What is the difference between watts and watt-hours?

Watts measure power, the instantaneous rate at which a device pulls electricity. Watt-hours measure energy, how much total electricity is stored or consumed over time. One watt used for one hour equals one watt-hour.

How do you convert watts to watt-hours?

Multiply the device's wattage by the number of hours it runs. A CPAP drawing 55W with its heated humidifier for 8 hours uses 440Wh. To go the other way, divide watt-hours by watts to get runtime in hours.

Is a higher watt-hour rating always better?

Not necessarily. A higher Wh rating means longer runtime, but it also means more weight, higher cost, and longer recharge times. Size the battery to your actual daily load plus a safety buffer rather than buying the biggest number on the shelf.

Why do solar generators have two wattage numbers?

The two figures are continuous output and surge (peak) output. Continuous watts is what the inverter can deliver all day long. Surge watts is a brief spike the unit can handle for a second or two when a motor or compressor starts up.

What does "1500Wh" mean on a solar generator?

1,500Wh is the battery's rated energy. After inverter losses, plan on about 1,275Wh of AC output (1,500 x 0.85). That runs a modern full-size fridge averaging 63W, the ENERGY STAR median, for about 20 hours.

How many watt-hours does a fridge use per day?

ENERGY STAR-certified full-size models (18 to 28 cubic feet) use about 1.0 to 1.9kWh a day, a median of 1.5kWh, because the compressor cycles and averages roughly 40W to 80W. Older fridges can use far more; a plug-in watt meter gives your real figure.

How many watts do I need for home backup?

Add up the running watts of everything you want on at once and add 20%. A fridge, lights, Wi-Fi and an occasional microwave come to about 1,750W, so look for about 2,100W continuous, and a surge rating that covers the fridge compressor's start-up (1,200W on Honda's chart) on top of whatever else is running.

Watts vs Watt-Hours Explained (2026)

Watts vs watt-hours in one line: watts (W) are how much power something draws or a power station can deliver at a given moment, and watt-hours (Wh) are how much energy is used or stored over time, so Wh = W x hours. A CPAP drawing 20W without its humidifier uses 160Wh over 8 hours; a power station's usable energy is about its rated Wh x 0.85 after inverter losses.

Why does the difference matter when buying a solar generator?

Mixing the two up leads to the two most common sizing mistakes. One buyer sees 1,500 watts on a unit, assumes it will run a fridge all night, and finds the battery holds far less energy than needed. Another pays for a 3,600Wh unit to run a CPAP that needs roughly 160Wh a night without its humidifier, or about 440Wh with it. Both mistakes come from confusing watts with watt-hours.

Watts (W) and watt-hours (Wh) measure two completely different things. Watts describe how much power a device pulls at a single instant. Watt-hours describe how much energy is stored or consumed over time. Mixing them up is like confusing the horsepower of your car with the size of its gas tank. Both matter. Neither substitutes for the other.

This guide breaks the concepts apart, shows you the formula that connects them, and walks through a four-step method for sizing a solar generator without guesswork.

Infographic explaining watts vs watt-hours: watts measure instantaneous power like a speedometer, watt-hours measure stored energy like a gas tank — Pin this to reference when comparing solar generators.

Watts: the instantaneous draw

A watt is a unit of power. It tells you how hard an electrical device is pulling at any given moment. Think of it as the speedometer reading on your car: useful right now, but it says nothing about how far you can drive.

Every appliance has a running wattage (what it pulls during normal operation) and many also have a surge wattage (a brief spike when a motor or compressor kicks on). Start-up surges last a moment but can be several times the running draw: Honda's wattage chart lists 1,200W to start an ENERGY STAR fridge that runs at 132 to 192W.

Running wattage examples

CPAP machine: 8–20W without the heated humidifier, up to about 55W with it (ResMed's figures for the AirSense 10)
LED light bulb: 8.5–10W for a 60W-equivalent bulb
Laptop: up to 60W while charging (the 15-inch MacBook Air's adapter)
Wi-Fi router and modem: 10–20W (Champion's chart)
Full-size refrigerator: 132–192W while the compressor runs (Honda)
Microwave: about 1,500W for a model with 1,000W of cooking power (Honda)
1,500W space heater: 1,500W continuous
Window AC (10,000 BTU): about 670W running for an ENERGY STAR model, up to 2,400W to start (Champion)

Your solar generator's continuous AC output rating must exceed the sum of the running watts you plan to use at once. Its surge rating must exceed the largest single surge event. A unit with a 1,000W surge rating cannot start a compressor that needs 1,800W, no matter how full the battery is.

Watt-hours: the energy tank

A watt-hour is a unit of energy. One watt-hour equals one watt of power used for one hour. It is the gas tank equivalent: total stored energy available before the battery is empty.

A unit rated at 1,000Wh does not deliver 1,000Wh to your AC outlets. Two losses eat into that number:

Cutoff before empty: the battery management system stops before the LiFePO4 cells are fully drained, to protect them.
Inverter losses: converting the battery's DC to AC costs some energy as heat, and proportionally more at very light loads.

