To size a solar generator, calculate four things: the energy your devices use in watt-hours (Wh), the watts (W) they draw when running together, the startup surge of motors or compressors, and the solar energy you can replace each day. Battery capacity follows from your energy target: divide load Wh by conversion efficiency and the fraction of battery capacity you plan to use. Then add a separate planning contingency if needed.
Key Takeaways
- Add the Wh needed for your chosen backup period. For cycling appliances such as refrigerators, use measured energy or an EnergyGuide estimate instead of assuming the compressor runs all day.
- Choose an inverter that covers simultaneous running watts and the relevant startup surge. Its W rating and the battery’s Wh rating answer different questions.
- Estimate battery nameplate Wh as load Wh ÷ (conversion efficiency × usable battery fraction). An 85% efficiency and 80% usable fraction give a combined factor of 0.68.
- Size panels from your location and season, expected system losses, and the station’s solar input limits. Compare daily solar harvest with daily load energy.
The Four Numbers to Calculate
Daily energy (Wh) determines battery runtime. Simultaneous running power (W) determines the inverter’s continuous rating. Startup surge (W) determines whether motors can start. Solar recovery (Wh per day) determines how quickly you can replace used energy. Work through them in that order before shopping.
Step 1: Calculate Daily Energy Use
List only the devices you expect to use during the outage, camping trip, or off-grid day. Record each device’s power and its hours of use. For a steady load, W × hours = Wh: a 60W fan used for 3 hours needs 180Wh. Add the Wh values for the full period you want to cover. For more than one day without dependable recharging, multiply your daily energy target by the number of days.
Build a Load Worksheet
- Record watts from a product label, manual, or reliable measurement. For a device with several operating modes, use the mode you plan to run.
- Record hours of operation, including short bursts as fractions of an hour. Ten minutes is about 0.17 hours.
- Mark which devices can run at the same time and which have a motor or compressor startup surge.
- For cycling loads, enter measured Wh for the entire period or a defensible average draw. Do not multiply a compressor’s on-cycle wattage by 24 hours unless it truly runs continuously.
Refrigerators and Other Variable Loads
A refrigerator, air conditioner, or pump may draw much more while running than it averages over a whole day. Use a plug-in energy meter when practical, or divide the refrigerator’s EnergyGuide annual kWh by 365 and multiply by 1,000 for a daily Wh planning figure. Actual use varies with room temperature, door openings, and settings. Check startup surge separately; the fridge energy figure does not establish the inverter size. My refrigerator backup guide compares runtime scenarios in more detail.
What if the Label Shows Volts and Amps?
For DC equipment, V × A gives W. For AC equipment, V × A gives volt-amps (VA), which may differ from real watts because of power factor. Prefer a watt rating or a reliable watt measurement for energy calculations. For motor loads and critical equipment, verify the operating and startup requirements with the device documentation or a qualified electrician.
Two Worked Energy Examples
These retained appliance figures illustrate the worksheet, not a promise about your particular devices. The refrigerator entries below assume an average draw of 90W across the stated period. Verify that average from a meter or a daily-energy estimate before using either total to buy a power station.
| Scenario | Appliance | Running/average W | Time | Energy |
|---|---|---|---|---|
| 3-hour | Refrigerator | 90W average assumed | 3h | 270Wh |
| 3-hour | Microwave | 1,000W input assumed | 0.17h | 170Wh |
| 3-hour | Lighting | 100W | 3h | 300Wh |
| 3-hour | Box fan | 60W | 3h | 180Wh |
| 24-hour | 50-inch LED TV | 150W | 4h | 600Wh |
| 24-hour | Refrigerator | 90W average assumed | 24h | 2,160Wh |
| 24-hour | Microwave | 1,000W input assumed | 0.5h | 500Wh |
| 24-hour | Desktop computer | 200W | 9h | 1,800Wh |
| 24-hour | Total | — | — | 5,060Wh |
The 3-hour example totals 920Wh: 270 + 170 + 300 + 180. The 24-hour example totals 5,060Wh: 600 + 2,160 + 500 + 1,800. If your refrigerator’s measured or label-derived energy differs, replace that row and recalculate the total and battery size. A daily-energy value already includes its duty cycle; do not apply a second cycling adjustment.
Step 2: Size the Inverter and Battery
Choose the Inverter’s Continuous and Surge Ratings
Add the running watts of devices that might operate at the same time, not every appliance you own. In the 3-hour example, the assumed simultaneous load is 90 + 1,000 + 100 + 60 = 1,250W. In the 24-hour example, it is 150 + 90 + 1,000 + 200 = 1,440W. A 1,500W continuous inverter barely exceeds those illustrative totals; allow room for uncertainty and confirm the exact microwave input and refrigerator startup behavior before selecting a model.
For surge, check the manufacturer’s startup specification or a suitable measurement. The inverter must handle the running devices plus the startup event that can occur while they are on. A broad extra wattage percentage does not substitute for this check. The pure versus modified sine wave inverter guide explains another compatibility factor for AC devices.
Similar: 3 Best 1,000W Solar Generators (Complete Analysis)
Calculate Required Battery Capacity
Required battery nameplate Wh = load Wh ÷ (conversion efficiency × usable battery fraction).
