Off-Grid Inverter Sizing Calculator

Frequently Asked Questions

How do I size an off-grid inverter?

Add the continuous wattage of everything that can run simultaneously and pick an inverter rated at or above that sum, with at least 20% headroom. A house running a 1,200 W AC, 200 W fridge, and 400 W of lights and electronics needs ≥ 2,160 W continuous.

What about surge from motor loads?

Compressors and pumps draw 3–7× their running watts for a fraction of a second on start-up. A 1,000 W well pump can spike to 3,000–5,000 W, so the inverter's surge rating (usually 2× continuous for 5 s) must cover it.

Is bigger always better?

No - oversized inverters have higher idle (tare) losses, sometimes 20–60 W just running with no load. That can add up to 0.5–1.4 kWh/day, which matters on a small off-grid bank.

Pure sine vs modified sine - does it matter?

Pure-sine is required for variable-speed motors, microwaves, induction cooktops, CPAPs, and most modern electronics. Modified-sine units are cheaper but can hum, run hot, or fail to start sensitive loads.

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Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

An inverter converts battery-bank DC into household AC, and it has two ratings you must satisfy: continuous watts (what it can deliver indefinitely) and surge watts (what it can deliver for a few seconds to start motors). A 1,200 W refrigerator pulls only its 120 W running load most of the time, but its compressor draws 3–7× running watts for the fraction of a second the motor accelerates from stop. Under-sizing the surge rating means the inverter trips on every fridge cycle. This tool sums your steady resistive load (lights, electronics, heaters) and your motor / compressor load, applies a 3× surge factor to the motor portion, and recommends a continuous size with user-set headroom (default 25%).

Formula / Method

Continuous watts = resistive W + motor W (running). Surge watts = resistive W + (motor W × 3) - the 3× factor is a conservative average across modern compressor and induction-motor loads; soft-start units can be as low as 1.5× while old single-phase induction motors approach 6×. Recommended inverter continuous rating = continuous × (1 + headroom %). Always round up to the next standard size (300, 600, 1000, 1500, 2000, 3000, 4000, 5000, 6000, 8000 W…). A quality inverter will list its surge rating separately - confirm it exceeds your calculated surge value, not just the continuous.

Worked Example

1,200 W resistive (toaster, lights, electronics) plus 800 W of motor loads (fridge compressor + well pump running). Continuous = 1,200 + 800 = 2,000 W. Surge = 1,200 + (800 × 3) = 3,600 W. With 25% headroom on continuous: 2,000 × 1.25 = 2,500 W - pick the next standard size, a 3,000 W inverter. Confirm its surge rating ≥ 3,600 W (most 3 kW pure-sine inverters surge to 6 kW for 5 seconds, so it fits). If your inverter is rated only 3,500 W surge, you must either upgrade to a 4 kW model, or add a soft-start to the well pump to drop its inrush.

Common Mistakes

Sizing only for continuous and ignoring surge - the inverter trips every time the AC compressor cycles.

Using nameplate watts for motors - nameplates list running watts; surge is 3–6× that.

Choosing a modified-sine inverter for sensitive electronics - furnaces, variable-speed pumps, and some chargers buzz, run hot, or fail on modified-sine output.

Forgetting DC input cable sizing - a 3 kW inverter on a 24 V bank pulls 125 A; you need 2 AWG or larger and an appropriately sized fuse.

Stacking too many loads - an inverter can pass surge for the fridge OR the pump, but if both start in the same second, the surge sum often trips it.

Tips & FAQ

Pure sine or modified sine? Always pure sine for a house. Modified sine is only acceptable for resistive loads (heaters, incandescent lights) and simple universal-motor tools. What headroom? 20–25% is standard; 30–40% if you expect load growth or plan to add an EV charger or heat pump. Low-frequency vs. high-frequency: low-frequency (toroidal transformer) inverters surge harder and longer - better for well pumps and welders. High-frequency inverters are lighter and cheaper, fine for normal household loads. 48 V over 12 V: at 5 kW continuous, a 48 V bus draws 104 A versus 417 A at 12 V - the cables, fuses, and disconnects shrink dramatically. Idle draw: inverters consume 15–50 W just being on. A 30 W idle draw is 720 Wh/day - meaningful on a small off-grid system. Many inverters offer a power-save / search mode. Stacking: two inverters can be stacked in parallel (more capacity) or in series (split-phase 120/240 V) when the brand supports it - useful for growing systems. Don't buy by price alone: a $200 no-brand 3 kW inverter probably delivers 1.5 kW in practice. Stick with Victron, Schneider, Outback, Magnum, and similar tier-one brands. Always verify against the inverter datasheet; this tool is a planning estimate.

Off-Grid Inverter Sizing Calculator

Frequently Asked Questions

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