Generator Sizer

Overview

Generators are sold on one number — "7500 watts!" — and that number is usually surge rating, not what you can run. Meanwhile the appliances that actually matter in an outage are motors: a refrigerator that runs at 150 W needs 450+ W for the two seconds its compressor starts, a ½ HP well pump that runs at 1,000 W needs 3,000 W to start. Stack two motor starts on top of your base load and a "7,500 W" generator that really delivers 6,000 W continuous trips its breaker, and the fridge never gets its turn.

scripts/generator_sizer.py does the audit nobody does: per-appliance running watts, starting-surge multipliers, a priority-tier load-shed plan, the honest generator class for your list, fuel burn per hour and per storm, and the transfer-switch / CO-safety verdicts — before the storm, not during it.

When to Use

• Buying a portable or standby generator ("will a 3,500 W run my house?").
• Outage prep for a storm: which appliances can run, in what rotation.
• Sizing a generator for a well pump, furnace blower, sump pump, or medical device (the four loads that force a class jump).
• Deciding gasoline vs propane vs tri-fuel, and how much fuel a 48-hour outage actually consumes.
• Checking whether a neighbor's/friend's generator offer covers your needs.

Don't use for: whole-house automatic standby + ATS design (needs a licensed electrician's load calculation per NEC 220), solar/battery systems, or RV electrical planning — those have different load models.

How It Works

1. Appliance catalog. Built-in table of ~30 common loads with running watts, surge multiplier, and priority tier (TIER 1 life-safety: furnace blower, sump, fridge, medical; TIER 2 comfort; TIER 3 luxury). Override or add your own wattages from nameplates.
2. Load sheet. Sums running watts; computes worst-case simultaneous surge = base load (all non-motor) + the single largest motor surge + running watts of motors already spinning. Staggered-start doctrine: you never start two compressors in the same second on purpose.
3. Class verdict. Snaps required continuous watts to the real market classes: 2,000 W inverter → 3,500 W → 7,500 W → 10–13,500 W → 22 kW standby, with what each class realistically runs (a 2,000 W inverter runs a fridge + lights + internet, period).
4. Fuel model. Gasoline burn ≈ 0.05 gal/h base + ~0.095 gal/h per delivered kW at load (so a 7,500 W unit at 50% ≈ 0.4 gal/h); propane burns ~2× by volume with a 20 lb tank (4.7 gal) factored in. Output: hours per tank and gallons needed for your storm duration at your actual load fraction — not the marketing number at 25% load.
5. Safety gates. 240 V loads (well pump, furnace on 240) demand a transfer switch or breaker interlock — backfeeding a dryer outlet is an electrocution hazard for line workers and illegal. CO doctrine: 20+ ft from any window/door/vent, never in a garage or under a deck, battery CO alarm indoors. Inverter-class needed for sensitive electronics if THD matters (CPAP, oxygen concentrator).

Using the Script

# essentials for a storm: fridge + freezer + sump + furnace blower + lights/router
python3 scripts/generator_sizer.py \
  --appliances fridge freezer sump-pump furnace-blower lights-10 router \
  --storm-hours 48

# adding a well pump (the classic class-jumper) with custom watts
python3 scripts/generator_sizer.py \
  --appliances fridge freezer well-pump furnace-blower lights-10 \
  --custom microwave:1100:1.5 --fuel propane --storm-hours 24

# what does my existing 3500W generator actually carry?
python3 scripts/generator_sizer.py --appliances fridge sump-pump lights-10 router cpap \
  --generator-watts 3500 --check-only

Outputs: tiered load sheet (running W, surge W, tier), worst-case surge event, required continuous watts, generator class verdict with the "skipped" appliances if your --generator-watts is too small, staggered start order, fuel plan (gal/h at your load fraction, hours per tank, gallons per storm), and a safety checklist with the transfer-switch flag.

Common Pitfalls

1. Sizing by surge rating. "7,500 W" units deliver ~6,000 W continuous. Size to continuous watts + one motor surge, never the box number.
2. Forgetting starting surge entirely. Motors need 2–3× for 1–3 seconds. Two motors starting together (fridge + sump after a power blip) is exactly when generators stall.
3. Assuming "240V plug = house wiring." Any connection to home wiring legally requires a transfer switch or listed interlock. Backfeed cords kill linemen and melt dryer outlets.
4. Fuel math from the manual. Runtime quotes are at 25% load. Your real load fraction doubles or triples the burn; a 48 h storm on a 7,500 W genset at 50% needs ~20 gal of gasoline stored safely.
5. Running it "just for an hour" in the garage. CO kills at low concentrations; generators run outdoors 20+ ft from openings, with a battery CO alarm indoors. More deaths come from CO than from wiring.
6. Ignoring altitude. Gasoline engines lose ~3% output per 1,000 ft above sea level — at 8,000 ft a "7,500 W" is a 6,000 W. The script takes --elevation-ft.
7. Forgetting the furnace blower is a motor. Gas furnace heat needs electricity for the blower (400–800 W, 3× surge). "I have gas heat" households discover this in the first outage.

Verification Checklist

• Every Tier-1 appliance entered with nameplate watts (not guesses).
• Largest-motor surge added to total running load for the sizing check.
• Generator class ≥ required continuous watts with ≥15% headroom.
• 240 V loads → transfer switch/interlock identified; no backfeed plan.
• Fuel plan covers storm duration at REAL load fraction + 25% reserve.
• CO safety: 20 ft clearance, outdoor only, battery CO alarm indoors.
• Elevation derate applied if above 3,000 ft.
• Dry-run test: run the generator + top-3 loads for 30 min before storm season, log voltage sag on motor starts.