Suppose your AC conversion efficiency is 0.85 and you intend to use 80% of the battery’s nameplate capacity. Divide by 0.85 × 0.80 = 0.68. The 20% reserve is represented by the 0.80 usable fraction. It must not be added to a 15% conversion loss as a flat 35% markup.
| Scenario | Load energy | Battery before contingency | With optional 10% |
|---|---|---|---|
| 3-hour | 920Wh | 1,353Wh | 1,488Wh |
| 24-hour | 5,060Wh | 7,441Wh | 8,185Wh |
| Assumptions | 85% AC efficiency | 80% usable battery | 10% optional margin |
These are planning estimates under the stated assumptions. Actual usable capacity and AC efficiency vary by model, load, battery condition, temperature, and idle draw. Confirm the product’s output limits and usable energy; then round upward to a suitable capacity class. You can check a candidate with my portable power station runtime calculator. Battery cycle life also matters if you plan to discharge the system often.
A Simple Amp-Hour Example
My TalentCell LiFePO4 battery is rated at 12V and 24Ah. Multiplying those ratings gives 288Wh of nameplate energy. That conversion explains the label; it does not mean an AC appliance will receive all 288Wh after system losses.
Step 3: Size Solar Recharging
A first estimate is daily solar harvest (Wh) = panel array W × local peak-sun hours for the intended season × a stated system derate. Peak-sun hours change by location and season. Shade, panel angle, temperature, wiring, controller behavior, and weather affect the result. Your station’s solar voltage, current, and wattage limits also cap what an array can deliver; check panel open-circuit voltage in the cold conditions relevant to your setup.
For example, a 400W array with 4 peak-sun hours and a 0.75 derate yields about 1,200Wh per day (400 × 4 × 0.75). That is an estimate, not guaranteed output. If your daily load is 1,500Wh, the 300Wh daily deficit will drain the battery over time unless another charging source makes up the difference. My solar-panel sizing guide covers local sun, losses, and array planning; the charge-controller sizing guide covers voltage and current checks.
I have seen some portable panels deliver roughly 70–80% of their rated output in favorable sun with my own setup. That observation does not establish a universal derate for your location or season. The testing methodology explains how I distinguish firsthand measurements from sourced specifications.
Other Factors to Consider
- Ports: Check AC, USB, and regulated DC outputs against your actual devices.
- Expandability: Verify compatible batteries and required cables for the exact model and generation. Added batteries increase energy capacity; they do not automatically increase inverter output.
- Battery wear: Consider chemistry, rated cycle life, and how deeply you will discharge the battery.
- Waveform and electrical setup: Confirm the inverter and connection method suit sensitive equipment; follow the manufacturer’s instructions.
- Portability and recharge time: Larger batteries weigh and cost more, and a small solar input may take several days to refill them.
If you are choosing between brands after sizing the system, see my BLUETTI versus EcoFlow, Goal Zero versus Jackery, and BLUETTI versus Goal Zero comparisons for the model-level differences.
Can You Run a Solar Generator Continuously?
Possibly, if the exact power station supports the intended pass-through or EPS operation and incoming energy covers the loads plus conversion and charging losses. A pass-through feature alone cannot keep a battery full. If daily load energy exceeds daily recharge energy, the battery will eventually empty.
Check the manual for simultaneous charging and discharging limits, transfer behavior, input and output ratings, thermal limits, and any restrictions on critical equipment. A portable power station is not automatically a substitute for a dedicated home backup installation.
Quick Decision Checklist
- Choose the devices and hours of backup, then total their Wh.
- Add the running W of devices used together and verify the largest relevant startup surge.
- Divide load Wh by efficiency × usable fraction; apply an optional contingency only afterward.
- Estimate local seasonal solar harvest and compare it with daily energy use.
- Check the exact power station’s ports, inverter ratings, solar input limits, compatible batteries, operating guidance, and weight.
Frequently Asked Questions
How many watts should my solar generator be?
Choose continuous inverter output above the watts your devices may draw together, then verify the inverter can handle the relevant startup surge. Separately size the battery in Wh for your runtime target.
How many watt-hours do I need for an outage?
Add the Wh your essential loads use during the planned outage, then divide by conversion efficiency × usable battery fraction. At 85% and 80%, respectively, 920Wh of AC load calls for about 1,353Wh of nameplate battery capacity before extra contingency.
How do I size a solar generator for a refrigerator?
Use measured daily energy or its EnergyGuide annual kWh ÷ 365 to estimate Wh. Confirm compressor startup surge separately and account for other devices that may run at the same time.
How much solar do I need?
Divide the daily Wh you need to replace by local seasonal peak-sun hours and a realistic derate, then check the station’s solar voltage, current, and wattage limits. For 1,200Wh per day, 4 peak-sun hours, and a 0.75 derate, a 400W array is the starting calculation.
Is a bigger system always better?
More battery capacity extends runtime, but it also adds cost and weight. Choose enough continuous W, surge W, Wh, and solar recovery for your verified loads, then add a reasonable planning margin.
Similar: Can You Charge an Electric Car With a Solar Generator?
Final Answer
Start with your load worksheet. Use the same numbers to check simultaneous running watts, startup surge, battery Wh after efficiency and usable-capacity losses, and daily solar recovery. The right size is the system that covers all four requirements for your intended use.
Source and method note: The example appliance figures are illustrative. Use labels, manuals, EnergyGuide data, or measurements for your equipment; the battery and solar results show their assumptions. Site testing observations are identified separately from planning estimates.
Below I have some comparisons of solar generators from different brands. These companies have models in several different sizes, so you can review what size is ideal for you:
- Bluetti vs. EcoFlow – Collective Solar Generator Deep Dive
- Goal Zero vs. Jackery – Battle of the Brands (Cost/Specs/Analysis)
- Bluetti vs. Goal Zero – Solar Generator Matchup (In-Depth)