If you’ve started shopping for a solar generator (also called a portable power station), you’ve probably noticed something frustrating fast: every listing brags about watt-hours, watts, and “peak power” like you’re supposed to already know what those numbers mean for your situation.
Buy too small, and your fridge shuts off at 2 a.m. Buy too big, and you’ve paid an extra $500–$1,000 for capacity you’ll never touch.
The good news is that sizing a solar generator isn’t guesswork. It comes down to two numbers: how much power your devices draw and how much energy you need to store, and a short formula ties them together.
As an ANCE-certified solar technician who’s specified systems ranging from single-panel backup kits to full off-grid arrays, I’ll walk you through exactly how to calculate the right size for your case, whether that’s a weekend campsite, a CPAP machine, or whole-home blackout backup.
The Two Numbers That Actually Matter
Before you look at a single product page, understand the difference between these two specs.
Watts (W)
This is power, the rate at which energy is used at any given moment. It determines whether the generator’s inverter can handle your device at all.
A generator rated for 500W of continuous output can’t run a 1,500W space heater, no matter how big its battery is.
Watt-hours (Wh)
This is energy, the total amount of power stored in the battery. It determines how long your devices will run before the unit is empty.
A 1,000Wh generator running a 100W device will last roughly 10 hours (before accounting for inverter losses).
Get the watts wrong, and your device won’t turn on. Get the watt-hours wrong, and it turns off sooner than you expected. You need to size for both.
Step 1: List Every Device You Plan to Run
Grab a notebook or a spreadsheet and list every device you might power, along with two figures for each:
- Running watts: the steady-state power draw (usually printed on a label, sticker, or in the manual)
- Starting/surge watts: the brief spike some motor-driven devices need to start (fridges, well pumps, power tools, air conditioners). This can be 2–3x the running wattage for a second or two.
If the wattage isn’t listed, you can calculate it from amps and volts printed on the device: Watts = Volts × Amps.
Most household devices in the U.S. and Mexico run on 120V, so a device pulling 5A draws roughly 600W.
Typical Power Draw by Device
| Device | Running Watts | Starting Watts |
|---|---|---|
| Smartphone charging | 5–10W | — |
| Laptop | 30–65W | — |
| LED light bulb | 8–12W | — |
| Wi-Fi router | 10–20W | — |
| CPAP machine (no humidifier) | 30–60W | — |
| Mini fridge | 60–100W | 200–300W |
| Full-size refrigerator | 100–200W | 800–1,200W |
| Box fan | 50–100W | — |
| Microwave | 600–1,200W | — |
| Coffee maker | 800–1,200W | — |
| Window AC unit (5,000 BTU) | 450–600W | 1,200–1,800W |
| Space heater | 1,200–1,500W | — |
| Well pump (1/2 HP) | 1,000W | 2,000–3,000W |
If you’re weighing whether your solar setup can handle a refrigerator specifically, see our full breakdown of refrigerator power draw and how to size for it.
Step 2: Calculate Your Continuous Power Requirement (Watts)
Add up the running watts of everything you’ll run at the same time, then check that figure against the highest single starting-watt spike you’ll experience (since that spike briefly stacks on top of whatever’s already running).
Example: a weekend camping setup.
- Mini fridge running: 80W
- LED lights: 20W
- Phone charging: 10W
- Laptop: 50W
- Total running load: 160W
- Mini fridge starting surge: 250W (replaces its 80W running draw for a moment)
- Peak load: 160W − 80W + 250W = 330W
You’d want a solar generator with a continuous output rating of at least 300–500W and a surge rating above 330W to comfortably run this setup, with headroom for inverter inefficiency and future devices.
Rule of thumb
Size your generator’s rated (continuous) wattage output to at least 20–30% above your calculated peak load.
Inverters lose some efficiency (typically 85–95%), and you don’t want to run right at the ceiling constantly, which shortens component life and gives you no room for adding a device later.
Step 3: Calculate Your Energy Storage Requirement (Watt-Hours)
This is the step people skip, and it’s the one that determines whether you’re out of power by dinnertime. Multiply each device’s running watts by the number of hours per day you’ll use it, then add it all up.
Formula: Watts × Hours of Use = Watt-hours needed per day
Example: the same camping setup, run for 24 hours.
| Device | Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| Mini fridge (cycles ~50% of the time) | 80W | 12 | 960 |
| LED lights | 20W | 5 | 100 |
| Phone charging | 10W | 2 | 20 |
| Laptop | 50W | 4 | 200 |
| Total | 1,280 Wh/day |
Add a 20–25% buffer for inverter losses and battery degradation over time, and you land at roughly 1,500–1,600 Wh needed per day.
