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Inverter Sizing Calculator

Inverter size by array capacity and by connected load.

Your measurementsEvery assumption editable01

Deliberately oversizing the array against the inverter. 1.15–1.25 is common.

For an off-grid or backup inverter — the steady draw it must hold.

Motor loads draw several times their running watts at startup.

Inverter sizeCalculated live02

Result

6.67kW inverter

6.67 kW by array, 5.3 kW by load — take the larger

Array size
8 kW DC
DC to AC ratio
1.2 : 1
Inverter by array
6.667 kW AC
Continuous AC load
3500 W
Startup surge headroom900 W motor at 3×
+1800 W
Inverter by load
5.3 kW AC
Recommendedwithin the usual DC/AC range
6.67 kW

Oversizing the array relative to the inverter is deliberate, not a mistake. An array almost never produces its nameplate output, so a 1.2 ratio keeps the inverter working in its efficient band far more of the time. The cost is clipping — losing the very top of production on the brightest days — which is usually a smaller loss than the efficiency gained.

A grid-tied inverter is sized by the array; an off-grid or backup inverter is sized by the loads it must carry. Where a system does both, the larger of the two figures governs, which is what the result takes.

Surge matters more than continuous rating for anything with a motor. A well pump or compressor drawing three times its running watts for a second will trip an inverter sized only on steady load, however comfortable the continuous figure looks.

A grid-tied inverter is sized by the array; an off-grid or backup inverter is sized by the loads it carries. This calculates both and takes the larger, because a system doing both jobs has to satisfy both.

Why use this tool?What it does differently03

Why use this tool?

Both sizing methods

By array through the DC/AC ratio, and by load including surge. The larger governs.

Explains oversizing

A DC/AC ratio above one is deliberate — it keeps the inverter in its efficient band far more of the time.

Surge accounted for

A motor drawing three times its running watts will trip an inverter sized on steady load alone.

Clipping flagged

Warns when the ratio is high enough to lose real production on peak days.

How this worksThe method04

How this inverter sizing calculator works

Array-based sizing divides DC array capacity by the DC/AC ratio. Ratios between 1.15 and 1.25 are conventional, and the deliberate oversizing exists because an array almost never produces nameplate output.

Load-based sizing adds the continuous AC draw to the extra current the largest motor pulls while starting. That surge, not the steady figure, is what determines whether an inverter holds up when a pump or compressor kicks in.

The trade in oversizing is clipping — losing the very top of production on the brightest days. Below about 1.3 that loss is smaller than the efficiency gained from running the inverter nearer its rated output most of the time.

How to use itStep by step05

How to use it

  1. Step 1: Enter the array size

    DC nameplate capacity from the panel calculator.

  2. Step 2: Pick a DC/AC ratio

    1.15 to 1.25 for most grid-tied systems.

  3. Step 3: Add the AC loads

    For off-grid or backup, the continuous draw plus the largest motor.

  4. Step 4: Take the larger figure

    A system doing both jobs must satisfy both.

Example usageWorked figures06

Example usage

A grid-tied array
8 kW DC at a 1.2 ratio needs a 6.67 kW inverter. The array is deliberately larger than the inverter.
With backup loads
3,500 W continuous plus a 900 W motor at 3× surge is 5.3 kW by load — below the array figure, so the 6.67 kW inverter still governs.
A heavy motor load
Swap in a 2,500 W well pump at 5× surge and the load figure jumps to 13.5 kW, which now governs over the array.
Frequently asked questionsCommon questions07

Frequently asked questions

What size inverter do I need?

For grid-tied, array capacity divided by a DC/AC ratio around 1.2 — an 8 kW array wants roughly a 6.7 kW inverter. For off-grid, size on continuous load plus motor surge instead.

Why is the array bigger than the inverter?

Because an array rarely produces nameplate output. Oversizing keeps the inverter working in its efficient range far more of the time, and the clipping lost on peak days is smaller than the efficiency gained.

What is clipping?

When the array briefly produces more than the inverter can convert, the excess is lost. At ratios below about 1.3 it costs only a small fraction of annual production.

Does surge really matter?

Yes, and it is the commonest sizing failure. A well pump drawing three to five times its running watts for a second will trip an inverter that looks comfortable on continuous rating.

Can one inverter do grid-tie and backup?

Hybrid inverters do both, but they must satisfy both sizing rules — which is why this takes the larger of the two figures rather than either alone.

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