A practical planning figure is usable Wh = rated Wh x 0.85, the factor Jackery uses in its runtime formula, so a 1,000Wh unit delivers about 850Wh of AC output; at very light loads it can be less. DC output through the 12V or USB ports skips the inverter, so you keep more of the rated capacity.

The formula that ties them together

The math is simple: watts multiplied by hours equals watt-hours. Rearranged, watt-hours divided by watts equals runtime in hours.

Worked examples

CPAP at 20W (no humidifier) for 8 hours = 160Wh per night; with the heated humidifier at 55W, 440Wh. A 500Wh station (about 425Wh usable) covers two or more nights without the humidifier and almost one with it.
Refrigerator: the compressor draws 132 to 192W while it runs, but it cycles, so ENERGY STAR-certified full-size models average about 63W, or 1.5kWh a day (the median). At that rate a 1,500Wh station (about 1,275Wh usable) covers about 20 hours. An older fridge can use far more.
Microwave at 1,500W for 5 minutes = 125Wh. Running the microwave is cheap on energy but demanding on peak output.
1,500W space heater for 4 hours = 6,000Wh. This is why heating off-grid with resistive loads is brutal. You need a 6kWh+ expandable unit and serious solar input to sustain it.

Notice how the microwave and the space heater pull the same peak wattage but consume vastly different energy. That is the entire point of keeping the two specs separate in your head.

Why marketing numbers mislead buyers

Manufacturers know that bigger numbers sell. The spec sheet is where that instinct meets technical reality.

"1,500W" could mean two different things

Some listings use "1,500W" to describe the AC inverter output (the running wattage the unit can deliver). Others use it loosely in marketing copy to imply battery capacity. They are not the same. Always confirm which spec is which by looking for the units: W for power output, Wh for energy storage.

Peak vs continuous

A unit might advertise "3,000W peak" in bold and list "1,500W continuous" in smaller text. The peak figure is surge handling, good for a few seconds. The continuous figure is what runs your appliances all night.

DC versus AC output watts

Total output watts sometimes sum DC ports and AC ports together. You cannot pull all of that through the AC inverter. Check the AC inverter rating specifically.

Amp-hours vs watt-hours

Older RV and marine gear advertises capacity in amp-hours (Ah). To convert, multiply by the battery voltage: Ah times volts equals Wh. A 100Ah 12V battery stores 1,200Wh of raw energy. A 100Ah 24V battery stores 2,400Wh. Never compare Ah numbers across different voltages.

How to size a solar generator in 4 steps

Four steps, with the formula at each stage:

Step 1: List every device and its running watts. Use the nameplate on the appliance or a Kill-A-Watt meter for accuracy. Our appliance power guide has typical values for common household loads.
Step 2: Sum the running watts of devices used simultaneously. That total is the minimum continuous AC output your unit must deliver. Add a 20% buffer.
Step 3: Identify the highest surge draw. Usually this is a compressor (fridge, AC, well pump). Your solar generator's surge rating must exceed this number with margin.
Step 4: Sum daily watt-hours and multiply by days of autonomy. Divide by 0.85 to account for usable capacity and inverter losses: rated Wh needed = (daily Wh x days) / 0.85. For 1,500Wh a day and 2 days: 3,000 / 0.85 = about 3,530Wh.

Common mistakes

Buying by watts alone. A 2,000W unit with a 500Wh battery runs a microwave for 15 minutes and quits. The output rating tells you nothing about runtime.
Ignoring the 85% usable rule. LiFePO4 cells last longer when not cycled to zero. The BMS usually cuts off before 100% depth of discharge. Plan for 85% and you will not be surprised.
Forgetting inverter losses. Running everything through the AC outlets costs energy; power 12V and USB devices from the DC ports where you can.
Overlooking surge on motor loads. Refrigerators, well pumps, and air conditioners can draw several times their running watts for a moment at start-up. A unit that handles continuous load but lacks surge headroom will trip its inverter.

Quick reference: what each Wh tier covers

200–700 Wh: Phones, tablets, one to three nights of CPAP without the humidifier, a laptop work session, LED lights and a fan. Camping and short emergencies.
700–1,500 Wh: A fridge for most of a day, router and laptop, a coffee maker, CPAP for several nights. Weekend off-grid or single-day outages.
1,500–3,600 Wh: Heavy appliances in rotation, a fridge plus a router and lights for about half a day to a day and a half, short bursts of microwave or coffee maker. Short-term home essentials backup.
3,600+ Wh (expandable): Multi-day home backup, well pump starts, running a chest freezer plus fridge plus essentials. Paired with 1,000W+ of solar input, this is essential-circuits backup.

Once you stop asking "how many watts does this generator have" and start asking "how many watts continuous, how many watts surge, how many usable watt-hours," the buying decision becomes a math problem instead of a marketing pitch.

Current examples: a 1,024Wh unit such as the EcoFlow DELTA 3 (1,800W output) runs a fridge and essentials for half a day or more; a 3,600Wh unit such as the EcoFlow DELTA Pro (3,600W output) covers essentials for a day or more. Our picks are based on published specifications, pricing, warranty terms and owner reports.

EcoFlow DELTA 3Buy at EcoFlow Check price on Amazon