How to Size a Home Battery Using Simple Math (Without Overthinking It)

Sizing a solar system becomes fairly intuitive once you understand annual kilowatt-hours, sun hours, and simple shortcuts like the Phi method. Battery sizing, however, is a different conversation altogether. Batteries are not about how much energy you produce over a year. They are about when you use energy and how you want to manage the most expensive or most critical hours of the day.

For most homeowners, the single most important driver of battery sizing is Time-of-Use pricing. In many utility territories, the highest rates fall between roughly 4 pm and 9 pm, though the exact window depends on your utility and rate plan. This guide focuses first on battery sizing for TOU savings, then explains how backup power changes the math.

A simple and surprisingly effective starting point is to look at your annual energy usage and smooth it out into an average. The process looks like this. Take your total annual kilowatt-hours and divide by 365 to get average daily usage. Then divide that number by 24 to estimate your average hourly consumption.

This averaging approach intentionally smooths out spikes from air conditioning, cooking, or short bursts of high usage. While some utilities provide fifteen-minute or hourly interval data, starting with an average avoids overcomplicating the process and gives you a realistic baseline for most homes.

Once you know your average hourly usage, estimating peak window consumption becomes straightforward. Multiply your average hourly usage by the number of peak hours you want to cover. For many homeowners, that is a five-hour window in the evening.

As an example, if your average hourly usage is 1.5 kilowatt-hours, a five-hour peak window equates to about 7.5 kilowatt-hours of energy. Since most residential batteries start around ten kilowatt-hours, this immediately explains why a ten kilowatt-hour battery is often considered the practical minimum for meaningful TOU savings.

Below is a simple reference chart using this averaging method with a five-hour peak window. It is designed to give homeowners a quick gut check before moving into detailed modeling.

Annual Usage (kWh/year)	Avg Hourly Usage (kWh/hr)	Peak Window (5 hrs) kWh	Typical Battery Size
6,000	0.7	3.5	~10 kWh (minimum class)
9,000	1.0	5.0	10–13.5 kWh
12,000	1.4	7.0	~13.5 kWh
15,000	1.7	8.5	13.5–20 kWh
18,000	2.0	10.0	20+ kWh or two batteries

The chart intentionally leans conservative. Batteries perform best when they are not being fully drained every single day. Having a bit of buffer improves longevity, flexibility, and day-to-day comfort.

This also explains why battery capacities around 13.5 kilowatt-hours are so common. Companies like Tesla have access to enormous amounts of real-world household energy data, and their battery sizing reflects what works well for a large percentage of homes. For many households, a battery in this range aligns closely with evening usage during peak TOU hours.

For homeowners primarily focused on bill savings, a single battery in this size class is often enough to offset the most expensive part of the day without overspending. It is not meant to power everything indefinitely. It is meant to handle the hours that matter most financially.

The math changes once backup power becomes a priority. During an outage, the battery is no longer just covering a four or five hour peak window. It may need to support the home overnight, into the morning, and potentially through multiple days if weather limits solar production.

Because of this, many homeowners who prioritize backup choose to install two batteries. The additional capacity provides flexibility and peace of mind, especially if the goal is to maintain a relatively normal lifestyle during an outage. That said, a single battery can still perform extremely well in outages if the homeowner understands their loads and is willing to manage usage, particularly overnight.

One of the simplest ways to understand battery runtime is to convert kilowatt-hours into watt-hours. A 13.5 kilowatt-hour battery equals 13,500 watt-hours. From there, runtime becomes intuitive. At a steady 1,500 watts of consumption, that battery lasts roughly nine hours. At 500 watts, it can last more than a full day. The battery capacity does not change. Your consumption does.

This is why monitoring matters so much. Modern battery systems from manufacturers like Tesla, FranklinWH, and Enphase Energy include whole-home consumption monitoring. These tools allow homeowners to see real-time usage, understand which loads matter most, and observe how quickly batteries discharge under different conditions.

Once energy behavior becomes visible, decisions about battery quantity and sizing become far easier and far more confident.

Battery sizing does not need to be perfect. It needs to be appropriate. If your primary goal is TOU savings, size the battery to comfortably cover your peak window. If your primary goal is backup, size for overnight usage and build in margin. If you want both, be honest about which one matters more.

A properly sized battery should feel helpful rather than stressful. Understanding how and when your home uses energy is the most reliable way to get there.