Two numbers, and people only check one
Watts — continuous and surge — decide what will run at all. If a load's starting draw exceeds the inverter's rating, it does not run for a short time; it does not run.
Watt-hours decide how long it runs once it is running.
A 1,500Wh battery with a 1,000W inverter will not start a freezer. A 1,000Wh battery with an 1,800W inverter will start it and run it most of the day. The larger number on the box is the wrong one to shop on.
Step 1 — list what has to keep running
Not what you would like. What you would be genuinely unhappy to lose. Most households write a much longer list than they need, and then buy for it.
| Load | Running watts | Starting watts | Hours a day |
|---|
| Refrigerator | ~150 average | 500-1,000 | 24 (cycling) |
|---|
| Freezer | 100-500 | 500-1,000 | 24 (cycling) |
|---|
| Router and modem | ~20 | — | 24 |
|---|
| LED lighting | 30-60 | — | 5-6 |
|---|
| Laptop | 45-65 | — | 6-8 |
|---|
| CPAP, humidifier off | 30-60 | — | 8 |
|---|
| Furnace fan (½ HP) | 300-800 | 800-1,600 | 6-8 (cycling) |
|---|
| Sump pump | Varies | High | Depends entirely on rain |
|---|
All figures except the fridge average are Champion Power Equipment's published ranges. The 150W fridge average and the 85% inverter efficiency used below are this site's stated planning assumptions, listed on the methodology page. The full chart has the rest.
Step 2 — the watts number
Take the largest single starting wattage on your list, then add the running watts of everything that will already be on when it starts.
Worked example. A fridge, lights, a router and a laptop:
- Fridge running average: 150W
- Lights: 50W
- Router and modem: 20W
- Laptop: 60W
- Running total: 280W
- Fridge starting surge: 1,000W at the top of the published range
- Peak demand: 280 − 150 + 1,000 = 1,130W
So an 1,800W inverter covers it with real margin. A 1,000W one does not.
The trap is two motors. If a chest freezer is on the list, both compressors can call within a second of each other, and the peak becomes roughly 280 − 150 − 300 + 1,000 + 1,000, which is nearly 2,000W. That is the entire reason the 2kWh tier exists, and why surge deserves its own page.
Step 3 — the watt-hours number
Multiply each load's running watts by the hours it actually runs, remembering that compressors cycle rather than run continuously — a fridge is on roughly a third of the time, which is where the 150W average comes from.
Same household, over 24 hours:
- Fridge: 150W average × 24h = 3,600Wh
- Lights: 50W × 6h = 300Wh
- Router: 20W × 24h = 480Wh
- Laptop: 60W × 8h = 480Wh
- Total: 4,860Wh
- Divide by 85% inverter efficiency: about 5,700Wh of battery
Which is more than any portable battery on this site. That number is not a mistake — it is the honest arithmetic, and it is why the next step matters more than the first two.
Step 4 — decide how long, then load-shed
The 5,700Wh figure assumes 24 hours of everything. Almost nobody needs that, and the fix is not a bigger battery.
- Run the fridge in blocks. A closed fridge holds temperature for hours. Powering it four hours in twelve keeps food safe and cuts its share from 3,600Wh to roughly 1,200Wh.
- Drop the laptop to essential use.
- Light one room rather than the house.
Do that and the same household's day comes down to roughly 1,800-2,200Wh — squarely inside the 2kWh tier, and inside 1kWh for a single overnight.
Match the purchase to the outage
Capacity is not a preference. It falls out of two numbers: how long your power is usually off, and what you refuse to lose while it is.
| Your outage lasts | What has to keep running | Realistic product class |
|---|
| Under 4 hours | Router, phones, a lamp | 300-500Wh is genuinely enough
Power stations under $500 |
|---|
| 4 to 24 hours | Fridge, lights, devices | 1kWh with an 1,800W inverter
Best under $1,000 |
|---|
| 1 to 2 days | Fridge, freezer, comms, furnace blower | 2kWh with 2,400W or more
Best 2000Wh power stations |
|---|
| 3 days or more | All of the above, sustained | 2kWh plus solar or a generator
Generator vs power station |
|---|
The jump people get wrong is the last one. No portable battery covers a multi-day outage on capacity alone — what covers it is a recharge path, whether that is panels or an engine.
Step 5 — check the recharge path
Two limits in your station's manual, both capable of making a correctly-sized battery useless:
- Maximum solar input. A unit that accepts 200W accepts 200W regardless of how much panel you own, which caps your daily allowance. The solar arithmetic works back from daily watt-hours.
- AC recharge time. Real outages flicker. A battery that refills in an hour uses those windows; one that takes six hours misses them.
Four households, sized
| Household | Peak watts needed | Daily Wh (with load shedding) | Buy |
|---|
| Apartment, comms only | ~200 W | ~500 Wh | 300-500Wh |
|---|
| House, fridge and essentials | ~1,130 W | ~1,500 Wh | 1kWh, 1,800W inverter |
|---|
| House, fridge and freezer | ~2,000 W | ~2,200 Wh | 2kWh, 2,400W+ inverter |
|---|
| Rural, plus a well pump | Well above 2,000 W | ~3,000 Wh | A generator, with a battery indoors |
|---|
Calculated from the published figures above with this site's stated assumptions. Your appliances differ — an old fridge in a hot garage is well above the planning average, a new one below it.
Buy one size up, once
The most common regret in this category is under-buying, and the reason is that people size to the calculation exactly and then discover the calculation was optimistic — an appliance draws more than its label, or the outage lasts longer than expected, or a second thing turns out to matter.
Margin is cheap at purchase and impossible afterwards, and expansion only adds hours, never watts. If you are between two tiers, take the larger one.
