Two Powerwalls (27 kWh) Backup Runtime at 800W Load
Two Tesla Powerwalls (27 kWh combined usable capacity) will provide approximately 33.75 hours at an 800W load.
How to use this tool
- Enter the battery's usable capacity in kWh (from its spec sheet).
- Estimate your average backed-up load in watts during an outage.
- Read the backup runtime in hours.
- Check the runtime in days for multi-day outage planning.
- Lower the load (back up fewer circuits) to extend runtime.
See how many days two stacked Powerwalls will keep your home running during a power outage at a moderate 800W load.
Frequently asked questions
- How long will a 10 kWh battery last?
- It depends entirely on your load. Backing up a 500 W essentials load (fridge, lights, Wi-Fi), a 10 kWh usable battery lasts about 20 hours. A 1,000 W load halves that to 10 hours.
- Should I use nameplate or usable capacity?
- Use usable capacity. Batteries hold back a reserve to protect the cells, so the deliverable energy is less than the nameplate rating. The spec sheet usually lists both.
- What's a typical essentials load during an outage?
- A refrigerator, LED lighting, phone/laptop charging, and internet equipment often average 300–600 watts combined, since the fridge compressor only runs part of the time. Adding well pumps, AC, or electric heat raises it sharply.