Battery & storage

Battery size for 48 hour backup

Enter the loads you want backed up for two full days with zero recharge. Get a battery size built for extended outages.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your loads
Add up the running watts of everything you want powered for both days.
3 Battery characteristics

How it works

Three steps to a 48 hour battery size

Step 1

Total your loads

Add up running watts for everything you want backed up across both days. The 24 hour backup tool uses the same load math for a shorter window if you want to compare.

Step 2

Check whether 48 hours fits your region

Storm-prone regions with a history of multi-day outages are the most common reason to plan for 48 hours instead of 24. The storm season planning tool can help calibrate this to your area.

Step 3

Consider solar recharge instead of doubling capacity

If solar can recharge your battery during the day between two overnight periods, you may not need to double your battery size at all. The solar recharge sizing tool models this directly.

Understanding 48 hour backup sizing

Why this calculator assumes zero recharge

This tool doubles the raw energy math from a 24 hour target, assuming no solar or grid input across the entire 48 hour window. This is the most conservative possible planning assumption and will overstate the actual battery need for most households, since daylight hours in between the two nights typically provide at least some solar recharge if a solar array is connected, a distinction the Department of Energy's inverter basics guide explains as part of how grid-tied systems handle backup power. The solar recharge sizing tool models a more realistic scenario where daytime production offsets some of this need.

Why some households choose 48 hours over 24

Regions with a documented history of multi-day outages after major storms, or households where even a single additional day without power creates real hardship, often plan around a 48 hour target from the start rather than treating it as an upgrade later, consistent with outage-duration statistics published by the U.S. Energy Information Administration. The storm season planning tool and wildfire PSPS planning tool both help calibrate expected outage duration to your specific region.

Buying upfront versus expanding later

Doubling battery capacity from a 24 hour to a 48 hour target roughly doubles the battery portion of your system cost, so it's worth confirming with an installer whether purchasing all batteries together offers better pricing than adding a second unit later, a comparison the quick financing comparison tool in our Financing category can help frame. Inverter capacity and available panel space also factor into whether expansion later is straightforward or requires additional electrical work. The battery expansion planning tool covers this decision in more depth.

What to do with this number next

If 48 hours still feels short for your area's typical outage pattern, the 72 hour backup tool and 1 week backup tool extend the same math further. Otherwise, the battery inverter pairing tool is a useful next step to confirm your inverter can handle the resulting battery count. For the inverter sizing that supports this duration, the off-grid inverter sizing calculator in our Inverters & Equipment category is worth checking too.

Questions

Frequently asked

A 600 watt critical loads circuit running continuously for 48 hours needs about 34.8 kWh of nameplate battery capacity, or roughly 3 batteries at 13.5 kWh each. This doubles the battery need compared to a 24 hour target using the same load.

Regions with a history of multi-day outages after major storms, or households with limited tolerance for even a second day without power, often plan around 48 hours instead of the shorter 24 hour baseline. The storm season planning tool can help gauge which target fits your region's typical outage pattern.

Yes, significantly. If solar can recharge the battery during the daylight hours in between the two overnight periods, the battery only needs to bridge each individual night rather than storing two full days of energy upfront. The solar recharge sizing tool models this more realistic scenario.

This depends on installer pricing and whether your inverter has room for future expansion built in. Some installers offer better per-unit pricing when batteries are purchased together, while others charge a premium for return visits to add capacity later. The battery expansion planning tool walks through this tradeoff.

Related tools

Keep sizing your backup

24 hour backup 72 hour backup 1 week backup Solar recharge sizing Storm season backup planning Battery expansion planning Battery inverter pairing All battery & storage tools