What Size Solar Generator Do I Need?
Calculate the inverter output, startup surge, battery watt-hours and solar-panel input needed to run your essential appliances during a power outage.
The Four Numbers That Determine Solar Generator Size
In This Guide
Solar Generator Size Recommendations
These ranges are starting points. Actual size depends on measured appliance power, required runtime and whether the battery can be recharged during the outage.
Small Essentials
- Phones and radios
- LED lights
- Router or modem
- Short CPAP support
- Small electronics
Basic Outage Kit
- Refrigerator for limited time
- Lighting and internet
- Medical devices
- Fans
- Brief small-appliance use
Household Essentials
- Refrigerator and freezer
- Multiple rooms of lighting
- Communications
- Selected cooking loads
- Longer overnight reserve
Selected Home Circuits
- Multiple essential circuits
- Well or sump pump
- Furnace blower
- More daily cooking
- Multi-day expansion
The Solar Generator Sizing Formulas
Use separate calculations for inverter output, battery capacity and solar recovery.
1. Continuous Output
Required inverter watts = simultaneous running watts × safety marginA 20% to 25% margin helps avoid operating continuously at the inverter’s maximum rating.
2. Battery Capacity
Required battery Wh = daily appliance Wh ÷ usable efficiencyUsing 80% to 85% usable efficiency accounts for inverter loss, standby consumption and reserve.
3. Solar Panel Array
Panel watts = daily Wh ÷ (peak sun hours × solar efficiency)A planning efficiency of about 70% to 80% helps account for heat, angle, cable and charging losses.
Watts vs. Watt-Hours
Watts measure the rate at which an appliance uses power. Watt-hours measure how much energy it uses over time. A 100-watt load running for five hours consumes 500 watt-hours.
Surge Watts
Startup surge is not added to daily energy use. It is compared with the inverter’s brief surge rating to determine whether the appliance can start without overloading the system.
Solar Generator Size Calculator
Add each appliance, its running watts, hours used per day and startup surge. Use measured values whenever possible.
Build Your Emergency Load
The result is a planning estimate. Verify labels, manuals or watt-meter measurements before purchasing.
| Appliance | Running Watts | Hours/Day | Startup Surge | Remove |
|---|---|---|---|---|
Typical Appliance Wattage Ranges
These are broad planning ranges—not substitutes for the actual label, manual or watt-meter reading.
| Appliance | Typical Running Range | Startup Consideration | Best Planning Method |
|---|---|---|---|
| Refrigerator | About 100–300W while compressor runs | Can require several times running watts briefly | Measure with a watt meter over 24 hours and verify surge |
| Chest freezer | About 80–250W while compressor runs | Compressor surge | Measure cycling energy and startup peak |
| LED light bulb | About 5–15W each | Minimal | Add all bulbs used at the same time |
| Router and modem | About 10–35W combined | Minimal | Read adapters or measure continuously |
| CPAP | Often about 30–100W | Heated humidifier increases use | Use the medical-device manufacturer’s battery guidance |
| Television | About 50–200W | Usually modest | Check label or measure normal viewing mode |
| Microwave | About 1,000–1,800W input | High short-duration load | Use input watts, not advertised cooking watts |
| Coffee maker | About 800–1,500W | Usually low surge but high heat load | Multiply watts by brief daily runtime |
| Electric space heater | Often 750–1,500W | Large continuous drain | Avoid relying on portable batteries for prolonged resistance heating |
| Furnace blower | About 300–1,000W | Motor startup surge | Verify system voltage, controls and professional connection method |
| Sump pump | About 500–1,500W | Potentially large motor surge | Use pump nameplate and measured startup |
| Well pump | Varies widely; often 1,000W+ | High surge and possibly 240V | Use motor data and a qualified electrician or pump professional |
Solar Generator Sizing Examples
These examples show the calculation process using assumed values. Replace them with your measured loads.
Example 1: Refrigerator, Lights and Internet
- Refrigerator: 150W × 8 hours = 1,200Wh
- Lights: 40W × 6 hours = 240Wh
- Router: 25W × 12 hours = 300Wh
At 85% usable efficiency, battery size is about 2,047Wh. A 2–3kWh unit with enough surge capacity is the practical range.
Example 2: CPAP, Lights and Phone Charging
- CPAP: 60W × 8 hours = 480Wh
- Lights: 20W × 5 hours = 100Wh
- Phones: 30Wh total
At 80% usable efficiency, battery size is about 763Wh. A 1kWh unit provides a reasonable margin, subject to the CPAP manufacturer’s requirements.
Example 3: Refrigerator, Freezer and Sump Pump
- Refrigerator: 1,200Wh/day
- Freezer: 900Wh/day
- Sump pump: 800W × 1 hour total = 800Wh
At 80% usable efficiency, battery size is about 3,625Wh. The inverter must also handle the highest motor startup surge and any simultaneous loads.
How Many Solar Panels Do You Need?
Panel sizing should replace the energy used each day—not merely match the power station’s battery capacity.
