Short answer
Make a list of the critical items you want to power, find each item’s running watts and any motor starting (surge) watts, then size your generator or battery system to handle the highest expected peak plus some headroom. For generators, choose a unit with at least 20–30% extra capacity over your calculated peak. For battery backups, size the inverter to handle the peak watts and size the battery (Wh) to support the total hours you’ll run those loads.
How to size backup power the right way
Step 1: Decide what you’ll power
Common critical loads and typical wattages (check your nameplates/manuals):
- Refrigerator/freezer: 120–250W run, 600–1200W start
- Sump pump (1/2 HP): 800–1000W run, 1500–2000W start
- Gas furnace blower: 400–700W run, 800–1400W start
- Wi‑Fi/router + modem: 15–30W
- LED lights (room or two): 20–100W total
- CPAP: 30–70W
If you plan to use a microwave, well pump, or window A/C, include them. Motors and compressors have start surges.
Step 2: Gather running vs. starting watts
- Look at the appliance label or manual.
- If watts aren’t listed, use amps × volts (A × 120V) to estimate watts.
- For motors with only running watts listed, estimate start watts as 2–3× running.
Formulas:
Running watts = sum of all items you’ll run at once
Peak/Surge watts ≈ Running watts + (Largest starting watts – that item’s running watts)
Battery capacity needed (Wh) ≈ (Total watts × hours) ÷ (inverter efficiency × usable battery fraction)
Use 0.9 for inverter efficiency and 0.8 for usable battery (LiFePO4), so multiply by ~1.4
Step 3: Do the math (example)
Scenario: fridge (150W run/900W start), sump pump (900W run/1800W start), furnace blower (600W run/1200W start), lights/router (60W).
- Running watts = 150 + 900 + 600 + 60 = 1,710W
- Largest extra surge = pick the biggest start minus run:
- Fridge: 900 – 150 = 750W
- Sump pump: 1800 – 900 = 900W (largest)
- Furnace: 1200 – 600 = 600W
- Peak watts ≈ 1,710 + 900 = 2,610W
Generator sizing: choose at least 20–30% headroom.
- 2,610W × 1.25 ≈ 3,260W. A 3500–4500W generator is a good fit.
Battery backup sizing:
- If you want 4 hours of runtime at an average of 700W (you won’t run everything continuously), energy needed = 700W × 4h = 2800Wh.
- Adjust for losses/usable capacity: 2800Wh × 1.4 ≈ 3900Wh. Choose a ~4 kWh battery. The inverter must handle the peak (≥ 3000W surge, ≥ 2000–2500W continuous for this scenario).
Choosing equipment
- Portable generators:
- 2000W inverter generator: quiet, clean power (<5% THD) for electronics; good for fridge + lights + router.
- 3500–4500W open-frame or inverter: covers a fridge, sump pump, furnace blower, lights. Dual-fuel models add flexibility.
- 5000–7000W: supports more circuits and 240V loads (well pump). Expect more fuel use and noise.
- Battery power stations (LiFePO4 preferred):
- 1 kWh: router + lights for many hours, or a fridge for ~8–12 hours intermittent.
- 2–3 kWh: fridge + electronics for a day, light duty pump cycles.
- 4–6 kWh: better for multi-load support. Ensure inverter is pure sine wave with adequate surge rating.
Tools and materials
- Appliance manuals or labels; notepad or spreadsheet
- Plug-in watt meter (Kill A Watt) for 120V loads; clamp meter for circuit-level checks
- Heavy-duty extension cords: 12 AWG for 15A–20A runs, outdoor-rated
- For generators: transfer switch or interlock kit, inlet box (e.g., NEMA L14-30), 30A generator cord
- CO alarm, fuel cans with stabilizer, weather cover for storage
Safety considerations
- Never run a generator indoors or in a garage. Place outside, at least 20 ft from doors/windows, exhaust pointing away from the house. Use a CO detector.
- Do not backfeed your panel with a “suicide cord.” Use a listed transfer switch or interlock with a proper inlet and breaker.
- Use grounded, outdoor-rated cords; avoid overloading with daisy-chains. Keep cords fully uncoiled to prevent overheating.
- For generators, check whether the neutral is bonded in the unit and follow the transfer switch manufacturer’s instructions.
- Battery power stations should be kept dry, ventilated, and used within their rated limits. Avoid cheap modified sine wave inverters for appliances with motors.
Tips for best results
- Sequence startup: turn on the biggest motor load first, let it stabilize, then add others. This manages surge on smaller units.
- Consider soft-start kits for well pumps or window A/Cs to reduce starting surge.
- Inverter generators are quieter and produce cleaner power for electronics.
- Rotate and stabilize fuel. Test your setup monthly for 10–15 minutes under load.
- Label which circuits are on the transfer switch and keep a load plan card with wattages.
Common mistakes
- Underestimating motor starting amps and tripping the generator/inverter.
- Sizing battery by watts only and forgetting runtime (Wh/kWh).
- Using light-gauge cords that drop voltage, causing motors to overheat.
- Trying to run electric water heaters, ovens, or central A/C on small backups—these are heavy loads.
- Ignoring total harmonic distortion (THD) for sensitive electronics; stick to inverter generators or pure sine inverters.
When to call a pro
- Installing a transfer switch/interlock and inlet, or connecting 240V loads like a well pump.
- Verifying grounding/bonding arrangements for your specific generator model.
- Designing a larger battery system (stackable power stations or LiFePO4 banks) integrated with home circuits.
Rough costs
- 2000W inverter generator: $400–$900
- 3500–4500W generator: $500–$1,200 (add $100–$300 for dual-fuel)
- Transfer switch/interlock + inlet/cord: $300–$700 materials; $400–$1,200 labor
- Power stations: ~$500–$900 per kWh (LiFePO4), inverter included
Take 45–90 minutes to list loads, read labels, and run the numbers. A little planning upfront ensures your backup actually carries the loads you care about without tripping or damaging equipment.