Inverter Size Calculator

To find an inverter's DC input current, divide the AC load in watts by the inverter's efficiency, then divide by battery voltage. A 1,000 W load on a 90%-efficient inverter draws 1,000 ÷ 0.90 = 1,111 input watts; at 12V that is 1,111 ÷ 12 = about 92.6 amps from the battery. That single number decides your cable gauge, fuse rating, and whether your battery bank can source the current without its voltage collapsing.

This is why battery voltage matters so much. Running the same 1,000 W load at 48V instead of 12V drops the DC current to roughly 23 A — one-quarter the current, one-quarter the cable copper, and far smaller fuses. High-current 12V systems need short, fat cables and beefy fuses; higher-voltage banks make wiring cheaper and safer.

The calculator uses two familiar power relationships:

The first step accounts for the energy the inverter itself wastes as heat. A 90% inverter must pull 1,111 W from the battery to deliver 1,000 W of AC — the extra 111 W is conversion loss. The second step is Ohm's power law rearranged (I = P ÷ V): once you know DC watts and DC voltage, current follows directly.

The fuse recommendation applies the NEC continuous-load rule of 125%: a circuit carrying current for three hours or more should be protected at 125% of that current. So a 92.6 A draw calls for roughly a 115–125 A fuse or breaker, and cable rated to carry it. Always fuse the positive lead as close to the battery as practical to protect against a dead short.

To go the other direction — from DC amps back to AC watts, or to plan a whole bank — pair this with the battery runtime calculator and the power consumption calculator.

Example 1 — 1,000 W load, 90% inverter, 12V: Input watts = 1,000 ÷ 0.90 = 1,111 W. DC current = 1,111 ÷ 12 = 92.6 A. Fuse at 92.6 × 1.25 ≈ 116 A → a 125 A fuse with 2 AWG cable on a short run.

Example 2 — 2,000 W load, 92% inverter, 24V: Input watts = 2,000 ÷ 0.92 = 2,174 W. DC current = 2,174 ÷ 24 = 90.6 A. Notice the current is nearly the same as Example 1 despite double the load — because voltage doubled. Fuse ≈ 113 A.

Example 3 — 3,000 W load, 95% inverter, 48V: Input watts = 3,000 ÷ 0.95 = 3,158 W. DC current = 3,158 ÷ 48 = 65.8 A. A 3,000 W load moves only 66 A at 48V — this is why large systems standardize on 48V. Fuse ≈ 82 A.

Example 4 — 4,000 W surge on a 12V bank: During start-up the same divide-by-voltage math applies: 4,000 ÷ 0.90 ÷ 12 ≈ 370 A momentarily. Your battery's continuous and peak discharge ratings, plus the BMS current limit on a lithium bank, must allow this brief spike.

Inverter efficiency is the ratio of AC watts out to DC watts in, and it is never 100%. Typical figures:

• Peak efficiency 90–96% for quality pure sine inverters at moderate load (30–80% of rating).
• Lower efficiency at very light loads because of fixed overhead: the inverter's own electronics, display, and cooling fans draw idle power whether you use 5 W or 500 W. A 3,000 W inverter idling to power a 10 W phone charger can be under 50% efficient.
• Lower efficiency near maximum load as resistive (I²R) losses in the transformer and MOSFETs rise with current.

The practical lesson: match inverter size to load. An oversized inverter wastes energy idling; a maxed-out inverter runs hot and inefficient. Enter a realistic efficiency — 90% is a safe default, 92–95% for premium units, 85% for budget or modified-sine models. The no-load (idle) draw matters for off-grid systems that stay on all day: even 20–40 W of idle consumption adds up to 0.5–1 kWh per day, which your battery must supply. When idle loss dominates, some systems add a low-power "eco mode" that sleeps the inverter until a load appears.

The DC input current from this calculator directly sets your cable gauge and overcurrent protection. High-current DC is unforgiving: voltage drop and heat both scale with current, and a short circuit on an unfused battery lead can deliver thousands of amps.

