Plan standby power using real appliance running watts. Account for surges and diversity for accuracy. Export results, compare sizes, and pick confidently today power.
Use this estimator for planning only. Always confirm final sizing with a licensed electrician and local requirements.
| Load | Running watts | Qty | Duty (%) | Surge (×) |
|---|---|---|---|---|
| Central AC (compressor + blower) | 3500 | 1 | 70 | 4.5 |
| Refrigerator | 700 | 1 | 60 | 3.0 |
| Well pump | 1000 | 1 | 30 | 4.0 |
| Lighting (whole home) | 800 | 1 | 80 | 1.0 |
| Kitchen outlets / small appliances | 1200 | 1 | 40 | 1.5 |
1) Diversified running load
For each load group: Running_div(W) = Running_watts × Quantity × (Duty% / 100). Total diversified running is the sum across all load groups.
2) Largest additional surge
For each load group: Starting(W) = (Running_watts × Quantity) × Surge_multiplier. Extra surge is max(0, Starting − Running_full). The calculator uses the maximum extra surge across loads.
3) Peak demand and headroom
Peak(W) = Total_running_div + Max_extra_surge, then Peak_with_headroom = Peak × (1 + Headroom%/100).
4) Derating adjustment
A simplified model is applied: altitude derate ≈ 3% per 300 m, temperature derate ≈ 1% per 5°C above 25°C, capped at 30%. Required watts are inflated by 1 / derate_factor.
A practical sizing worksheet starts with a complete load inventory. Separate continuous essentials (router, lighting, fridge) from discretionary loads (dryer, oven). Typical essential baselines range from 600–1,800 W, while large intermittent equipment can dominate the peak. Record nameplate watts or measured values, then group identical items with quantity to reduce entry errors.
Adding every watt usually overstates real simultaneous demand. Diversity is modeled by multiplying each load group by a duty percentage that reflects expected overlap. For example, 800 W of lighting at 80% contributes 640 W to diversified running. When duty is applied across many loads, diversified totals often land 20–45% below nameplate sums for lived-in homes.
Motors and compressors briefly draw higher current at start. A refrigerator might use a 2–3× multiplier, while pumps and HVAC compressors may require 3–6×. This calculator treats surge as a worst-case for a load group and adds only the largest “extra surge” to the diversified running total. If two large motors can start together, increase headroom or model them as one combined load.
Engine-driven generators produce less power in thin or hot air. A simple planning rule is about 3% reduction per 300 m elevation and about 1% per 5°C above 25°C, capped here at 30%. At 1,500 m and 40°C, the combined reduction is roughly 20%, meaning a 18 kW requirement may need about 22.5 kW installed.
Most residential equipment is rated in watts, but generator catalogs list kW and sometimes kVA. kW is real power; kVA accounts for power factor. With PF 0.90, 20 kW corresponds to about 22.22 kVA. Current estimates help sanity-check service limits: divide required watts by 240 V for approximate amperes.
After headroom and derating, round up to a common generator class to avoid operating at maximum output. Many whole-home installations fall near 14–26 kW, but electric cooking and water heating can push higher. Verify with transfer switch ratings, fuel supply, and a licensed electrician’s load calculation before purchase. Also consider soft-start kits for HVAC to cut surge and reduce required standby capacity during outages significantly.
1) What does the recommended kW represent?
It is the calculated minimum continuous rating after adding the largest surge, your headroom percentage, and the derating adjustment. Choose the suggested standard size at or above this value.
2) Why is duty percentage included?
Duty models how often a load is expected to run during an outage. It prevents over-sizing from assuming every appliance runs at full power simultaneously, while still capturing realistic overlap.
3) How should I set surge multipliers?
Use 1.0× for heaters and electronics, 2–3× for refrigerators, 3–5× for pumps, and 3–6× for HVAC compressors. If you know LRA or starting watts, convert them to a multiplier.
4) Do I include cooking and water heating loads?
For whole-home coverage, include any electric range, oven, or water heater that may operate. For essential-only backup, exclude them and focus on HVAC, pumps, refrigeration, lighting, and outlets.
5) What if two motors can start together?
This model adds only the single largest extra surge. If simultaneous starts are likely, increase headroom, add a combined motor row, or use soft-start devices to reduce peak starting demand.
6) Is the kVA value required for selection?
Many residential generators are specified in kW. kVA helps when comparing alternator ratings or mixed inductive loads. Enter a realistic power factor so the kVA estimate aligns with your equipment mix.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.