How to Size a Solar Generator: A Step-by-Step Worked Example

Sizing a solar generator correctly is the most important step before making a purchase. If you underestimate your needs, you will run out of power during an outage. If you overestimate, you might end up with a heavy, expensive unit that is larger than necessary. This guide walks through a complete worked example, powering a phone, laptop, LED lights, a compact refrigerator, and a CPAP machine, to show you exactly how to calculate the output rating and battery capacity you need.

Step-by-Step Sizing Worked Example

Step 1: List Devices with Running Watts

Start by listing every device you plan to power simultaneously or within the same day. The most reliable method is to check the manufacturer’s label on the device, which usually lists the power in watts (W) or amps (A) and volts (V). If only amps and volts are listed, multiply them (Watts = Volts x Amps). For devices with variable power draw, like a laptop charger, use the highest input rating listed.

Reading labels can be tricky. Some devices list “VA” instead of “W.” For practical purposes with most household loads, VA and Watts are roughly equivalent, though for some power supplies they can differ. A watt meter removes this guesswork entirely. Another practical option is to use a plug-in watt meters to measure the actual draw of a device during typical use. This is especially useful for an appliance like a refrigerator, whose compressor cycles on and off.

Note: the wattages above are example values to show the method, not measurements of any specific product. A CPAP with a heated humidifier can draw much more than one without; always check your own devices.

Step 2: Find Surge Watts for Motors and Compressors

Many devices with motors or compressors draw significantly more power for a split second when they start up. This is called surge power or starting watts. The most common example in a home backup setup is a refrigerator. The compressor in a fridge can briefly draw several times its running watts. In this example we assume the compact fridge surges to 350 W. You must ensure the solar generator’s inverter can handle this peak surge, or the generator will overload and shut down.

For a deeper explanation of why this matters, see our guide on surge vs continuous power.

Devices rarely start at exactly the same moment, so the realistic peak is everything else running (140 W) plus the fridge’s startup surge (350 W): about 490 W. The inverter’s surge rating must be higher than that. A common rule of thumb is to look for an inverter rated for at least double the running wattage of your largest motor-driven appliance to comfortably handle the startup surge.

Step 3: Estimate Hours of Use Per Day

Be honest about how many hours each device will run. A fridge cycles on and off, so while it is plugged in 24 hours a day, the compressor might only run for 8 hours total. A CPAP machine runs all night. Laptops and phones charge for a few hours.

Step 4: Calculate Watt-Hours Per Day

This is the core of the sizing calculation. Watt-hours (Wh) represent the total energy consumed. Multiply the running watts by the hours of use for each device.

For a clear explanation of this unit and how it differs from amp-hours (Ah), read watt-hours vs amp-hours.

Step 5: Add Losses and a Buffer

No electrical system is 100% efficient. You need to account for inverter conversion losses and build in a safety buffer.

• Inverter Efficiency: Inverter efficiency around 85 to 90 percent is a common planning assumption. This means you need to draw more energy from the battery than your devices consume. To account for this, divide your total Wh by the efficiency factor (e.g., 0.85).
  Adjusted Wh = 1,310 Wh / 0.85 = 1,541 Wh.
• Usable capacity: lithium power stations let you use most of their rated capacity, though not quite all of it in practice, and lead-acid batteries should not be drained deeply. Check the manufacturer’s usable-capacity figure; the 20% buffer below also covers this.
• Safety Buffer: A common safety buffer is 20% for unexpected loads, slightly longer usage, or battery degradation over time.
  Buffer = 1,541 Wh x 1.20 = 1,849 Wh.

Final Energy Requirement: You need a solar generator with a battery capacity of roughly 1,850 Wh.

Step 6: Choose Output Rating and Battery Capacity

• Continuous Output: The inverter must be able to handle the total running watts. Your total running watts were 210 W. A generator with a 300 W to 500 W continuous output would cover this easily.
• Surge Output: The inverter must handle the highest surge load. The realistic peak is about 490 W. Choose an inverter whose surge rating is comfortably above that; in practice, most power stations with 300 to 500 W of continuous output list surge ratings well beyond it, but check.
• Battery Capacity: You need a battery rated for at least 1,850 Wh. Solar generators are almost always rated in watt-hours. Look for a model with a capacity of 2,000 Wh or slightly more to have comfortable margin for the buffer.
• Waveform: For a refrigerator and a CPAP machine, a pure sine wave inverter is strongly recommended to ensure the motors run smoothly and efficiently. Modified sine wave can cause some motors to run hot or create electrical noise.

