ThisCalculator

Everyday

Home Battery Backup Sizing for Essential Appliances

Updated · Standard formula, shown below

Home Battery Backup Sizing for Essential Appliances

This guide helps you determine the right size (in kilowatt-hours, kWh) for a home battery backup system that will keep your essential appliances running during an outage. You’ll list your appliances, calculate their daily energy use, and factor in your outage goals to get a target battery capacity. Before you start, gather a list of the appliances you want to power and their wattage ratings (usually on a label or in the manual).

What You’ll Need

  • List of essential appliances (fridge, lights, internet router, medical devices, etc.)
  • Wattage rating for each appliance (from the label or manual)
  • Estimate of how many hours per day each appliance runs during an outage
  • Calculator or phone
  • Notepad and pen

Step-by-Step

  1. List your essential appliances. Write down every appliance you want to keep running during an outage. Common ones: refrigerator, freezer, lights, Wi-Fi router, laptop charger, sump pump, well pump, CPAP, electric blanket, and a few outlets for phone charging.
  2. Find the wattage for each appliance. Look for a label that says “wattsâ€� or “Wâ€�. If you only see amps (A) and volts (V), multiply them: watts = amps × volts. For example, a fridge might draw 6.5 A at 120 V, so 6.5 × 120 = 780 W. If the label shows only “kWh/yearâ€�, divide by 365 to get daily kWh, then divide by 24 to get average watts (but that’s an average, not the running draw – it’s okay for estimation).
  3. Estimate daily run time for each appliance. For each item, decide how many hours per day it will run during an outage. A refrigerator runs about 8 hours per day (compressor cycles), lights might run 5 hours, a router runs 24 hours, a sump pump might run 1 hour if it’s rainy. Be realistic.
  4. Calculate daily energy use for each appliance. Multiply watts by hours to get watt-hours (Wh) per day. For the fridge: 780 W × 8 h = 6,240 Wh. For lights: 60 W × 5 h = 300 Wh. For router: 10 W × 24 h = 240 Wh.
  5. Add up all daily watt-hours. Sum every appliance’s Wh. For example: 6,240 + 300 + 240 + 500 (laptop) + 1,200 (sump pump) = 8,480 Wh.
  6. Convert to kilowatt-hours. Divide by 1,000: 8,480 Wh ÷ 1,000 = 8.48 kWh. This is your daily energy need.
  7. Decide how many days of backup you want. Multiply your daily kWh by the number of days. For 2 days: 8.48 × 2 = 16.96 kWh. For 3 days: 25.44 kWh.
  8. Account for battery depth of discharge (DoD). Most lithium batteries shouldn’t be discharged below 20% (DoD 80%). Divide your target by 0.8. For 2 days: 16.96 ÷ 0.8 = 21.2 kWh. For 3 days: 25.44 ÷ 0.8 = 31.8 kWh. This is the rated battery capacity you need.
  9. Factor in inverter efficiency. Inverters lose about 10% when converting DC to AC. Multiply by 1.1. For 2 days: 21.2 × 1.1 = 23.3 kWh. For 3 days: 31.8 × 1.1 = 35.0 kWh. This is your final target.
  10. Round up to a standard battery size. Batteries come in sizes like 10 kWh, 13.5 kWh, 15 kWh, 20 kWh, 30 kWh. Choose the next size up from your target. For 23.3 kWh, a 25 kWh or 30 kWh system works. For 35 kWh, go for 40 kWh.

Common Mistakes

  • Using starting watts instead of running watts. Motors (fridge, sump pump) draw several times more power when starting. If you size only for running watts, the battery might trip. Use a surge rating for the inverter, but for battery capacity, running watts are fine.
  • Forgetting to include the inverter’s own consumption. Most inverters use 20-50 W just being on. That adds up over 24 hours: 30 W × 24 = 720 Wh/day. Add that to your total.
  • Underestimating run time. People assume the fridge runs 24/7, but it cycles. However, during a hot day or if you open it often, it runs more. Use 10 hours to be safe.
  • Ignoring battery degradation. Batteries lose capacity over time. If you plan to keep the system for 10 years, add 20% extra capacity now.
  • Not accounting for solar recharge. If you have solar panels, you can recharge the battery during the day, so you might need less capacity. But if the outage is at night or cloudy, you still need enough for a full day.

Tips

  • Use a watt meter. A plug-in watt meter (like a Kill A Watt) gives you actual draw. Measure over a day to get real daily usage.
  • Prioritize loads. You don’t need to power everything. Focus on fridge, medical devices, and lights. Add comfort items only if budget allows.
  • Consider a subpanel. Have an electrician install a critical loads subpanel that only feeds your essential circuits. That way, the battery doesn’t try to power the whole house.
  • Think about surge capacity. Even if your daily energy is low, the inverter must handle the starting surge of your largest motor. Check the inverter’s peak power rating.
  • Plan for heating/cooling. Electric heaters and AC units draw 1,500-5,000 W. They will drain a battery fast. If you need heat, consider a gas furnace or space heater with a lower setting.

Troubleshooting

  • My battery runs out before expected. Check if any appliance is running longer than you estimated. Use a watt meter to verify. Also, check if the inverter is drawing more than expected. Lower the thermostat on the fridge, unplug unused devices.
  • My fridge won’t start. The inverter might not have enough surge capacity. Check the inverter’s peak watt rating. If it’s too low, you need a larger inverter, not a larger battery.
  • The battery never fully charges. If you have solar, the panels might be undersized. If grid charging, the charger might be too small. Check the charge rate in the manual.
  • I calculated 20 kWh but the system gives me only 16 kWh usable. That’s normal – the rated capacity is higher than usable due to DoD. Always use the usable capacity when planning.
  • My sump pump runs constantly during a storm. That can double your daily usage. Add extra capacity or consider a higher-capacity pump that runs less often.

Frequently Asked Questions

How many kWh do I need to run a refrigerator?

A typical refrigerator uses about 1.5-2 kWh per day. That’s roughly 150-200 W running for 8-10 hours. For a 2-day backup, you’d need 3-4 kWh just for the fridge.

Can I run my whole house on a battery backup?

Yes, but it would require a very large battery (50-100 kWh) and a large inverter. Most people choose to power only essential circuits to keep costs manageable.

How long can a 10 kWh battery run a house?

It depends on usage. If you use 2 kWh per day (fridge, lights, router), it lasts 5 days. If you run an AC (3,500 W), it lasts less than 3 hours.

Do I need a generator instead of a battery?

Batteries are quiet, clean, and automatic, but they have limited duration. Generators can run for days but require fuel and maintenance. Many people use a battery for short outages and a generator for longer ones.

How do I know if my appliances are 120V or 240V?

Large appliances like electric dryers, ovens, and well pumps often use 240V. Check the plug – a 240V plug is larger and has 4 prongs. Your battery system must support 240V if you need those.

Can I add more batteries later?

Most modular systems allow expansion, but check the manufacturer’s limits. Plan for future expansion by choosing a compatible system.

Related

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.