Inverter Size Calculator Widget
Help readers choose an inverter that will actually start their fridge. They list up to four appliance types with watts, quantity and surge factor, and the widget returns the running load, the continuous and surge ratings needed and the next common inverter size in VA.
Live preview
Under the widget on your page: Powered by A2Z Tools
Embed code
<iframe src="https://a2z.tools/embed/w/inverter-size-calculator" title="Inverter Size Calculator by A2Z Tools" width="100%" height="940" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>
A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.
<div data-a2z-widget="inverter-size-calculator" data-height="940"></div> <script async src="https://a2z.tools/embed.js"></script>
Adds a small script (what it does) that sizes the widget to fit its content, loads it lazily and keeps it isolated from your page's CSS.
Works with
How it works
Running load is the sum of every appliance's watts times its quantity. Motors and compressors draw several times their running power for a moment when they start, so each line has a surge factor (starting watts divided by running watts; 1 for lights and electronics). The widget assumes only one motor starts at a time and adds the largest single start-up excess, watts x (factor - 1), to the running load to get the surge the inverter must survive. For the continuous rating it adds the headroom you choose (25% by default) and divides by the power factor to convert watts to volt-amperes, the unit most inverters and UPS units are sold in. The suggested size is the smallest common rating at or above that VA - from 300 VA up to 50 kVA. A fridge (150 W, 3x), a TV, six 10 W lamps and two 75 W fans come to 460 W running, 760 W surge and an 800 VA unit.
Calculation method
- Running load = sum of (watts x quantity)
- Surge = running load + max over loads of watts x (surge factor - 1)
- Continuous rating = running load x (1 + headroom / 100); VA = continuous rating / power factor
- Suggested size = smallest of 300, 500, 600, 800, 1000, 1200, 1500, 2000, 2500, 3000 ... 50,000 VA that is >= VA
Worked examples
Home backup during power cuts
Inputs: Fridge 150 W x1 (3x surge), TV 100 W x1, LED lamps 10 W x6, fans 75 W x2; PF 0.8; 25% headroom
Result: 800 VA inverter; 460 W running; 575 W continuous; 760 W surge
460 x 1.25 = 575 W; / 0.8 = 718.75 VA -> 800 VA; surge 460 + 150 x 2 = 760 W.
Single 1 kW tool
Inputs: 1000 W x1; PF 0.8; 25% headroom
Result: 2000 VA inverter
1000 x 1.25 / 0.8 = 1562.5 VA; the next common size is 2000 VA.
An estimate for planning, not an electrical design. Installation, wiring and protection should be done by a qualified electrician.
Limitations
- Suggested sizes are a typical market ladder, not a specific product range; check the chosen inverter's continuous and surge figures.
- Does not size cables, fuses, breakers or the battery current drawn by the inverter.
- Power factor is one figure for the whole load; mixed motor and electronic loads vary.
Where publishers use it
- Inverter and UPS shops listing home-backup models by VA
- Off-grid cabin and van-life build guides
- Solar installers' pages explaining why a 1 kW inverter cannot run a 1 kW pump
- Electrical-trade blogs teaching watts versus VA
- Power-cut preparedness articles for homes and small offices
Questions
Why is the inverter size in VA bigger than my watts?
Inverters are limited by current, so they are rated in volt-amperes. With a power factor of 0.8, 575 W of load needs 575 / 0.8 = 719 VA, which rounds up to an 800 VA unit.
What surge factor should I use?
Use the starting watts on the appliance label or manual divided by its running watts. Compressor fridges and air conditioners often need around 3x for a fraction of a second, pumps 2-3x; resistive heaters, LED lights and electronics are 1x.
How much headroom should I add?
20-25% keeps the inverter out of continuous full-load operation, which runs hot and shortens its life. Add more if you expect to add loads later.
What if two motors start together?
The widget assumes one start at a time, which is normal when the fridge and pump cycle independently. If two can start at once, add the second one's surge excess yourself or raise its factor.
Does this tell me battery size or backup time?
No - it sizes the inverter's power. For how long a battery bank lasts, use the inverter backup or solar battery calculators.
Do I need a pure sine wave inverter?
Modified or square-wave inverters are cheaper, but motors, compressors, microwave ovens, induction hobs, laser printers and many chargers run hotter, noisier or not at all on them. A pure (true) sine output matches mains power and is the safe choice for mixed household loads.
Can an inverter run a kettle, toaster or iron?
Yes, if it is big enough: these are resistive, so the surge factor is 1 and the power factor close to 1, but a 2,000 W kettle alone needs a 2,000 W inverter plus headroom - and draws about 170 A from a 12 V battery, so the battery and cabling matter as much as the inverter.
What is inrush current?
The brief spike when a motor, transformer or large power-supply capacitor is first energised. Fridges and pumps show it as a surge lasting a fraction of a second to a few seconds; inverter datasheets give a separate surge or peak rating, often for 5 seconds or less, to cover it.
Cite or recommend this tool
If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.
A2Z Tools Inverter Size Calculator https://a2z.tools/embed/inverter-size-calculator
<a href="https://a2z.tools/embed/inverter-size-calculator">A2Z Tools Inverter Size Calculator</a>
[A2Z Tools Inverter Size Calculator](https://a2z.tools/embed/inverter-size-calculator)
Inverter Size Calculator by A2Z Tools - https://a2z.tools/embed/inverter-size-calculator
Related widgets
-
Backup time of a home inverter or UPS battery bank, or the battery Ah needed for the hours you want.
-
Battery bank kWh and Ah for days of autonomy, depth of discharge and system voltage, with battery count.
-
Generator kW and kVA from running watts and the largest motor's starting surge.
-
kVA, kVAR and phase angle from kW and power factor, plus the capacitor kVAR to reach a target.
-
Watts, volt-amps and amps for DC and single-phase AC circuits, with power factor.
-
Payback year, lifetime net savings and ROI of a solar system, with tariff rises and panel degradation.