What Size Home Battery Do I Need? — A Quick Answer

Quick Answer

The right battery size depends less on the size of your house and more on how much energy you want the battery to supply between charging opportunities. Before anything else, it helps to know: how much electricity you use, when you use it, whether you have solar, and whether the battery is mainly for self-consumption, backup, or tariff shifting.

This page is deliberately short. It won't tell you an exact model or capacity to buy — nobody can do that responsibly without your actual consumption data — but it will get you to a defensible starting range in a few minutes, and point you toward the deeper material once you're ready for it.

The 3 Numbers to Find First

Almost every home-battery sizing question comes back to some combination of these three numbers. You don't need all three for every goal — which ones matter depends on whether you're optimizing for solar self-consumption, backup power, or both.

A utility bill or utility app usually gives you daily or monthly consumption directly. Evening/overnight use and backup targets are less standard — you may need to estimate them from an hourly usage view if your utility provides one, or by adding up the loads you'd want covered during that window.

Simple Sizing Logic

These are simplified first estimates, not final numbers:

For solar self-consumption, that's usually:

For backup, it's a different calculation:

Example 1 — Solar Self-Consumption

Illustrative example:

• Daily consumption: 18 kWh
• Daytime consumption: 10 kWh
• Evening + overnight consumption: 8 kWh
• Primary goal: solar self-consumption

An 8 kWh usable-energy requirement could be a reasonable first comparison point. The nominal (headline) capacity you'd actually need to buy may be somewhat higher, depending on the battery's usable fraction and reserve settings — this isn't a conclusion that the household "needs exactly 8 kWh."

It's also worth checking how consistent that 8 kWh evening/overnight figure is across the year. A household with high air-conditioning use in summer evenings, or one that shifts to more indoor lighting and heating load in winter, may see this number move seasonally — worth a quick sanity check against a few different months of bills rather than a single one.

Example 2 — Backup

A separate illustrative example, using the backup formula above:

• Average critical-load power: 0.6 kW
• Desired backup duration: 6 hours

That 3.6 kWh covers the energy side of backup sizing only — actual backup design also depends on peak (not just average) power, inverter capability, and which circuits are wired for backup.

What This Quick Estimate Doesn't Account For

A few things these simplified formulas intentionally leave out, because they vary too much by product and household to generalize safely:

• Usable versus nominal capacity. A battery's headline kWh number is rarely the full amount you can actually draw on — manufacturers typically reserve some margin to protect battery health.
• Round-trip efficiency. Some energy is lost converting and storing power, so a bit more solar surplus is needed to fully charge the battery than the battery's own capacity suggests.
• Degradation over time. Usable capacity typically declines gradually over a battery's service life — a size that fits comfortably in year one may fit more tightly several years later.
• Tariff structures. If your goal includes shifting usage away from peak-price hours rather than just self-consumption, sizing logic changes and depends on your specific tariff.

The full sizing guide walks through usable capacity and round-trip efficiency with worked examples.

Battery Size Is Not Only kWh

A battery with enough stored energy can still fall short if it can't deliver enough power at once. Energy (kWh) answers "how long," while power (kW) answers "how much at once" — both matter, and the full sizing guide covers this distinction in more depth.

5, 7.5, 10, 13.5 or 15+ kWh?

These are common reference points in residential storage, not a menu to pick from directly — where your own target usable energy from above falls between them is a far better guide than which number sounds right. Picking a size just because it's the most commonly advertised one, without checking it against your own numbers, is one of the more common sizing mistakes.

The full sizing guide walks through what each range typically suits, and 7.5 kWh vs 10 kWh and is a bigger battery always better dig into two of the most common comparisons — useful next reads once you have a rough target range from this page.

When to Get a Professional Assessment

A rough self-estimate is a good starting point for conversations, not a replacement for one. It's worth getting a professional load assessment before purchasing if your home has unusual consumption patterns (a home office with heavy equipment, a heat pump, an EV charger), if you're unsure how to read your own usage data, or if backup power for specific critical circuits is a priority — an installer can measure or model these more precisely than a rough estimate can.

Quick Decision Checklist

• ☐Rough daily electricity use (from a bill or utility app)
• ☐Evening/overnight consumption, if you have solar
• ☐Whether backup power matters to you, and which loads
• ☐Whether you have solar, and roughly how much daytime surplus it produces

Frequently Asked Questions