Inverter Size Calculator

Find what size inverter you need from the appliances you will run — continuous watts, startup surge, and the DC current your battery bank must supply.

The formula, every assumption, and the primary sources behind this tool are published on this page.

Input mode

Inverter efficiency (0.7–1)

Battery voltageThe DC bus the inverter will draw from. 12 V is typical for vans and boats; home off-grid systems are usually 24 V or 48 V.

Reset

Results

Enter Hours/day above 0 for at least one appliance, then compute. Checking a continuous-run box alone does not add an appliance.

How it works

An inverter has to cover two numbers: the watts everything draws while running, and the brief surge when a motor starts. Loads you mark as running three hours or more at a stretch get a 125% factor from the National Electrical Code; a 15% floor covers nameplate derating in heat.

continuous_running_w = Σ running_w where marked continuousnoncontinuous_w = running_w − continuous_running_wnec_continuous_w = noncontinuous_w + continuous_running_w × 1.25required_continuous_w = max(nec_continuous_w, running_w × 1.15)required_surge_w = running_w + max(surge_w − running_w)surge_required_w = required_surge_w / 1.5recommended_size_w = smallest tier ≥ max(required_continuous_w, surge_required_w)dc_input_a = recommended_size_w / (battery_v × inverter_efficiency)

running_w: Sum of running watts for every selected appliance, if they are on together.
continuous_running_w: Running watts of loads you marked as 3+ hours at a stretch — NEC / NFPA 70's definition of a continuous load. Daily hours are not used for this call.
noncontinuous_w: Running watts of selected loads not marked continuous, taken at 100%.
nec_continuous_w: NEC mix: noncontinuous watts + 125% of continuous watts (210.19(A)(1) / 210.20(A)).
required_continuous_w: Continuous rating to buy: the larger of the NEC mix and 115% of the running total.
required_surge_w: Running total plus the single largest startup increment (same peak_w figure as the appliance-wattage and power-station-size tools).
surge_required_w: Continuous rating needed just to start the load: surge ÷ 1.5, the bottom of the published peak band. Sizing has to clear this too, or the inverter carries the running load but never spins the motor up.
recommended_size_w: Smallest common inverter size (300 / 600 / 1,000 / 1,500 / 2,000 / 3,000 / 5,000 W) at or above BOTH the continuous need and the rating needed for startup.
dc_input_a: Battery current at the recommended size: size_w ÷ (battery_v × 0.90). Used to pick DC cable later.

Assumptions

The 125% factor comes from NEC 210.19(A)(1) and 210.20(A): noncontinuous load plus 125% of continuous load. NEC Article 100 / NFPA 70 calls a load continuous when the maximum current is expected to continue for three hours or more at a stretch, and NEC 690.8(A)(3) rates a stand-alone inverter at the power it can deliver for three hours or more. The 125% rule is written for conductors and overcurrent devices; applying it to inverter watts is the same headroom convention, not a listing of inverter models.
Hours/day is only “I run this.” Three one-hour sessions do not make a continuous load. A refrigerator that stays plugged in still cycles, so plugged-in time is not maximum current for three hours. Check the stretch box only if the load itself runs at full draw for 3+ hours without a break.
The 1.15 floor covers temperature derate on the inverter's own nameplate. Victron Phoenix continuous ratings drop 8–17% from 25 °C to 40 °C, then 1.25% per °C above 40 °C. When every load is short-run the NEC factor is 1.0×, so the floor is what keeps you off a bare-rated inverter. A sealed van or engine bay still needs extra.
Published inverter peaks run from about 1.5× the continuous rating (Victron Phoenix VE.Direct 1600 VA: 1,450 W continuous / 2,200 W peak) up to 2.5×. Because the bottom of that band is what you might actually receive, sizing here assumes only 1.5× — so the recommended size still starts the load on the weakest unit in the band. Assuming 2× would put a refrigerator's 2,175 W inrush on a 1,000 W inverter that peaks to 1,500 W. Nameplate peaks also come with a duration limit; read it.
Startup watts on motor loads are an engineering estimate (~3× running), not a published figure from the appliance source. Exact inrush varies by model. Resistive and electronic loads use running = surge.
Peak wattage assumes only one motor starts at a time — a standard inverter-sizing heuristic. If two compressors can start together, add both surges.
DC input current uses the recommended inverter size and your inverter efficiency (0.90 default). Cable size must still satisfy voltage drop and ampacity.

FAQ

What size inverter do I need?

Add the running watts of everything that will be on together. Mark a load continuous only if it stays on for three hours or more at a stretch — those watts are multiplied by 1.25. A 15% floor covers heat. Then check startup: a refrigerator that runs at 725 W can surge near 2,175 W, which needs a 1,450 W continuous rating to cover at the bottom of the published peak band. Pick the next common size that clears both numbers — for that fridge it is 1,500 W, not the 1,000 W the running watts alone suggest.

Why not just multiply everything by 1.2?

A flat 1.2× is a vendor rule of thumb with no published basis. The Code's 125% factor only applies to loads that run three hours or more at a stretch, and inverter nameplates already derate in heat. This tool applies 1.25× to the loads you mark continuous, then takes the larger of that result and 1.15× the whole running total.

Does a bigger continuous rating always start a motor?

No. Surge capability is a separate rating, often 1.5–2.5× continuous and only for a second or two. A 1,000 W inverter that only peaks at 1,500 W will not start a 2,175 W fridge compressor even though 725 W running looks fine. Read the surge line on the spec sheet.

Sources

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