Battery Backup Size Calculator
Pick what you want to keep running and for how long to get the battery capacity and inverter size you need.
Method: Watts × hours for each load, plus about 19% for inverter and depth-of-discharge losses; the inverter covers all loads at once plus the largest startup surge.
Battery and inverter
2.5 kWh · 900 W
A portable power station covers this.
| Energy used | 2,040 Wh |
| Battery to buy (with losses) | 2,500 Wh |
| Running load | 230 W |
| Largest startup | Refrigerator (+450 W) |
| Inverter (peak + 20%) | 900 W |
- • Capacity includes about 19% for inverter losses and keeping the battery from fully draining.
- • Loads are averaged over the hours they actually run. A fridge cycles on and off, so it uses far less than its startup rating.
What common loads use
| Appliance | Running watts | Typical hours a day |
|---|---|---|
| Refrigerator | 150 W | 8 |
| Chest or upright freezer | 100 W | 8 |
| Wi-Fi router + modem | 20 W | 24 |
| LED lights | 60 W | 6 |
| Phones and laptop charging | 60 W | 5 |
| TV | 120 W | 4 |
| CPAP machine | 50 W | 8 |
| Furnace fan | 400 W | 8 |
| Sump pump (⅓ HP) | 800 W | 2 |
How it works
Battery size is energy: watts times hours, added up over everything you want to run, then increased for about 19% of losses. The inverter has to handle everything running at once plus the single biggest startup surge. A battery that holds enough energy but has a small inverter will shut off when a pump or compressor starts.
More battery backup guides
Frequently asked questions
What size battery to run a refrigerator?
A refrigerator averages about 150 watts and uses roughly 1.2 kWh a day, so a 1.5 to 2 kWh power station keeps it going for a day.
Battery backup or generator?
A battery is silent, needs no fuel and can sit inside, but it holds a fixed amount of energy. A generator runs as long as it has fuel. Many households pair a battery with solar.
Why do I need a bigger inverter than my loads?
Motors such as refrigerators, sump pumps and well pumps draw two to three times their running watts for a moment when they start.