Introduction
This case study is hypothetical and illustrative — a worked example showing how the sizing method applies to a realistic household, not a report of an actual installation. Names, locations, and circumstances are fictional; the math follows the standard method used throughout this guide series.
Meet a fictional household — the Millers, a family of four in a two-story suburban home with municipal water, a gas furnace, and a basement sump pump. After a 30-hour outage last storm season left them with a spoiled freezer and a nervous night watching the sump, they decided to size a generator properly. Here’s their process, start to finish. To run your own numbers, use the Generator Size Calculator.
The Situation
The Millers’ outage priorities, decided in advance: keep food safe (kitchen fridge + garage freezer), keep the basement dry (sump pump — their street floods in heavy rain), keep heat running in winter (gas furnace needs its fan and controls), keep a few lights and phones charged, and be able to heat food (microwave, used sparingly). Central AC, dryer, dishwasher, and EV charging were explicitly ruled out — Tier 3 loads for outage purposes, following the prioritization method in the estimating guide.
The Load Inventory
They walked the house with the nameplate method and built this list:
| Appliance | Running W | Starting W |
|---|---|---|
| Kitchen refrigerator (nameplate 5.8 A @ 120 V) | 700 | 2,200 |
| Garage freezer (older, nameplate 4.2 A @ 120 V) | 500 | 1,500 |
| Gas furnace fan + controls | 600 | 1,400 |
| Sump pump (nameplate 8.3 A @ 120 V) | 1,000 | 2,000 |
| Microwave | 1,100 | 1,100 |
| 12 LED lights + chargers + router | 250 | 250 |
| Total running | 4,150 |
Two things surprised them: the old garage freezer drew more than expected (a good catch — it was also a candidate for replacement), and the “small stuff” — lights, chargers, router — added up to a non-trivial 250 W. Both are classic inventory lessons from the common mistakes guide.
The Calculation
Step 1 — running total: 4,150 W.
Step 2 — largest surge: the refrigerator’s 2,200 W starting figure. Extra surge = 2,200 − 700 = 1,500 W. Subtotal = 4,150 + 1,500 = 5,650 W.
Step 3 — 20% headroom: 5,650 × 1.20 = 6,780 W → ~6.8 kW.
Had they ignored surge, they’d have shopped for ~5 kW and stalled on the first fridge startup. Had they added every surge, they’d have demanded ~9.5 kW and overspent by thousands. The disciplined middle — the method in the formula guide — landed at 6.8 kW.
The Decision
A 6.8 kW requirement sits at the top of the portable band, so they shopped 7–7.5 kW units. Their choice framework, drawn from the fuel and equipment guide:
- Generator: a 7.5 kW dual-fuel portable — gasoline for convenience, propane for the indefinite storage that suited their “use it twice a year” reality.
- Connection: an electrician-installed manual transfer switch with an outdoor inlet box (~$900 installed in their fictional market), so the hardwired furnace fan and sump pump — the two loads that justified the whole project — could run without extension cords through windows. This middle path is detailed in the DIY vs. professional guide.
- Fuel plan: two 20-lb propane tanks (stored indefinitely) plus 10 gallons of stabilized gasoline rotated through the car — enough for roughly 48 hours at their expected load, per their burn-rate math.
- Safety: battery-backed CO detectors on both levels, generator placement marked with paint 20 ft (6 m) from the house, exhaust direction noted in their written outage plan.
Total fictional project cost landed around $2,200 — generator, transfer switch install, cords, detectors, and fuel storage — versus $9,000+ quotes they’d received for standby systems they didn’t need. The cost guide puts these bands in context.
Lessons Worth Stealing
- Decide the tiers before listing loads. Ruling out central AC and the dryer is what kept this a $2,200 portable project instead of a $9,000 standby project.
- Read nameplates, don’t trust memory. The garage freezer surprise changed their number by ~500 W.
- The hardwired loads drove the connection decision. Furnace fan + sump pump justified the transfer switch; without them, cords would have sufficed.
- Fuel math is part of sizing. A generator with 12 hours of fuel is a 12-hour plan — they sized the fuel stock to a 48-hour target.
- Write the plan down. Startup order (fridge, wait, sump, wait, furnace), refueling schedule, and placement were taped inside a kitchen cabinet — because nobody thinks clearly at 2 a.m. in a blackout.
What-If Variations
The Millers’ answer changes if the assumptions change — which is exactly why the method matters more than the number. What if they’d insisted on central AC? Their 3-ton unit drew ~3,500 W running with ~9,000 W starting; adding it would have pushed the requirement past 12 kW — out of portable territory entirely and into a standby installation at roughly four times the project cost. Ruling it out was a $7,000 decision. What if they’d skipped the transfer switch? They’d have saved ~$900 but lost the furnace fan and sump pump — the two hardwired loads that motivated the project — reducing the system to a cord-based setup that couldn’t do its core job. What if the garage freezer dies? Their inventory method means recalculating takes five minutes: drop 500 W running and its 1,500 W surge, and the requirement falls to ~6.2 kW. A good inventory is a living document, not a one-time chore.
Related Guides
- Complete Guide — the full sizing walkthrough with appliance tables.
- Estimating Guide — build your own load inventory.
- Formula Explained — the math behind this case study.
- Cost Guide — typical price bands for each approach.
- Pre-Storm Checklist — readiness list for your own household.
Get Your Number
Reminder: this case study is hypothetical and illustrative. Your household’s loads, priorities, and costs will differ — but the method transfers exactly. Build your inventory, run it through the Generator Size Calculator, and make your decisions from your own numbers.
Disclaimer: Calculator results are planning estimates only. Always verify wattage against your equipment nameplates; actual loads vary by model and condition. All figures in this case study are illustrative. Generator connections to home wiring and transfer equipment must be installed by a licensed electrician in compliance with the National Electrical Code and local code. Never operate a generator indoors or in enclosed spaces.
