Lauren HollandBy Lauren Holland

Updated November 2026

A home backup power supply keeps the essentials running when the grid goes down, but only if it is sized correctly. In this guide you will total the watt-hours your critical loads need, add a realistic safety margin, and match that number to a battery size and charging method. By the end, you will have a working figure you can use to compare units with confidence.

Start With What You Need to Keep Running

Most people buy a backup unit based on brand reputation or a number on the box, then discover mid-outage that it cannot run the refrigerator overnight. The better approach is to list the loads you actually care about first. For most households, that means the refrigerator or freezer, a CPAP machine, a sump pump, a router and modem, a few lights, and phone charging. Anything you would not want to lose during a multi-day outage belongs on the list.

Shopping for one? See our guide: Top 5 EcoFlow Power Stations with Solar Panel Options for Home Backup (2026).

Be selective. Electric dryers, electric water heaters, central air conditioning, and electric ranges draw far more power than a portable battery can practically supply, and they are usually better handled by a whole-home system or a generator. Keeping the list to true essentials is what makes a portable home backup power supply affordable and workable.

Gather the Nameplate Data

Each appliance has a label, usually on the back or inside the door, listing its voltage (V) and amperage (A) or its wattage (W). If you only see amps, multiply volts by amps to get watts. Some labels list running watts and starting watts separately. Motors, including refrigerator compressors and sump pump motors, draw a brief surge at startup, often two to three times their running wattage. Note both numbers.

Calculate Your Watt-Hour Requirement

Battery capacity is measured in watt-hours (Wh), so you need to convert each appliance into energy used over time. The formula is simple:

Daily watt-hours = Running watts × Hours of use per day

Refrigerators cycle on and off, so their real consumption is much lower than their running wattage multiplied by 24 hours. A common approach is to use the manufacturer’s stated annual energy use (kWh per year) divided by 365 to get a daily figure, or to estimate roughly 30 to 50 percent duty cycle for compressor appliances. Check the spec sheet before relying on an estimate.

Appliance Typical Running Watts Estimated Hours Per Day Approximate Daily Wh
Refrigerator (compact) 80 to 150 W running 8 to 12 (cycling) 600 to 1,200 Wh
CPAP machine 30 to 60 W 8 240 to 480 Wh
Sump pump (1/3 HP) 500 to 800 W running 0.5 to 1 (intermittent) 250 to 800 Wh
Wi-Fi router and modem 10 to 25 W 24 240 to 600 Wh
LED lights (5 fixtures) 40 to 60 W total 5 200 to 300 Wh
Phone charging 20 W per charge 2 40 Wh

These ranges are illustrative. Your own appliances will vary, so use the nameplate and spec sheet values wherever you can. Treat the table as a starting framework, not a final answer.

Add Up Your Daily Total

Sum the daily watt-hours for every item on your list. Suppose the refrigerator needs 900 Wh, the CPAP needs 360 Wh, the router needs 400 Wh, the lights need 250 Wh, and phone charging needs 40 Wh. That is 1,950 Wh per day.

Apply a Safety Margin

Batteries do not deliver 100 percent of their rated capacity. Inverter conversion, heat, and battery aging all take a share. Manufacturers often specify usable capacity below nominal capacity, and inverter efficiency commonly falls somewhere in the range of 80 to 90 percent. A practical rule is to multiply your daily total by 1.25 to 1.4 to account for these losses and a buffer for unexpected use. Using the example above, 1,950 Wh × 1.3 gives roughly 2,535 Wh, so you would look for a unit with comfortably more than 2,500 Wh of usable capacity for one day of coverage.

Decide How Many Days You Need to Cover

Your target outage length changes the math. Many households plan for one to two days, which covers most storm-related interruptions. Others, especially those relying on medical equipment in areas with long winter or hurricane outages, plan for three to five days. Multiply your adjusted daily figure by the number of days you want to cover.

Keep in mind that a single unit may not carry multi-day loads alone. You may need to conserve, run the refrigerator only part of the time, or recharge the battery during the day.

Plan How to Recharge the Battery

A battery is only half the system. Recharging determines how long you can run outages without fuel or grid power. Common options include:

  • Wall outlet charging: Fastest when grid power is available, typically several hours from empty to full on larger units.
  • Solar input: Panels feed the battery directly. Output depends on panel wattage, sun hours, and weather. Many owners report that a few panels can add a meaningful amount of energy per sunny day, but overcast weather can cut that substantially.
  • Car charging: Some units accept 12V DC input, useful for short top-ups but slow.

For multi-day outages, pairing battery capacity with solar input is the most common strategy. Check the maximum solar input wattage a unit accepts and make sure your panels fit within it. If you are adding panels, the full sizing process is covered in the Solar Power Backup System for Home: A Step-by-Step Sizing Guide (2026).