Example Solar Calculation
A household using 1,800Wh per day with four peak sun hours and 75% system efficiency needs approximately:
1,800Wh ÷ (4 × 0.75) = 600 watts of solar panels.
Why Real Output Is Lower Than Panel Rating
Clouds, haze, smoke, winter sun angle, high panel temperature, shade, cable loss, dirt and charger conversion reduce production. A 600W array may deliver much less than 600W for substantial portions of the day.
Check Electrical Limits
The array’s open-circuit voltage, operating voltage, current and total wattage must remain within the power station’s PV-input limits. Series connections increase voltage; parallel connections increase current.
Plan for Poor Weather
Solar panels may produce little energy during the storm that caused the outage. Keep enough battery reserve, an alternate charging method or a load-reduction plan for several poor-sun days.
Sizing for Refrigerators, Pumps and Medical Equipment
Refrigerators and Freezers
Measure 24-hour energy and startup surge. Room temperature, door openings and thermostat settings change the result.
Well and Sump Pumps
Pumps can have large startup surges and may use 240V. Confirm motor data and professional connection requirements.
CPAP and Medical Devices
Use the device manufacturer’s approved battery guidance and include humidifier, oxygen, alarm and nighttime reserve requirements.
Furnace Blowers
Confirm voltage, motor startup, controls and transfer method. A battery cannot safely power a hardwired furnace through an improvised cord.
Electric Heating
Resistance heaters consume energy rapidly. Use safe-room insulation and non-electric emergency heat planning rather than assuming a battery will heat the house all night.
Cooking Appliances
Microwaves, kettles and hot plates draw high power but may run briefly. Include their input watts and realistic daily minutes.
Sizing a Solar Generator for Home Circuits
Selected-circuit backup requires more than a large portable battery.
Choose the Circuits
List the refrigerator, freezer, furnace, well pump, sump pump, lights, outlets and medical equipment that must remain available. Exclude central air conditioning, electric water heating, electric ranges and other large loads unless the system is specifically designed for them.
Confirm Voltage
Many portable power stations provide only 120V. Well pumps, dryers, ranges and some HVAC equipment require 240V. A 120V unit cannot power a 240V load without compatible system hardware.
Use Transfer Equipment
A qualified electrician should install and verify any inlet box, manual transfer switch, interlock or smart home panel. Never connect a portable power source to a wall outlet to energize home wiring.
Allow Expansion
An expandable system can start with essential circuits and add battery capacity later. Confirm expansion-battery availability, compatibility and maximum supported capacity before buying.
Common Solar Generator Sizing Mistakes
Buying by Watt-Hours Alone
The inverter may still be too small to start a motor or run simultaneous loads.
Ignoring Startup Surge
Compressors and pumps may overload a system that appears large enough based on running watts.
Assuming Rated Solar All Day
Panel rating is measured under standardized conditions, not storm clouds, winter shade or poor orientation.
Using 100% Battery Capacity
Conversion losses, standby draw and reserve mean the full label capacity is not available at AC outlets.
Using Appliance Watts as Hours
Energy equals watts multiplied by time. A 1,500W kettle used for six minutes consumes about 150Wh, not 1,500Wh.
Ignoring Connection Requirements
Hardwired loads and home circuits need approved transfer equipment—not improvised backfeeding.
Solar Generator Sizing Checklist
Complete this list before selecting a model or panel bundle.
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Use the calculator first, then compare power stations that meet the output, surge, capacity and solar-input requirements.
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Solar Generator Sizing FAQ
What size solar generator do I need for a refrigerator?
Many households start with a 1–2kWh battery and an inverter that can handle the refrigerator compressor’s startup surge. Measure 24-hour energy and startup power because models vary widely.
Is a 2,000-watt solar generator enough for a house?
A 2,000-watt inverter can run many essential plug-in appliances, but the battery capacity determines runtime and the system may not support 240V appliances or all household circuits.
How many watt-hours do I need for a power outage?
Add each appliance’s watts multiplied by hours of use, multiply by the desired days, then divide by expected usable efficiency. Many essential household plans fall between 2,000Wh and 4,000Wh per day.
How much larger should the inverter be than the load?
A common planning approach is to add a 20% to 25% margin above simultaneous running watts, while separately ensuring the surge rating covers motor startup.
How many solar panels are needed for a 2,000Wh battery?
It depends on how much energy must be replaced and the available sun. Replacing 2,000Wh with four peak sun hours at 75% system efficiency requires roughly 670 watts of panels.
Can a solar generator power a well pump?
Possibly, but many well pumps require 240V and high startup surge. Use the pump motor data and consult a qualified electrician or pump professional before selecting a system.
Should I use the appliance’s maximum watts or average watts?
Use simultaneous running watts for inverter sizing, maximum startup surge for surge sizing and average energy over time for battery-capacity sizing.
Can I connect a solar generator to my house through an outlet?
No. Never backfeed a home through a receptacle. Use direct plug-in loads or compatible transfer equipment installed by a qualified electrician.
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