That points you toward a solar generator in the 1,000–1,500 Wh capacity class if you’re recharging daily with sun, or larger (2,000 Wh+) if you want a full day or two of backup without any recharge at all.
Solar Generator Sizing by Use Case
| Use Case | Typical Capacity Needed | Typical Continuous Output |
|---|---|---|
| Phones, laptops, small electronics only | 200–500Wh | 100–300W |
| CPAP machine overnight backup | 300–600Wh | 100–150W |
| Weekend camping (lights, fridge, devices) | 1,000–1,500Wh | 300–600W |
| Short blackout backup (fridge, Wi-Fi, some lights) | 1,500–2,500Wh | 600–1,000W |
| Multi-day home backup (fridge, lights, fans, chargers) | 2,000–5,000Wh | 1,000–2,000W |
| Running a window AC unit | 2,000Wh+ | 1,500–2,000W (with surge headroom) |
| Whole-home essential circuits (fridge, AC, well pump, etc.) | 3,000Wh+ (often paired with expansion batteries) | 3,000W+ |
If you’re deciding between a battery-only backup system and a full solar generator, this comparison of solar batteries vs. generators for backup value breaks down the trade-offs in cost and runtime.
And if the portable power station route isn’t cutting it for your energy needs, our roundup of the best portable power stations compares current models by capacity, weight, and recharge speed.
Don’t Forget Recharge Time (If You’re Pairing With Solar Panels)
Sizing the battery is only half the equation if you plan to recharge via solar panels rather than a wall outlet.
A 1,000 Wh battery paired with a 100 W solar panel will need roughly 10–14 hours of strong, direct sunlight to fully recharge and more on cloudy days.
As a rule, match your panel wattage so that a full recharge is realistically achievable within your available daylight hours, not just on a lab-perfect sunny afternoon.
Most manufacturers list a “max solar input” spec. Check that your panel array doesn’t exceed it and that it comes reasonably close to it if you need fast recharges.
Common Sizing Mistakes to Avoid
Ignoring starting watts
A generator that easily handles a fridge’s running wattage can still shut down or trip if it can’t handle the compressor’s starting surge.
Sizing for “typical” use instead of worst-case
If you might someday run a space heater or window AC, size for that scenario now. Retrofitting later usually means buying a second unit.
Forgetting inverter and battery losses
Real-world usable capacity is typically 85–90% of the rated watt-hour figure, and it drops further as the battery ages, particularly with lithium-ion chemistries.
LiFePO₄ batteries hold their capacity longer over repeated cycles than standard lithium-ion, which matters if you’re sizing for years of regular use rather than occasional backup.
Assuming full sun for solar recharging
Cloud cover, panel angle, and season can cut solar input by 50% or more. Don’t count on textbook conditions.
FAQ: Sizing a Solar Generator
How many watt-hours do I need to power my house for a day?
A typical U.S. household uses 28,000–30,000Wh per day, far beyond what a portable solar generator can supply.
Most home backup setups instead focus on essential circuits: refrigerator, some lighting, Wi-Fi, and phone charging, which usually run 2,000–4,000 Wh per day. For true whole-home coverage, you’re looking at a stationary battery system, not a portable unit.
What size solar generator do I need to run a refrigerator?
Plan for at least 1,000–1,500 Wh of capacity and 600W+ of continuous output to run a standard refrigerator for a full day, accounting for its compressor’s starting surge. A more efficient mini fridge can be run on a smaller 500–800Wh unit.
Can a solar generator run a CPAP machine all night?
Yes, most CPAP machines draw only 30–60W without a humidifier, so even a compact 300Wh solar generator can typically power one for 5–8 hours, with room to spare for a phone charge.
Is it better to size up or size down when I’m unsure?
Size up. A slightly oversized solar generator costs more upfront but gives you flexibility to add devices, handles unexpected surge loads safely, and degrades more slowly since it’s rarely run near its maximum capacity.
An undersized unit that trips or dies mid-use is a bigger problem than a few hundred extra watt-hours you don’t use every time.
The Bottom Line
Sizing a solar generator comes down to two calculations: add up your running and surge watts to size the inverter’s continuous output, then multiply watts by hours of use to size the battery’s watt-hour capacity and pad both with a comfortable buffer.
Do that math before you shop, and you’ll avoid the two most common regrets: a unit that can’t start your fridge, or one that runs dry halfway through the day.
