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.
Results
Enter hours for the appliances the inverter will run, then compute.
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 continuous
noncontinuous_w = running_w − continuous_running_w
nec_continuous_w = noncontinuous_w + continuous_running_w × 1.25
required_continuous_w = max(nec_continuous_w, running_w × 1.15)
required_surge_w = running_w + max(surge_w − running_w)
recommended_size_w = smallest tier ≥ required_continuous_w
dc_input_a = recommended_size_w / (battery_v × 0.90)
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).
recommended_size_w- Smallest common inverter size (300 / 600 / 1,000 / 1,500 / 2,000 / 3,000 / 5,000 W) at or above the continuous need.
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.
- Do not assume every inverter can surge to 2× its continuous rating. Published Victron peaks run from about 1.5× (Phoenix VE.Direct 1600 VA: 1,450 W continuous / 2,200 W peak) to 2.5×. If this tool flags surge as the binding constraint, read the nameplate surge watts and duration — or step up a size.
- Startup watts on motor loads are an engineering estimate (~3× running), not a DOE figure. 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 at 90% efficiency and the battery voltage you picked. Cable size still has to pass a 3% voltage-drop check on the DC run.
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. Pick the next common size that clears the continuous number, and confirm the inverter's published surge clears the peak.
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
- IAEI Magazine — Conductor sizing and overcurrent device ratings (NEC 125% continuous-load rule; stand-alone inverter 3-hour rating)
- Victron Energy — Phoenix VE.Direct 250–1600 VA datasheet (continuous vs peak watts, temperature derate)
- U.S. DOE Energy Saver — Estimating appliance and home electronic energy use
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