Understanding a few simple formulas can take the guesswork out of small-scale solar. Whether you are sizing a portable power station for a weekend trip or figuring out how long a panel will take to recharge a battery, the math is straightforward. This guide walks you through the key equations every beginner should know, using only the information found in publicly available educational materials and manufacturer documentation. Each formula is followed by a worked example and a note on why real-world results almost always differ from the paper calculation.

Short version: Watt-hours = watts × hours; runtime estimate = battery watt-hours ÷ device watts (minus losses); charge time = battery watt-hours ÷ effective panel watts × 1.2; amp-hours to watt-hours = amp-hours × voltage. Real results always vary.

Formula 1: Watt-hours (energy consumed)

The most basic solar calculation is figuring out how much energy a device uses over time. The U.S. Energy Information Administration explains that electricity use over time is measured in watthours (Wh). One watthour is the energy of one watt used for one hour. Virginia Cooperative Extension’s energy series provides a practical formula: (Wattage × Hours Used per Day) ÷ 1000 gives daily kilowatt-hour consumption. For smaller setups, you can skip the division and simply multiply watts by hours to get watt-hours.

Example: A 10 W LED light running for 3 hours consumes 10 × 3 = 30 Wh.

Why real results differ: Virginia Cooperative Extension notes that many appliances have a range of settings, so the actual power consumed depends on the setting used at any one time. In addition, many appliances continue to draw a small amount of power when they are switched off, creating “phantom loads” that add a few watt-hours to total consumption. These factors mean your measured energy use may be slightly higher than the simple multiplication suggests.

Formula 2: Estimating runtime from a battery

Once you know how many watt-hours a device needs, you can estimate how long a battery will keep it running. The basic idea is to divide the battery’s capacity in watt-hours by the device’s power in watts. If a battery stores 500 Wh and you plug in a 50 W device, the theoretical runtime is 500 ÷ 50 = 10 hours.

However, that number assumes perfect efficiency. Victron Energy’s Wiring Unlimited guide points out that when you run an AC load through an inverter, the inverter itself consumes some energy, so the power drawn from the battery is higher than what the load sees. The guide gives an example where a 2400 W AC load on a 12 V battery draws 200 A from the battery, ignoring inverter inefficiencies. In practice, those inefficiencies reduce the usable energy, making the real runtime shorter than the simple division suggests.

Example: A 500 Wh battery powering a 50 W device gives a paper runtime of 10 hours. With inverter losses, you might get noticeably less, especially if the inverter is small or running near its limit.

Why real results differ: inverter losses are only one part; not all of a battery’s rated capacity may be usable, depending on the product. Always treat the formula as a starting point, not a precise prediction.

Formula 3: Solar charge time estimate

Charging a portable power station with a solar panel is rarely as fast as the panel’s wattage suggests. Anker SOLIX’s solar charging guide offers a straightforward formula that accounts for real-world energy losses:

Estimated charging time = Battery capacity (Wh) ÷ Usable solar input (W) × 1.2

Usable solar input is the lower of two numbers: the combined rated wattage of your panels, or the power station’s maximum solar input limit. If you connect 800 W of panels to a station with a 600 W input limit, the usable input is 600 W. The 1.2 multiplier is a planning factor that Anker includes because panels rarely maintain their rated output throughout an entire charging cycle.

Example: A 1000 Wh battery charged by a 200 W panel (and no input limit lower than 200 W) gives 1000 ÷ 200 = 5 hours, then 5 × 1.2 = 6 hours estimated charging time under favorable conditions.

Why real results differ: Anker’s guide explains that solar conditions and panel choice affect charging time. Sunlight intensity, cloud cover, panel angle, temperature, and even dust on the panel surface can reduce output. The 1.2 factor is a general planning number, not a promise. On a cloudy day, charging will take longer.

Converting amp-hours to watt-hours

Some batteries are labeled in amp-hours (Ah) rather than watt-hours. To compare them or use them in the formulas above, you need to convert. Victron Energy’s Wiring Unlimited book shows that power in watts equals voltage multiplied by current (P = V × I). Therefore, watt-hours equal amp-hours multiplied by the system voltage.

Example: A 12 V battery rated at 100 Ah stores 12 × 100 = 1200 Wh of energy.

Why real results differ: the conversion uses the battery’s nominal voltage, so it gives a theoretical energy content; check the product’s own Wh figure and usable-capacity notes where they are given.

Using location-based estimates with PVWatts

If you want to go beyond simple charge-time formulas and understand how much energy a panel might produce over a month or a year, the PVWatts calculator from the National Laboratory of the Rockies is a helpful tool. It estimates the energy production of a grid-connected photovoltaic system based on your location, system size, tilt, and azimuth. The calculator uses 30 years of historical weather data to provide an expected range of output, so you can see how seasonal changes affect generation. While PVWatts is designed for grid-connected PV systems, its location-specific estimates can still give beginners a rough sense of how many watt-hours a panel might produce in their area. As with all estimates, the actual output will vary with local weather and shading.

Key formulas at a glance

Formula Inputs What makes it less accurate
Watt-hours = Watts × hours Device wattage, hours used Actual wattage varies with settings; phantom loads add extra consumption
Runtime = Battery Wh ÷ Device W Battery capacity, device power Inverter losses; usable share of capacity
Charge time = Battery Wh ÷ Effective panel W × 1.2 Battery capacity, panel wattage, input limit Sunlight variability, panel angle, temperature, soiling
Amp-hours to watt-hours = Ah × V Amp-hours, nominal voltage Uses nominal voltage; usable capacity may be lower

These formulas are the foundation of most beginner solar planning. For a deeper dive into the tools and worksheets that build on them, visit our Beginner Calculators page. You can also explore the broader Calculators and Learning Tools section or read the companion article Solar Calculators for Beginners: Watt-Hours and Runtime. If you are just getting started with portable panels and power stations, our Portable Solar Basics guide and the Solar for Beginners hub are excellent next steps.

Frequently asked questions

Why does my solar panel charge slower than the formula suggests?

Real-world conditions such as clouds, panel angle, and temperature reduce a panel’s output below its rated wattage. Anker SOLIX’s planning factor of 1.2 accounts for typical energy losses, but actual charging time can be longer, especially on overcast days or when the panel is not pointed directly at the sun.

Do I need to use the 1.2 factor for all charge time estimates?

The 1.2 multiplier is a general planning number provided by Anker to account for common losses. It is a sensible starting point for portable solar setups, but it is not a precise prediction. If you have measured your panel’s real-world output in your specific location, you can adjust the factor accordingly.

How do I find the wattage of my device if it is not labeled?

Virginia Cooperative Extension suggests that if the wattage is not listed, you can estimate it by finding the current draw in amperes and multiplying that by the voltage used by the appliance. Most household devices in the United States use 120 volts. You can also use a clamp-on ammeter to measure the current while the device is running, then multiply by the voltage to get watts.

Ready to run your own numbers? Head over to the Beginner Calculators category for more step-by-step guidance.

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