Generator Size Calculator Widget
Put a generator sizing check on a dealer's product page or a storm-preparedness guide. Readers enter the running watts of everything they want to power and the biggest motor among them, and get the running and starting watts, kW and kVA and a suggested generator size.
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<iframe src="https://a2z.tools/embed/w/generator-size-calculator" title="Generator Size Calculator by A2Z Tools" width="100%" height="700" 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="generator-size-calculator" data-height="700"></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
Generators must cover two things: the steady running load, and the brief surge when a motor starts. The widget adds a headroom percentage to the total running watts for the first. For the second, it assumes the largest motor starts while everything else is already running, so the peak is the total running watts plus that motor's running watts times its starting multiplier minus one. Generac's sizing guide puts motor starting wattage at often two to three times the running figure, and notes a 3-ton central air conditioner can demand several times more; the presets use 3x for air conditioners, fridges and well pumps and 2x for sump pumps and tools, and a custom value can be entered from the nameplate. kW is watts / 1000 and kVA is kW divided by the power factor, 0.8 being the usual rating basis for generator sets. The suggestion is the smallest common size covering both the running and starting needs: 4,000 W including a 1,500 W air conditioner needs 4,400 W running and 7,000 W starting, so 7.5 kW.
Calculation method
- Running requirement = total running watts x (1 + headroom / 100)
- Starting requirement = total running watts + largest motor running watts x (multiplier - 1)
- kW = W / 1000; kVA = kW / power factor (0.8 default)
- Suggested = smallest of 2, 3, 3.5, 4, 5, 6, 7.5, 8, 10, 12, 13, 15, 17, 20, 22, 24, 26, 30 ... 500 kW that covers both requirements
Worked examples
Home essentials with central AC
Inputs: 4,000 W running in total, including a 1,500 W air conditioner at 3x; 10% headroom; PF 0.8
Result: Suggested 7.5 kW; 4,400 W running (5.5 kVA); 7,000 W starting
4,000 x 1.10 = 4,400; 4,000 + 1,500 x 2 = 7,000; smallest common size >= 7 kW is 7.5.
Workshop, no motor surge
Inputs: 9,000 W of heaters and lights; no motor; no headroom
Result: Suggested 10 kW
9 kW running and starting; next size up is 10 kW.
Hard-starting pump
Inputs: 3,000 W total including a 2,000 W pump at 5x; no headroom
Result: Suggested 12 kW
3,000 + 2,000 x 4 = 11,000 W starting.
An estimate for planning, not a substitute for an electrician's load calculation.
Limitations
- Assumes one motor starts at a time; simultaneous starts need a larger set or a soft starter.
- Starting multipliers are typical values; the motor's locked-rotor data or the appliance maker's figure is better.
- Altitude and high temperature derate generator output; check the maker's derating table.
Where publishers use it
- Generator dealers' product and buying-guide pages
- Hurricane, storm and power-cut preparedness checklists
- RV, camping and job-site power guides
- Small-business continuity planning for shops and clinics
- Electricians' sites explaining kVA, kW and starting surge
Questions
What is the difference between running and starting watts?
Running watts keep an appliance going; starting (surge) watts are the short burst a motor needs to get moving. Generac describes the start as often two to three times the running figure, and a 3-ton central air conditioner running at 3.5-5 kW can demand up to 15-18 kW to start.
Why does the suggestion cover the starting watts?
It is a conservative choice so the rated output can carry the start. Many portable generators publish a separate surge rating above their running rating; if that surge rating exceeds the starting requirement, a smaller set may work.
What is the difference between kW and kVA?
kW is real power; kVA is apparent power, which includes the current that reactive loads such as motors draw. Generator sets are commonly rated at a 0.8 power factor, so a 10 kW set is 12.5 kVA.
How do I find my appliances' watts?
Read the nameplate or manual. If it shows amps, multiply by the voltage: a 120 V appliance drawing 10 A uses about 1,200 W; a 230 V one drawing 5 A about 1,150 W.
Can I run the whole house on a portable generator?
Usually only selected circuits. Connect through a transfer switch or interlock installed by an electrician - never back-feed a house through a socket, which endangers utility workers.
Sources
- What Size Generator Do I Need? Sizing Guide - Generac . Starting (surge) wattage is often two to three times the running figure; a 3-ton central AC at 3.5-5 kW running can demand up to 15-18 kW to start.
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 Generator Size Calculator https://a2z.tools/embed/generator-size-calculator
<a href="https://a2z.tools/embed/generator-size-calculator">A2Z Tools Generator Size Calculator</a>
[A2Z Tools Generator Size Calculator](https://a2z.tools/embed/generator-size-calculator)
Generator Size Calculator by A2Z Tools - https://a2z.tools/embed/generator-size-calculator
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