Step 7: Plan Recharge

How will you replenish the 1,850 Wh you used?

• Solar Charging: If you rely on solar panels, you need to consider the available sunlight. Peak sun hours vary widely by location and season, and winter values are much lower. With 5 peak sun hours, replacing 1,850 Wh needs 1,850 ÷ 5 = 370 W of ideal output; allowing for real-world losses (assume about 75%), that becomes roughly 370 ÷ 0.75 ≈ 490 W of panels, and more in winter. Also check that your power station’s solar input limit is high enough. Read more about this in how long a solar generator takes to charge.
• AC Wall Charging: Most solar generators can be recharged from a standard wall outlet. This is the fastest method. A unit with a 200 W AC charger would need at least 9 to 10 hours to refill 1,850 Wh; faster AC charging shortens this.
• Car Charging: Many units also support charging from a vehicle’s 12V DC outlet, though this is much slower and best used for topping off.
• Running a Fridge: If the refrigerator is your primary concern, see our dedicated guide on running a refrigerator on a solar generator for specific tips on managing its energy draw and surge requirements.

Common Sizing Mistakes

• Forgetting Surge Watts: This is the most common error. A fridge or a well pump can trip the inverter on startup if the generator cannot handle the surge. Always check the surge rating of the inverter against the starting surge of your largest motor-driven appliance.
• Overestimating Solar Input: Solar panels rarely produce their rated wattage for a full hour. Factors like clouds, shading, heat, and panel angle reduce output. It is safer to plan on noticeably less than the panel’s rating in real-world conditions.
• Ignoring Standby Power: Many devices, like modems, routers, or the solar generator itself, draw a small amount of power 24/7. This “vampire load” can add up to several hundred watt-hours over a day. Account for these if they are essential.
• Assuming All Inverters Are the Same: A modified sine wave inverter might cause a CPAP motor to run hot or a fridge compressor to hum. Pure sine wave is the standard for sensitive electronics and motors. Always check the waveform of the generator you are considering.

FAQ

How do I find the running watts of my devices?

Check the label on the device or its power adapter. It will list the power in watts (W) or volts (V) and amps (A). Multiply volts by amps to get watts (Watts = Volts x Amps). For the most accurate measurement, especially for devices with fluctuating draws, use a plug-in watt meter to see the actual draw during operation.

What is the difference between running watts and surge watts?

Running watts (or continuous watts) is the power a device needs to stay on. Surge watts (or starting watts) is the extra power needed to start a motor or compressor. An inverter must be able to handle the surge, even if it only lasts for a fraction of a second. If the inverter cannot handle the surge, it will overload and shut down.

How much battery capacity do I need for a CPAP machine all night?

Multiply your CPAP’s actual power draw by the hours you sleep. Draw varies a lot by model and settings, and a heated humidifier can raise it considerably, so measure with a watt meter or check the manual. Always consult your device manufacturer and your doctor or medical supplier to ensure proper operation, and have a backup power plan in place.

Can I run a refrigerator on a solar generator?

Yes, but you must carefully size the generator for the fridge’s surge wattage. Startup surge varies widely between refrigerators, so check the manufacturer’s data or measure it. Ensure the inverter’s surge rating exceeds the fridge’s starting surge. For a detailed guide, see running a refrigerator on a solar generator.

Sizing a solar generator does not require an engineering degree, but it does require careful planning. By listing every device, calculating total watt-hours, and adding a realistic buffer for losses and surge power, you can confidently choose a portable power station that will meet your needs without overspending. Always verify your specific device requirements with the manufacturer’s documentation and prioritize safety in every connection. For a permanent home backup solution, never attempt to back-feed your home through a wall outlet. A licensed electrician must install a transfer switch or interlock and a power inlet for a safe and code-compliant connection.