Power inverter calculator: size, cable, fuse & battery

Plan a safe DC-to-AC system for your van, RV, boat, truck or cabin. List what you'll run and get the inverter rating, the DC current it pulls, the cable gauge and fuse that can handle it and the battery bank to match.

01System

Battery voltage

12 V24 V48 V

Battery type Inverter efficiency Cable length, one way Length unit Max. voltage drop Days of autonomy

02AC loads

Device	Qty	Watts	Surge W	h/day

All rows are assumed to run at the same time. Remove items you will never use together for a smaller (cheaper) inverter.

Recommended inverter––

Continuous load––

Peak surge––

DC current @ full rating–size cables & fuse for this

Fuse / breaker–ANL / Class-T near battery

Cable size (copper)––

Voltage drop––

Daily energy–incl. inverter losses

Battery bank––

How to size a power inverter

An inverter converts DC battery power (12, 24 or 48 V) into household AC power (120 V / 60 Hz or 230 V / 50 Hz). Choosing one involves four linked decisions: the inverter's continuous rating, its surge rating, the DC wiring and fuse that feed it and the battery bank that supplies the energy.

1. Continuous and surge watts

Add the wattage of every device that may run at the same time and add 20 – 25 % headroom; inverters run hot and inefficiently near their limit. Then check surge: motors, compressors and some power tools need two to seven times their running power for a moment. Most quality pure sine wave inverters can deliver about twice their continuous rating for a few seconds. The calculator increases the inverter size until both conditions are met.

2. DC current – the number that sets everything else

Power is voltage times current, so a low system voltage means high current. A 2,000 W inverter at full load draws roughly 185 A from a 12 V battery (2,000 ÷ 12 ÷ 0.9), but only 93 A at 24 V and 46 A at 48 V. High current needs very thick cable, big fuses and a battery that can deliver it. That is why systems above about 2,000 – 3,000 W are usually built at 24 or 48 V.

3. Cable gauge and voltage drop

Cables must satisfy two limits: ampacity (they must not overheat) and voltage drop (the inverter must see enough voltage to avoid low-voltage shutdowns). The calculator uses the full round-trip length (positive + negative) and copper resistance at 20 °C, and checks ampacity for 105 °C insulated cable. Keep the inverter as close to the battery as possible – doubling the cable length doubles the voltage drop.

AWG	mm²	Approx. ampacity (105 °C)	Typical 12 V use
8	8.4	80 A	up to ~600 W inverter, short run
6	13.3	120 A	~1,000 W
4	21.2	160 A	~1,500 W
2	33.6	210 A	~2,000 W
1/0	53.5	285 A	~2,500 – 3,000 W
2/0 – 4/0	67 – 107	330 – 445 A	3,000 W +, or move to 24 V

4. Fuse protection

Every battery-to-inverter cable needs a fuse or DC breaker within about 7 in (18 cm) of the battery terminal. Size it at about 125 % of the inverter's maximum DC input current and never above the cable's ampacity. Lithium batteries can deliver thousands of amps into a short circuit; use a fuse with a high interrupt rating, such as Class-T, on large LiFePO4 banks.

Pure sine wave vs. modified sine wave

A pure sine wave inverter reproduces grid-quality power and works with everything: laptops, CPAP machines, microwaves, induction cooktops, variable-speed tools and appliances with motors. A modified sine wave inverter produces a stepped waveform. It is cheaper, but motors run hotter and noisier, some chargers and medical devices fail, and efficiency is lower. Today the price difference is small; buy pure sine unless you only run simple resistive loads.

Battery capacity

Daily energy is each device's watts × hours, divided by the inverter's efficiency. Lead-acid batteries should not be discharged below about 50 % to achieve a reasonable life, while LiFePO4 can use 80 – 90 % of rated capacity. The battery also has to deliver the inverter's current: a lead-acid bank should not be discharged faster than about C/4, and a lithium battery's BMS must be rated for the continuous amps. The calculator checks both and recommends the larger capacity.

Installation checklist

Mount the inverter in a dry, ventilated location as close to the battery as possible.
Use fine-strand, tinned copper cable with crimped and heat-shrunk lugs.
Install a fuse at the battery and a battery disconnect switch.
Ground the inverter chassis according to the manual (vehicle chassis or grounding bus).
Use a GFCI outlet on the AC side and never connect an inverter to a building's wiring without a proper transfer switch.
In boats and RVs follow ABYC / NFPA 1192 or your local standards.

FAQ

What size inverter do I need to run a microwave?

A typical "1,000 W" microwave draws 1,400 – 1,600 W from the outlet. Choose a pure sine inverter of at least 2,000 W. On 12 V it will pull about 140 – 150 A while cooking, so use 2 AWG or larger cable.

How long will a 100 Ah battery run a 1,000 W inverter?

A 12 V 100 Ah LiFePO4 battery stores about 1,280 Wh, of which roughly 1,150 Wh is usable. At a 1,000 W load (about 1,110 W from the battery) that is roughly one hour. A lead-acid battery of the same rating gives less than half that.

Can I connect an inverter to my car's cigarette lighter?

Only small inverters (about 150 W or less). 12 V accessory sockets are usually fused at 10 – 15 A. Anything bigger must be wired directly to the battery with a fuse.

Should I choose 12 V, 24 V or 48 V?

12 V is simplest for vans and small systems up to about 2,000 W. 24 V suits mid-size RVs and boats, and 48 V is standard for cabins, homes and systems above 3,000 W because it reduces current, cable size and losses.

Why does my inverter shut down when an appliance starts?

Either the surge exceeds the inverter's peak rating, or the voltage drops too far because the cables are too thin or too long, or the battery cannot deliver the current. Check all three with the calculator.

About InvertRight

InvertRight is an independent, free calculator for sizing 12 V, 24 V and 48 V power inverters in vans, RVs, boats, cabins and off-grid systems, including DC current, cable gauge, fuse rating and battery capacity. We are not affiliated with any inverter or battery manufacturer.

Every formula on this page runs entirely in your web browser. The numbers you type are never uploaded, stored or shared. Our goal is simple: give you an honest starting point so you can compare product specifications with confidence and avoid paying for capacity you do not need – or buying something too small to do the job.

The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.

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Last updated: October 28, 2026

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