Free 12V Electrical Calculators for Campers, Boats & Off-Grid (2026)

Sizing a camper, boat, or off-grid solar system means doing the same five calculations every time: wire gauge, fuse size, battery bank capacity, inverter wattage, and solar array. Get any of them wrong and you either burn copper, brown out an inverter, or buy panels you do not need. To save you the spreadsheet, we built a free online calculator for each one — no signup, no paywall, both AWG and mm², 12V / 24V / 48V across the board.

This post is a quick tour of all five tools, what each one solves, and which you should reach for first depending on where you are in your build.

The same five numbers drive almost every DC build. The calculators below find each of them in under a minute.

The Five Calculators at a Glance

Calculator	What you put in	What you get out
Wire gauge & fuse calculator	Amps (or watts), system voltage, cable run length, drop target	Required AWG and mm², voltage drop, recommended fuse
Battery bank sizer	Daily Wh, days of autonomy, chemistry, system voltage	Required Ah, usable Ah after DoD, series/parallel layout
Inverter size calculator	List of AC appliances and their surge type	Continuous wattage, startup surge, DC current at your bus voltage
Solar panel calculator	Daily Wh demand, sun region, system voltage	Array wattage, panel count, minimum MPPT controller current
Embed page	A target tool slug	An <iframe> snippet for blogs, forums, and product pages

All five run client-side, work on phones, and use the same conventions as the VoltPlan diagram editor, so you can paste their results straight into a build sheet.

1. Wire Gauge & Fuse Calculator

Use this one first whenever you add a new circuit. Most camper and boat fires start at undersized cable or a fuse that does not match the wire it is supposed to protect, so this is the calculation with the highest safety stakes.

You enter:

• Load in amps or watts (toggle between the two).
• System voltage: 12V, 24V, or 48V DC.
• One-way cable length — the tool doubles it for round trip.
• Acceptable voltage drop: 3 % for inverters and DC-DC chargers, 10 % for general lighting.

You get:

• The smallest legal AWG and the equivalent mm² cross-section.
• Actual voltage drop in volts and as a percentage.
• A recommended fuse size that protects the wire (not the load).

If you would rather see the math, the wire gauge sizing guide walks through the formula step by step, and the fuse sizing and placement guide covers where the fuse actually goes on the cable.

Open the wire gauge calculator.

2. Battery Bank Sizer

Once you know your daily energy budget, this calculator turns it into an actual battery purchase: how many Ah at what voltage, in what series/parallel layout, accounting for the depth-of-discharge limit of your chosen chemistry.

You enter:

• Daily energy use in Wh — sum of every appliance × hours used.
• Days of autonomy you want without sun or shore power (1.5 to 3 days is typical for vans, 3 to 5 for boats).
• Chemistry: LiFePO4, AGM, gel, or flooded lead-acid. Each has a different safe DoD.
• Bus voltage: 12V, 24V, or 48V.

You get:

• Total nameplate Ah required at your bus voltage.
• Usable Ah after the chemistry-specific DoD ceiling is applied.
• A suggested series/parallel cell or block layout.

The big lever here is chemistry. Going from AGM (50 % DoD) to LiFePO4 (80 % DoD) cuts the bank you need to buy by roughly 38 % for the same usable energy — the LiFePO4 complete guide explains why and what trade-offs come with it.

Open the battery bank sizer.

3. Inverter Size Calculator

Almost everyone oversizes inverters out of nervousness, then loses 30–60 W to idle draw 24 hours a day. This calculator prevents that by computing two separate numbers — continuous load and startup surge — instead of just adding wattages on a napkin.

You enter:

• Each AC appliance you intend to run, with its rated watts.
• Whether it is resistive (kettle, toaster), motor-driven (compressor fridge, drill), or electronic (laptop, induction hob). The calculator applies the correct surge multiplier — typically 1× for resistive, 3–6× for motors.
• DC bus voltage so it can convert AC watts to DC amps for the wire calculator.

You get:

• Continuous wattage (sum of everything that can realistically run together).
• Worst-case surge wattage during a compressor or motor start.
• A recommended pure-sine inverter size with sane headroom.
• DC input current — feed this straight into the wire gauge calculator to size the cable from the battery to the inverter.

If a 1000 W inverter that "should" handle a fridge is tripping low-voltage cutoff, the marine inverters and shore power guide and the Renogy 1000 W inverter cautionary tale both show why surge headroom matters more than continuous rating.

Open the inverter size calculator.

4. Solar Panel Calculator

The fastest way to spend money on solar you do not need is to size by gut feeling. This calculator does the opposite — it works backward from your daily Wh, applies realistic peak-sun-hours for your region, and tells you the smallest array that closes the gap.

You enter:

• Daily energy use in Wh (the same number you fed into the battery sizer).
• Sun region — peak-sun-hours range from ~2.5 in northern Europe winter to ~6+ in southern Spain or Arizona summer.
• System voltage and a system-loss figure (cabling, MPPT efficiency, soiling).

You get:

• Required array wattage to fully recharge in one good solar day.
• Panel count for common 100 W, 200 W, and 400 W modules.
• Minimum MPPT charge controller current at your bus voltage.

Once you know array size, the question becomes wiring topology. Solar panels: series vs parallel covers the trade-off (higher Voc and longer cables vs higher current and shade tolerance), and the solar panel sizing guide goes deeper on real-world derating.

Open the solar panel calculator.

5. Embed Page (Bonus, for Bloggers and Forum Threads)

If you run a blog, YouTube channel, or a product page in this niche, you can drop any of the four calculators into your own site as an iframe. The embed page generates the snippet for you and every embed includes a rel="canonical" link back to voltplan.app, so you can use the tool to add genuine value to your readers without competing with the original page in search.

Which Calculator Should You Use First?

It depends on where in the build you are:

• Designing from scratch. Start with the battery bank sizer — the answer feeds everything else.
• Adding a new appliance. Use the inverter size calculator to update continuous and surge totals, then re-run the wire gauge calculator for the inverter feed.
• Planning a roof. Go straight to the solar panel calculator with your daily Wh.
• Replacing a single fuse or cable. The wire gauge & fuse calculator is the one you want.

A typical full-system pass takes about 10 minutes across all four tools, and the numbers stay consistent because every tool uses the same DC voltages, units, and rounding conventions.

Frequently Asked Questions

Are these calculators free? Yes. All four sizing tools and the embed page are free, do not require an account, and run entirely in your browser.

Do they work for 24V and 48V systems? All four sizing tools support 12V, 24V, and 48V DC. Inputs and outputs scale automatically — for example, doubling system voltage roughly halves DC current and lets you drop one or two wire-gauge sizes.

Do they handle metric (mm²) and imperial (AWG)? The wire gauge calculator outputs both AWG and mm² for every result, so you can match European terminal blocks and US wire spools without converting by hand.

What standards do they follow? Voltage drop is calculated against the standard 3 %/10 % targets used by ABYC for marine and most RV best-practice guides. Fuse recommendations protect the wire ampacity, not the load — that is the rule the fuse sizing and placement guide covers in detail.

Can I embed them on my own site? Yes. Use the embed page to generate an iframe snippet for any of the four calculators.

Where does the data go? Nowhere. All four calculators run client-side. We do not store your inputs or your design.

Next Step: Turn the Numbers Into a Diagram

Once the five calculators agree, the last step is drawing the system so a future you (or an electrician) can read it back. That is what VoltPlan does — paste the wire gauges, battery bank, inverter, and solar array into a single diagram with brand-specific components and pricing, then export PDF or PNG for your build folder.

The calculators tell you what to buy. The diagram tells you how it all connects.