How much battery do you actually need to ride out a three-day outage?

Every battery sales page leads with a capacity number, and almost every one of them is useless on its own. A 13.5 kWh battery can carry one house for four days and another house for six hours. The spread isn't the battery. It's the load.

So: four numbers. Your average daily consumption, your critical-load fraction, your expected solar recharge, and your outage duration target. Work those out and the product tier picks itself.

1. Start with daily kWh, not peak watts

Pull twelve months of bills and divide annual kWh by 365. That's your baseline. If you're in a cooling-dominated climate, also note your worst month — an Austin August runs roughly 1.7× the annual average, and outages love August.

Peak watts matter separately, for whether the inverter can start your compressor, but capacity sizing is an energy question. Keep the two apart or you'll end up buying a battery sized for a moment instead of a day.

2. Strip it down to critical load

During an outage you are not running the house as usual. You're running a smaller house inside it. In our fleet, the median critical load comes out to roughly a third of normal consumption:

• Refrigeration — 1.2–2.0 kWh/day. Non-negotiable, and the reason most people call us.
• Well pump or sump — 0.5–3.0 kWh/day. Non-negotiable if you have one.
• Lighting, routers, chargers — 0.8–1.5 kWh/day on LED.
• One HVAC zone or mini-split — 4–11 kWh/day. This is the whole argument.
• Medical equipment — varies; tell us at design time, it changes the topology.

Everything else — dryer, oven, EV charging, the second fridge in the garage — comes off the backup panel. Not because it can't be backed up, but because backing it up triples the battery bill for capability you'd use twice a decade.

Air conditioning is the line where partial-home backup becomes whole-home backup. Everything else is rounding.

— Linh Kapoor, Head of Install

3. Count the solar recharge, honestly

A battery alone is a fixed budget. A battery with solar is a budget plus income, and that changes the arithmetic entirely — which is why every Trew system ships with outage charging enabled rather than as an upsell.

Be conservative with the income side. Storm days are overcast days. We model outage-period production at 40% of clear-sky output for the first 48 hours after a named storm, and 75% for heat-wave or planned-shutoff outages, when skies are usually clear.

4. Put it together

Usable capacity needed = (critical daily load − daily recharge) × days of autonomy, then divide by the battery's usable depth of discharge. For the median fleet home targeting three days through a storm:

(9.1 kWh − 5.4 kWh) × 3 days = 11.1 kWh ÷ 0.9 DoD = 12.3 kWh usable — comfortably inside a single 13.5 kWh unit.

Add the air conditioner and critical load climbs to about 17 kWh/day. The same math returns 38.7 kWh, and you're in whole-home territory with three stacked units. That jump — one appliance, three times the battery — is the single most important thing to understand before you shop.

Three mistakes we see weekly

Sizing to the bill instead of to the outage

Offsetting 100% of annual consumption is a savings goal. It tells you almost nothing about whether you'll have a working refrigerator on day three.

Ignoring depth of discharge

Nameplate capacity minus reserve minus round-trip losses is frequently 15% below the number on the box. We quote usable kWh, and you should make everyone else do the same.

Backing up a panel nobody has reorganized

If the critical circuits are scattered across a 40-space panel, the cost of the electrical work can exceed the cost of the battery. Sometimes the right first move is a subpanel, not more storage.