Storm’s on the way, and you’re wondering whether a solar generator could keep your fridge cold and your phone charged. Then you pull up a spec sheet and it’s wall-to-wall watts, watt-hours and surge ratings. Don’t worry. The math is easier than it looks once you know which two numbers matter.
So how big a solar generator do you need to back up your home? For most people, a battery of roughly 1,000 to 3,000 watt-hours will cover the essentials for about a day. If you want to run central air or an electric range, you’ll need far more. Let’s work out your own number.
What’s the difference between watts and watt-hours?
Watts tell you how much power a device pulls at any given moment. Watt-hours (Wh) tell you how much energy a battery can store. Picture a tap and a bucket: watts are how fast the water flows, and watt-hours are how big the bucket is.
You’ll want to check both. The generator’s output rating has to be high enough to run your devices all at once, and its battery has to be big enough to keep them going for as long as you need.
Start by listing what you want to power
Resist the urge to back up the whole house on your first pass. Begin with the must-haves, then add extras if your budget and battery allow. A typical list looks like this:
- Refrigerator and freezer
- Phones, laptops and a Wi-Fi router
- LED lights
- A sump pump, if you have one
- A CPAP machine or other medical device
- A fan, space heater or window air conditioner (these are power hogs)
How do you find each device’s wattage and run time?
Check the label on the back or bottom of the appliance, or flip through the manual. Many list watts right on them, and if not, you can multiply volts by amps. Numbers vary a lot by model, but as rough guides, an LED bulb uses a few watts, a router about 10 to 20, a laptop 50 to 100, and a modern fridge averages around 100 to 200 watts while it’s running. A space heater, on the other hand, can pull 1,500 watts all by itself.
Next, guess how many hours a day each thing will really run. Your fridge won’t be working nonstop, since the compressor cycles on and off, so it’s often running about a third to half the time. If you’d like a more precise answer, an inexpensive plug-in energy meter will show your actual usage.
Doing the math on your daily watt-hours
Here’s the easy part. Multiply watts by hours to get watt-hours for each device, then add them all up. Here’s a sample day:
| Device | Watts | Hours | Watt-hours |
|---|---|---|---|
| Refrigerator (average) | 150 | 8 | 1,200 |
| Wi-Fi router | 15 | 24 | 360 |
| Laptop | 60 | 6 | 360 |
| LED lights | 30 | 5 | 150 |
| Phone charging | 20 | 3 | 60 |
That comes to about 2,130 Wh. These are just example figures, so swap in your own. Inverters lose a bit of energy when they convert power, so tack on 15 to 20 percent as a cushion. For this example, you’d shop for a battery of roughly 2,500 Wh or more.
Why output and surge ratings matter for solar generator size
Capacity is only half the story. Add up the running watts of everything you might use at the same moment, and make sure the generator’s continuous output beats that total.
Here’s the catch, though. Motors in fridges, sump pumps and well pumps need a burst of extra power to get going. That’s called surge or starting wattage, and it can run two to three times the normal figure. Pick a unit whose surge rating can handle your biggest motor.
Also, most outlets on a power station are 120V. If you want to run 240V gear like a well pump, a large air conditioner or an electric dryer, you’ll need a model with 240V output, and our guide to portable power stations with 240V output covers that category.
Planning how you’ll recharge
A big battery only helps if you can refill it. You can charge from a wall outlet when the grid’s up, from a car, or from solar panels. In real-world sun, panels rarely hit their label rating. Expect about half to three-quarters of it, and a lot less on cloudy days.
If you’re hoping to ride out several days without grid power, panel capacity matters as much as battery size. When you’re ready to size that side, take a look at portable solar panels for power stations.
Common sizing mistakes to avoid
- Trying to run heating or cooling appliances. Electric heaters, dryers and ovens drain a battery fast. Plan around them or skip them.
- Ignoring surge watts. A unit that handles your running load can still trip when a compressor kicks on.
- Forgetting efficiency loss. Usable capacity is always a bit lower than the number on the box.
- Plugging a generator into a wall outlet to power the house. This is dangerous. Use the unit’s own outlets or a properly installed transfer setup.
A rough size guide
- Under 1,000 Wh: phones, laptops, lights and a router for a night or two.
- 1,000 to 2,000 Wh: all of the above, plus a short fridge run or a CPAP for several nights.
- 2,000 to 3,000+ Wh: a fridge, electronics and lights for about a day, and expansion batteries are often available.
- Whole-home needs: usually a permanently installed battery system rather than a portable unit.
Frequently asked questions
How many watt-hours do I need to run a refrigerator?
A modern fridge averages around 100 to 200 watts while running, and it cycles on and off, so it’s often on for roughly a third to half the day. Work out your fridge’s own usage, then add a 15 to 20 percent cushion for inverter losses.
Can a solar generator run my whole house?
Portable units generally can’t. Whole-home needs usually call for a permanently installed battery system. A portable one is better suited to essentials like the fridge, lights, internet and phones.
Why does the surge rating matter?
Motors need extra power for a moment when they start, often two to three times their running wattage. If the surge rating’s too low, the unit can trip even though your total running load looks fine.
The bottom line
Add up the watt-hours for what you truly need, add a cushion, and make sure the output and surge ratings fit your biggest device. That’s how you find the right solar generator size for your home. Ready to compare? See our roundup of the best solar generators for home backup. And if you’d rather run most of your house, a permanent setup may suit you better, so our guide to whole home battery backup systems is a good next stop.
Not sure what else to get ready? Our power outage checklist walks you through the rest, and if you’re planning to take your setup camping, read up on powering a campsite or RV without a generator.