Generator Sizing
How to Size a Generator for Your Home (Simple Load-Based Guide)
Most homeowners overspend on backup power—or end up with a generator that can’t handle their needs—because they guess instead of calculate. Generator sizing isn’t about buying the biggest unit you can afford; it’s about understanding what your home actually needs to run during an outage.
This guide walks you through a simple, load-based method using real appliance wattage, startup loads, and a practical calculator—so you can choose a system that’s reliable, efficient, and right for your home.
About this guide: Backup Power Explained publishes practical, homeowner-focused guides to help people choose safe, code-aware backup power solutions without overspending.
- Which generator size class fits your essentials (2–4k, 5–9k, or 10k+ standby)
- Whether startup surge wattage could trip you up
- What to buy next (portable vs standby) based on your actual loads
Why Generator Size Matters
Undersized
- Tripping breakers
- Failed startups (especially motors)
- Frustrating “it should work” outages
Oversized
- Wasted money up front
- Fuel inefficiency (lightly loaded generators burn poorly)
- Bigger, louder equipment than you need
Key idea: sizing is about loads (what you actually run), not square footage. Two homes the same size can need totally different generator capacity.
Step 1 — Identify Your Essential Loads
Start by separating what you must run during an outage from what’s merely convenient. This one step prevents most overspending.
Must-run
- Refrigerator / freezer
- Furnace blower (or boiler controls)
- Sump pump
- Well pump (if applicable)
- Medical devices
- Modem/router (communications)
Nice-to-have
- Microwave / coffee maker
- TV / entertainment
- Extra lighting circuits
- Non-essential outlets
- Large electric cooking / dryer (often not practical)
Tip: If you’re using an interlock/transfer switch, you can choose to power only your essential breakers. That’s how most homeowners keep generator size (and cost) reasonable.
Step 2 — Running Watts vs Startup Watts
Generator sizing requires two numbers: continuous power and surge power. Motors are the big reason.
Running watts (continuous)
- The steady power a device uses once running
- Think “normal operating load”
Startup watts (surge)
- The short burst needed to start motors
- Most common with fridges, pumps, blowers, compressors
Why motors matter: a generator can run a load fine but fail at startup if the surge demand exceeds capacity. That’s why you size for worst-case startup, not average use.
Home Backup Power Load Calculator
Enter only the appliances you must run during an outage. A good calculator accounts for both running and startup wattage to estimate a safe generator size.
Calculator: Open the Load Calculator →
Use the calculator to total your running watts and identify your biggest startup surge. Then come back here to choose the correct generator category with headroom.
Safety note: Generator sizing and installation should follow local electrical codes. Always verify requirements with your local authority or a licensed electrician before installation.
Step 3 — Interpreting Your Results
- Total running watts = what your generator must support continuously
- Startup (surge) watts = the peak needed when motors kick on
- Headroom matters = you want margin so the generator isn’t pinned at 100%
Rule of thumb: size for the worst-case startup moment (for example, sump pump + fridge surge overlapping), then add practical headroom so the generator runs smoother and more reliably.
What Size Generator Do You Need?
Small Portable Generators (2,000–4,000W)
- Fridge + a few lights + router
- Minimal loads (careful with motor startup overlap)
- Best for “keep food cold + basic comfort”
Large Portable Generators (5,000–9,000W)
- Furnace blower
- Sump pump
- Multiple appliances / circuits
- Common “sweet spot” for many homes
Standby Generators (10,000W+)
- Whole-home or near-whole-home backup
- Automatic transfer switch
- Best for frequent outages or critical needs
Reality check: Whole-home electric cooking, dryers, and central A/C can push sizing higher fast. Many homeowners choose an “essentials panel” approach instead of powering everything.
Recommended Picks by Size (Trusted Options)
How these recommendations are chosen: Picks are based on real outage scenarios, startup surge behavior, noise/fuel tradeoffs, and homeowner usability — not marketing hype. Use your calculator result to pick the correct size class first.
2k–4k Inverter (Quiet + Efficient)
Best if your plan is “keep food cold + lights + internet” and you’re careful about motor startups overlapping. Ideal for apartments, smaller homes, or minimal outage loads.
- Works well for: fridge/freezer, lights, router, TV, small kitchen loads
- Watch out for: sump/well pump surges, multiple motor startups at once
- Why this class: quiet, fuel efficient, stable power (good for electronics)
5k–9k Dual-Fuel 120/240V (Most Homes)
The most common “right-sized” category for homes running real essentials: furnace blower + sump pump + fridge + lighting/outlets (with headroom).
- Works well for: furnace/boiler controls, sump pump, fridge/freezer, multiple circuits
- Why 120/240V matters: supports 240V loads (well pumps, some HVAC, etc.)
- Fuel flexibility: dual-fuel lets you pivot to propane during gas shortages
10k+ Standby Generator + ATS (Automatic)
For frequent outages, medical needs, or “I want the house to just work” reliability. Automatic transfer switch (ATS) restores power hands-free.
- Works well for: whole-home or near-whole-home coverage
- Best for: families, remote workers, critical loads, long outage regions
- Big idea: sizing is still load-based — don’t buy 24kW unless you truly need it
Common Generator Sizing Mistakes to Avoid
1) Sizing by square footage
- Homes with electric cooking, well pumps, or A/C can vary wildly
- Loads matter, not size
2) Ignoring startup surge
- Fridges, pumps, and blowers can surge 2–5× running watts
- You size for the “worst startup moment”
3) Running at 100% capacity
- Hotter, louder, rougher voltage regulation
- Less fuel efficient, less headroom for surges
4) Forgetting 240V needs
- Well pumps and some HVAC require 240V
- Make sure your generator and inlet/transfer setup supports it
Practical approach: if you’re unsure, build an “essentials breaker plan” first. That alone usually drops you into the right generator class.
Sizing Quiz (Quick Check)
5 Questions
Answer each question. Then click Score Quiz.
1) The biggest reason generators “fail” even when the wattage seems high enough is:
2) A smart first step to avoid overspending is to:
3) “Running watts” describes:
4) A 120/240V generator matters most because it:
5) A practical sizing approach is to: