Buying guide · Battery storage

How to Size a Home Battery

The guide we wish every customer got before their first quote.

Battery sizing has one honest input: what you need to keep running, for how long. Everything else — brand, capacity, whole-home vs essentials — falls out of that. Here’s the exact method we use, so you can walk into any quote conversation (including ours) knowing what the numbers mean. Figures below are illustrative; your home’s real numbers come from your own usage data.

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The method

Six steps to the right size.

1
List what can’t go dark
Refrigeration, internet and a workstation, some lights, medical equipment, a window of heating or cooling. Write it down — this list, not a product brochure, is the design document.
2
Get your real numbers
Your utility bill shows monthly kWh; smart meters show 15-minute detail. A typical Central Texas home runs 30–60 kWh a day — but an essentials-only list is usually a small fraction of that. Real data beats every rule of thumb.
3
Decide: essentials or whole-home
Essentials backup costs less and covers most real outages. Whole-home is seamless but needs more capacity — or smart load control, where a SPAN panel changes the economics: every circuit available, loads shed intelligently as the outage stretches.
4
Pick your duration target
Riding out a 2-hour blip is a different design than a 3-day ice storm. Home batteries commonly store roughly 10–15 kWh per unit — stack units for capacity, but see step 5 before buying more boxes.
5
Pair with solar if you can
The multiplier: solar recharges the battery every daylight hour of an outage, which can turn one day of stored capacity into indefinite runtime for modest loads. Sizing changes completely when the battery refills daily.
6
Verify the boring constraints
Panel capacity, wall space, temperature ratings, utility interconnection rules — the parts a licensed electrical contractor checks before quoting, because each can change the design.
In practice

A worked example — clearly illustrative.

A fridge, internet, lights, and a fan might draw 8–12 kWh across a full day — one-battery territory with careful loads. Add central air conditioning and you’re in a different conversation: compressor starting loads and multi-kW draws are exactly why step 1 comes before any brochure.

Brand-agnostic on purpose

Why brand comes last.

We’re certified across Enphase, FranklinWH, and SPAN, and we service Tesla Powerwall systems — because the right brand is the one that fits your loads, panel, and solar situation. A sizing method that starts with the brand is a sales process, not an engineering one. The full battery-vs-generator comparison lives here.

Battery questions? Tesla Powerwall vs FranklinWH vs Enphase, in plain English — read the comparison.

Asked often

Battery sizing, answered.

Can I just buy one battery and see how it goes?

You can — if the essentials list fits its capacity. The risk is buying twice: undersized backup fails on the day it matters. The load list tells you before the purchase does.

Do I need solar to make a battery worth it?

No — grid-charged backup works on its own. But pairing changes the math enough that if solar is ever in your plans, design for both now.

What about running my AC?

Possible with enough capacity or smart load management — and it’s the single biggest driver of system size. This is precisely the whole-home vs essentials decision.

What does the assessment cost?

The load review and sizing conversation is part of how we quote — request a visit and bring your electric bill.

Bring the load list. We’ll bring the math.

Your usage data plus one visit equals a battery sized for your actual life.
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