Power Basics
Watts, Volts, Amps, kWh, Surge
If you learn five terms—watts, volts, amps, kWh, and surge—you can size backup power correctly, avoid the “it should work” mistakes, and spend money where it actually matters.
Quick takeaway: Generators are sized mainly by watts + surge. Batteries are sized by watts + watt-hours (kWh).
Why “Power Basics” Matters
Most bad backup power buys come from mixing up these words: watts (power right now), watt-hours/kWh (energy over time), and surge (the startup kick for motors).
- You buy a generator that “should” run a fridge… but it won’t start it.
- You buy a battery “rated 2000W”… and it still dies fast.
- You underestimate loads by reading the wrong label on appliances.
Rule of thumb: make a load list first, then size the system to that list—not your hopes.
Use the Home Load Assessment.
The 5 Terms You Actually Need
1) Watts (W)
How much power something needs right now
Watts are the instant demand of an appliance while it’s running.
- A 600W sump pump draws ~600W while running
- A 1500W heater draws ~1500W while on
Why you care: Generator and inverter sizes are rated in watts.
2) Volts (V)
Electrical “pressure”
Most homes use 120V and 240V circuits.
- Lights/TV/fridge: usually 120V
- Well pump/dryer/range: often 240V
Why you care: If you need 240V loads, your generator + inlet/transfer setup must support it.
Start with the How to Size a Generator for Your Home.
3) Amps (A)
Electrical flow being pulled
Amps are how much current a device draws from a circuit.
Why you care: Breakers, wiring, and circuits are measured in amps (15A, 20A, 30A).
4) Watt-hours / kWh
How much energy over time
This is battery math: Watts = how hard right now. Watt-hours = how long.
Example: 100W × 10 hours = 1000Wh (1kWh)
Why you care: Batteries are sized by Wh/kWh. Use the Power Station Runtime guide.
5) Surge / Startup Watts
The “kick” motors need to start
Motors/compressors briefly draw more power at startup than during normal running.
- Fridges/freezers
- Well pumps / sump pumps
- Furnace blowers (often)
Why you care: #1 reason “the math was right” but the system still fails.
The Only Formulas You Need
Watt Formula
Watts = Volts × Amps
Examples
120V × 10A = 1200W • 240V × 20A = 4800W
Battery Runtime (estimate)
Runtime (hours) = Battery Wh ÷ Load W
Example
2000Wh ÷ 400W ≈ 5 hours
Real-world note: you’ll lose some energy to inverter inefficiency and cycling loads, so treat runtime as an estimate—not a guarantee.
Generator sizing shortcut: Add your running watts + the largest single startup surge (not every surge at once).
Then add a small buffer. Use the Home Load Assessment to do this cleanly.
Clean Reference Table: What These Units Mean
| Term | What it measures | Quick meaning | Where it matters most |
|---|---|---|---|
| Volts (V) | Electrical “pressure” | 120V vs 240V circuits | Whole-home loads, inlet/transfer setup |
| Amps (A) | Electrical flow | Current a device draws | Breakers, wiring, safe circuit use |
| Watts (W) | Power right now | Running demand | Generator / inverter sizing |
| Watt-hours (Wh) | Energy over time | How long you can run it | Battery runtime |
| Kilowatt-hours (kWh) | 1000Wh | Big-picture energy | Utility bills, large battery systems |
| Surge watts | Short startup spike | Motor “kick” | Starting fridges/pumps/blowers |
Running Watts vs Startup Watts (Real Example)
A fridge might be:
- Running: 150W–300W
- Startup surge: 600W–1200W (short burst)
If you’re building a backup plan around a fridge + freezer + sump pump, you must account for startup surge
or the generator/inverter may stall. This is why “rated watts” alone isn’t enough.
How to size correctly:
1) Add up your running watts (what stays on).
2) Add one big surge (largest motor start).
3) Add a small buffer.
Use the Load Assessment tool to do this without guessing.
1) Add up your running watts (what stays on).
2) Add one big surge (largest motor start).
3) Add a small buffer.
Use the Load Assessment tool to do this without guessing.
Common Power Mistakes That Break Backup Systems
Confusing watts with watt-hours
“2000W battery” tells you peak output, not runtime. Runtime comes from Wh/kWh.
Ignoring startup surge
Motors won’t start even though your running watt total looks fine.
Sizing to “everything” instead of “critical loads”
This is how you overspend. Pick the essentials first, then expand later.
Assuming everything is 120V
Many meaningful home loads are 240V and need the right generator setup.
Reality: The cleanest plan is often a combo: generator for motor loads + UPS/power station for electronics.
See UPS & Electronics Protection.
Appliance Reality Check
| Appliance | Running Watts | Surge Watts | Battery-Friendly? |
|---|---|---|---|
| LED lights | 10–100W | None | Excellent |
| Wi-Fi + modem | 15–60W | None | Excellent |
| Fridge | 150–300W | 600–1200W | Short-term |
| Microwave | 1000–1500W | Same | Drains fast |
| Space heater | 1500W | None | Poor choice |
| Sump pump | 600–1200W | 2–3× surge | Generator best |
| Well pump | 1000W+ | 2–3× surge | Generator best |
Numbers vary by model and age. Use your appliance label when available—and use typical values when it’s not.
The Load Assessment page is built for that.
Generator vs Battery: Why the Same Load Feels Different
Generators
- Great at handling surge
- Run as long as fuel is available
- Sized mainly by watts
If you need to start motors reliably (fridge, sump, well, blower), generators usually win.
Start here: How to Size a Generator for Your Home.
Batteries (Power Stations)
- Quiet, instant, no fumes
- Limited by stored energy (Wh/kWh)
- Best for electronics + short runs
Estimate runtime here: Power Station Runtime.
One-line takeaway: Generators solve power. Batteries solve time.
Recommendations (Simple Plans That Actually Work)
Plan A: “Essentials + Motor Loads” (Most homes)
- Identify critical loads (fridge, sump, blower, router)
- Add running watts + biggest surge
- Choose a generator size that starts motors reliably
Plan B: “Quiet Electronics Continuity” (Wi-Fi + devices)
- Use UPS for modem/router + desktop + networking gear
- Use a power station for longer runtime if needed
- Avoid heaters and big cooking loads on batteries
If you’re unsure which plan fits, do the assessment first. A correct load list makes every later decision easier.
What to do next
Now that you understand watts, volts, amps, kWh, and surge, the next step is simple: write down what you actually need to run, then size to that list.
Final tip: If your plan includes any motors (fridge, pump, blower), surge is the detail that makes or breaks your system.
Explore Generator Options
Answer one quick question to see what class of generator usually fits your situation — then learn how to size it properly.
What do you want to power during an outage?