Battery & storage

Battery storage size calculator

Enter the loads you want backed up, how many hours you want to cover, and your battery's depth of discharge and efficiency. Get a real kWh number in seconds.

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 at once.
How long you want power to last during an outage.
3 Battery characteristics
Most manufacturers publish this figure directly on the spec sheet.
Accounts for conversion loss charging and discharging the battery.

How it works

Three steps to a battery size

Step 1

Add up your loads

List the running watts of everything you want backed up at once. If you only want to cover essentials, the critical loads calculator walks through the typical short list appliance by appliance.

Step 2

Decide how many hours you need

Short outages usually run a few hours, but storm-prone regions often plan for a full day or more. The 24 hour backup tool and storm season planning tool both start from a fixed duration.

Step 3

Apply depth of discharge and efficiency

Your nameplate capacity needs to be larger than your usable energy need, since depth of discharge and round trip efficiency both take a cut. The usable vs nameplate calculator breaks this down further if you want to see each factor separately.

Understanding your battery size

Why this number is an estimate, not a quote

This calculator works from three inputs: how much power your loads draw, how long you want them running, and how much of your battery's rated capacity you can actually use. Real installations also depend on inverter sizing, panel compatibility, and local code requirements that vary by installer and jurisdiction, a distinction the Department of Energy's inverter basics guide covers on the hardware side, and the hybrid inverter sizing calculator in our Inverters & Equipment category covers directly. Use this number to compare against an installer's quote, not as a replacement for one.

Why nameplate and usable capacity are different numbers

A battery rated at 13.5 kWh nameplate does not deliver 13.5 kWh of usable energy. Depth of discharge limits, typically 90 to 100% for modern lithium batteries, and round trip efficiency losses of 8 to 12% both reduce the real number, figures consistent with NREL's residential battery storage performance data. A battery with 90% depth of discharge and 92% round trip efficiency delivers roughly 11.2 kWh of usable energy from that same 13.5 kWh nameplate rating. Check the depth of discharge calculator if you want to isolate just that one factor.

How many batteries you actually end up buying

Home batteries are sold in fixed nameplate sizes, most commonly around 10 to 13.5 kWh per unit, so your calculated need almost always rounds up to a whole number of units. A calculated need of 21.7 kWh nameplate means buying two 13.5 kWh batteries for 27 kWh total, not a custom-sized single unit. The modular battery stack tool covers how manufacturers handle stacking multiple units together. The hybrid inverter sizing calculator checks whether your inverter can handle this result.

Critical loads versus whole home coverage

Deciding what to back up changes this number more than any other input. A fridge, some lighting, and internet together might draw under 1 kW, while whole home backup including central air conditioning can draw 3 to 5 kW or more during compressor startup. The whole home sizing tool and HVAC backup tool both dig into higher-draw scenarios specifically.

What to do with this number next

Take your battery size to the battery inverter pairing tool to make sure your inverter can actually deliver that power, or to the cost per kWh comparison tool to see what different battery brands charge for the capacity you need. If an EV charger is part of your household load, the EV battery size vs solar panel count tool in our Solar + EV Integration category covers that added sizing question.

Questions

Frequently asked

Most homes covering critical loads need 10 to 20 kWh of battery storage, while whole home backup often needs 25 to 40 kWh. A home running 1.5 kW of critical loads for 12 hours needs about 21.7 kWh of nameplate capacity after accounting for depth of discharge and round trip efficiency losses.

Depth of discharge is the percentage of a battery's capacity you can safely use before recharging. Most lithium home batteries allow 90 to 100% depth of discharge, but manufacturers often recommend stopping around 90 to 95% to preserve battery lifespan over thousands of cycles.

Round trip efficiency losses, typically 8 to 12% for lithium batteries, mean some energy is lost converting power in and out of storage. A battery rated for 20 kWh nameplate might only deliver 17 to 18 kWh of usable energy once depth of discharge and efficiency losses are both applied.

Critical loads only, like a fridge, some lighting, and internet, typically need one battery under 15 kWh. Whole home backup including HVAC usually needs 2 to 3 batteries stacked together, since air conditioning alone can draw 3 to 5 kW.

Related tools

Keep sizing your battery

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