Check Output, Inverter, and Startup Surge

Capacity alone does not guarantee the unit will run your appliances. Check three more specifications:

  1. Continuous AC output (watts): The sum of your simultaneously running loads must stay below this figure. If your sump pump and refrigerator both run at once, add their running watts together.
  2. Surge rating: Motors need a brief surge to start. Make sure the unit’s peak or surge rating exceeds the highest starting wattage on your list.
  3. Waveform: Pure sine wave output is generally recommended for CPAP machines, medical devices, and modern electronics. Modified sine wave can cause problems with some motors and sensitive equipment.

Confirm Outlet and Transfer Options

Decide how you will connect. Plugging appliances directly into the unit’s outlets is simplest. Connecting the system to your home’s circuits through a transfer switch requires a licensed electrician and must comply with local electrical codes. Never backfeed a home circuit through a standard outlet, because it can endanger utility workers and damage equipment. A licensed electrician should handle any hardwired installation.

Common Mistakes to Avoid

  • Using rated capacity as usable capacity. Always apply an efficiency and safety margin.
  • Ignoring startup surge. A sump pump that runs fine on paper can trip an undersized inverter at startup.
  • Forgetting always-on loads. Routers, smoke detectors, and medical device chargers add up over 24 hours.
  • Assuming solar input will be available. Plan for cloudy days and reduced output in winter.
  • Overlooking battery storage conditions. Extreme heat or cold reduces battery life. Follow manufacturer storage and temperature guidance.
  • Skipping a test run. Run the full load for a few hours before you need it during a real outage.

Tips for a Reliable Setup

  • Keep a written list of your loads, their watts, and daily hours next to the unit.
  • Prioritize loads: during an extended outage, you may choose to power the CPAP and refrigerator first and delay everything else.
  • Charge the battery fully before storms are forecast.
  • Label outlets or cords so anyone in the household can operate the system.
  • If you rely on medical equipment, tell your utility company and your medical provider about your backup plan, and ask about medical priority programs where available.
  • EcoFlow Delta 2 Portable Power Station: A portable battery unit often considered for home emergency use because it pairs a mid-size battery with solar input options. View on Amazon
  • Jackery Explorer 1000 V2 Portable Power Station: A common pick for households needing a moderate daily energy budget with pure sine wave output. View on Amazon
  • Kill A Watt Electricity Usage Monitor: A plug-in meter that shows the real wattage and daily energy use of each appliance, so your calculations rest on measured data. View on Amazon

FAQ

How many watt-hours do I need for a home backup power supply?

It depends on your critical loads. Add up the daily watt-hours of each appliance you plan to run, then multiply by 1.25 to 1.4 for efficiency losses and a safety buffer. Multiply that by the number of days you want to cover.

Can a portable unit power a whole house?

Usually not for a full home, especially if you have central air conditioning, electric heat, or an electric range. Portable units are generally best for essential circuits. Whole-home backup typically requires a larger battery system or a generator.

How long will a refrigerator run on a backup battery?

It depends on the refrigerator’s efficiency, the battery’s usable capacity, and how often the compressor cycles. Use the appliance’s annual energy rating divided by 365 to estimate daily use, then divide the usable battery capacity by that figure.

Is a pure sine wave inverter necessary?

For most modern electronics and medical devices like CPAP machines, pure sine wave output is recommended by many manufacturers. Modified sine wave may work for simple resistive loads but can cause problems with motors and sensitive electronics.

Can I recharge the battery with solar panels during an outage?

Yes, if the unit accepts solar input and you have panels within its input limits. Output depends on sunlight, panel wattage, and weather, so solar charging supplements rather than guarantees full recharges.

Do I need an electrician to connect a backup system to my home?

Plugging appliances directly into a unit’s outlets does not require an electrician. Hardwired connections, transfer switches, and any link to your home’s circuits should be installed by a licensed electrician to meet local codes and protect utility workers.

How long do backup batteries last?

Lifespan depends on chemistry, cycle count, depth of discharge, and storage temperature. Consult the manufacturer’s specifications for rated cycle life, and store the unit within the temperature range it recommends.

Conclusion

A home backup power supply works best when it is sized from your actual loads rather than a marketing figure. List your essentials, convert each to daily watt-hours, add a safety margin, and confirm output, surge, and recharging options. Once those numbers line up, you can compare units with confidence. If your household includes medical equipment, consult your healthcare provider and a licensed electrician before finalizing your plan.

Ready to choose one?

Lauren Holland

About Lauren Holland

Lauren Holland is part of our editorial team and reviews and edits our solar generators for home backup & emergencies guides, checking product details against manufacturer specifications and owner feedback.