The mistake
Nearly everyone shops by price, buys the largest unit within budget, and then discovers it either does not start what they needed or dies hours too early. Size is not a budget decision. It is an arithmetic one, and the arithmetic takes about five minutes.
Step 1: list what must stay on
Not what you would like powered — what genuinely must be. For most households in an outage that is a fridge, internet, phone charging and some lighting. That is a far smaller list than people start with.
Step 2: add up the watt-hours
Multiply each item’s average draw by the hours you need it. Add them up.
| Item | Average draw | Over 24h |
|---|---|---|
| Full-size fridge | 100–200W avg | 1,000–2,000Wh |
| Wifi router | 10W | 240Wh |
| Laptop | 45–65W | ~300Wh |
| LED lights (4) | 40W | ~200Wh |
| Phone charging (2) | 10–20W | ~50Wh |
| CPAP | 30–60W | ~300Wh/night |
| Gas furnace blower | 300–600W | 1,500Wh+ |
| Starlink Mini | ~40W | ~960Wh |
A typical “keep the essentials alive” day lands around 1,500–2,500Wh. That is why 2,000Wh units are the popular bracket — it is not marketing, it is where the arithmetic points.
Step 3: check the inverter, not just the battery
Capacity decides how long. Output decides whether it starts at all. A 2,000Wh battery with a 1,000W inverter cannot run a 1,500W kettle for one second.
Add up the highest wattage you might run simultaneously, then add headroom for compressor surges. Most households want at least 1,800W of continuous output; 2,400W is comfortable.
Then add 30%. Advertised capacity is the cell rating; you will see 80–90% of it in practice, less in the cold. Batteries also lose capacity as they age. Sizing exactly to your calculation means being short within a year.
Quick answers
| If you want to… | Look for | Example |
|---|---|---|
| Charge devices camping | 300–500Wh | Entry units |
| Run a CPAP for 2 nights | ~1,000Wh | Delta 2 |
| Carry a fridge 24 hours | 2,000Wh+ | AC200L |
| Back up essential circuits | 2,500Wh+, 3,000W | Apex 300 |
| Full-time van with Starlink | 2,000Wh + 400W solar | AC200L |
When to buy expandable instead
If your answer sits near a boundary, or you suspect your needs will grow, buy into a system that accepts extra battery modules. It costs a little more now and saves replacing the whole unit later.