Use these principles:

• Fuse at 125% of continuous DC current, then round to the next standard fuse size. Place the fuse within 7 inches of the battery positive terminal per ABYC practice.
• Size cable for the fuse, not just the load, so the wire can carry fault current until the fuse blows. Keep runs short and voltage drop under 2–3%.
• Match battery discharge rating. A lithium BMS or lead-acid bank must support the continuous and surge amps. A 100 A continuous draw on a single 100Ah battery is a 1C rate — fine for lithium, hard on lead-acid.

DC current tells you the rate the battery discharges; amp-hours tell you how long it can sustain that rate. To size the bank, decide how many hours of runtime you need and work backward.

If a 1,000 W load pulls 92.6 A at 12V, one hour of runtime consumes about 92.6 Ah. Because you should not fully drain a battery, divide by usable depth of discharge: at 80% DoD you need 92.6 ÷ 0.80 ≈ 116 Ah of rated lithium capacity per hour, or roughly 185 Ah of lead-acid at 50% DoD. For three hours of runtime, triple it.

Higher-voltage banks reduce the amp-hour figure proportionally, which is another argument for 24V or 48V on larger systems. For a precise runtime that layers in depth of discharge and efficiency, use the inverter runtime calculator or the battery runtime calculator. To pick the inverter's wattage in the first place, start with the what size inverter do I need calculator.

System voltage is one of the most consequential early decisions, because it sets the DC current for every watt you draw and therefore the cost and safety of your wiring. The relationship is simple: for a fixed power, current is inversely proportional to voltage, so doubling the voltage halves the current.

• 12V suits small, simple systems — a single battery, a modest inverter under about 1,500 W, RVs and camper vans, and portable setups. It is the most common voltage for off-the-shelf 12V appliances, but high-power loads push current into the hundreds of amps, demanding very thick, short cable.
• 24V is the sweet spot for mid-size systems (1,500–3,000 W). Halving the current versus 12V shrinks cable and fuse sizes and reduces voltage drop, while 24V components remain widely available.
• 48V is standard for whole-home, large solar, and off-grid systems above about 3,000 W. At one-quarter the current of 12V, cable runs are dramatically smaller and cheaper, and 48V is the native voltage of most modern hybrid inverter/chargers.

A practical rule: keep continuous DC current under roughly 150–200 A wherever possible. If your load pushes past that at 12V, step up to 24V or 48V rather than fighting massive cable. The tables above show exactly how the amps fall as voltage rises — a 3,000 W load moves 278 A at 12V but only 69 A at 48V, a four-fold reduction in copper and connector stress.

1. Enter your AC load in watts. Use the running watts of everything on at once. Convert amps to watts first if needed.
2. Enter inverter efficiency. Use 90% as a default, 92–95% for premium pure sine, 85% for budget or modified sine.
3. Select battery voltage. 12V for small systems, 24V mid-size, 48V for large off-grid and whole-home banks.
4. Read DC input current. This is the amps your battery and cables must carry continuously.
5. Use the fuse and inverter recommendations to spec overcurrent protection and confirm your inverter has continuous headroom.

• Ignoring efficiency. Dividing watts by voltage alone understates DC current by 5–15%; always divide by efficiency first.
• Fusing to the load, not 125%. Continuous loads need the 25% margin or the fuse nuisance-trips.
• Undersized cable. Thin wire on a high-current 12V run overheats and drops voltage, starving the inverter.
• Battery cannot source the current. A small lead-acid bank sags under a big inverter's draw, tripping low-voltage cutoff.
• No fuse at the battery. An unprotected positive lead is a fire risk; fuse it within inches of the terminal.

The through-line in all of these is that DC current, not AC watts, drives the hardware. Your inverter may be rated in AC watts, but the battery, cables, terminals, busbars, and fuses all live on the DC side, where current can be five to ten times higher than the AC amps you are used to at the wall outlet. A 3,000 W inverter on a 12V bank moves nearly 280 A continuously — enough to weld with — so every DC connection must be torqued to spec, corrosion-free, and sized for both continuous and surge current. When in doubt, step the system voltage up to 24V or 48V to cut that current, or shorten and thicken the DC run. Verify the final numbers against your inverter's own installation manual, which lists the manufacturer's required cable gauge and fuse rating for each model.