves live 120V AC wiring, new 120V circuit installation, or modifications to the breaker panel. This work requires a licensed electrician and must meet applicable codes.
Converter or inverter failure, significant battery bank degradation, or any electrical fault you cannot clearly identify and safely isolate.
Anything beyond reading, measuring, and planning — and anything that is beyond your experience level.

Related Tools

🔥

Propane · Calculator

Propane Duration Calculator

Select your tank size, set the gauge level, and toggle on the appliances you're running to get estimated days remaining.

Use This Tool →

💧

Water System · Calculator

Water Tank Sanitization Calculator

Enter your fresh water tank capacity and get the exact amount of bleach needed to sanitize your system correctly.

Use This Tool →

🚗

Tires · Safety Tool

RV Tire DOT Date Checker

Enter the last four digits of your tire's DOT code to decode the manufacturing date and estimate how old your tires are.

Use This Tool →

Take It Further

The DIY RV Maintenance Blueprint

The Blueprint includes Electrical and Battery Systems as one of 17 field-ready RV maintenance guides, covering shore power, batteries, charging systems, fuse panels, and electrical safety. The 17 guides and Start Here overview provide over 80 printable pages of practical checklists, logs, and instructions. You also receive the bonus 20-page Basic Multimeter Workbook for RV Owners with guided tests for batteries, converter output, outlets, fuses, switches, wiring, and more.

View The Maintenance Blueprint — $47 Back To Free Resources