Short answer: sizing a battery backup system isn’t about picking a number off a spec sheet - it’s about matching stored capacity to the specific circuits you want protected, the loads those circuits actually pull, and whether solar is recharging the battery during the day. Two homes on the same block can need completely different systems. Here’s what actually goes into that number, and what a real consultation looks at before anyone gives you a quote.
What ‘Sizing a Battery’ Actually Means
A battery’s spec sheet has two numbers that matter more than the price tag: usable capacity, measured in kilowatt-hours (kWh), and continuous output, measured in kilowatts (kW). Capacity is how long the battery can run something. Output is how much it can run at once. A battery with plenty of stored capacity but low output can still trip and shut down if it’s asked to start a central air compressor and run the microwave in the same minute.
Most residential batteries on the market land somewhere around 10 to 13 kWh of usable capacity per unit, with continuous output around 5 kW and a short surge rating higher than that for startup loads. That’s enough to run a refrigerator, a furnace blower, internet gear, and a handful of lights and outlets for a full day or longer. It is not enough, on its own, to run central air through a hot Pueblo afternoon. That gap is exactly where the essentials-versus-whole-home decision comes in.
Essentials-Only Backup: What It Actually Covers
Essentials backup wires the battery into a dedicated sub-panel or a specific set of breakers covering the circuits that matter most during an outage: the refrigerator, the furnace fan and control board (not the burner itself, if it’s gas-fired - just the blower motor that moves the warm air), internet and networking equipment, a handful of designated outlets and lights, and anything medically necessary like a CPAP machine or a sump pump. Everything else on the panel, central air, the electric range, the dryer, EV charging, stays off until the grid is back.
For most Front Range homes, a single battery handles this comfortably, often stretching a full day or more if the connected loads are managed and solar is topping the battery back up each afternoon. This is the setup we spec most often, because it protects what actually causes hardship during an outage without paying for capacity that sits unused 360 days a year.
Whole-Home Backup: What Changes
Whole-home backup wires the battery, or batteries, into the main panel instead of a sub-panel, so every circuit in the house stays live, including the ones that pull real power. This is the setup that makes an outage genuinely unnoticeable - nobody in the house has to know the grid went down. But it runs on a very different power budget.
Central air conditioning alone can pull 3,000 to 5,000 watts continuously once it’s running, with a startup surge two to three times that for the first second or two. Put that against a single battery’s roughly 5 kW of continuous output and there’s almost nothing left over for the rest of the house. That’s why whole-home setups for anyone running central AC, an electric range, or EV charging almost always call for two or more batteries stacked together, not one.
The Loads That Blow Up a Sizing Estimate
These are the loads that turn a modest essentials system into a multi-battery whole-home system, and the ones we specifically ask about before quoting anything:
Central air conditioning - continuous draw plus a heavy startup surge; the single biggest line item in almost every whole-home sizing conversation.
Electric range or oven - short bursts, but high wattage while they’re running.
EV charging - a Level 2 charger alone can pull 7,000 to 11,000 watts, more than most single batteries can supply on top of anything else in the house.
Well pump - intermittent, but the startup surge on a pump motor is disproportionate to how often it actually runs.
Electric dryer or electric water heater - less common backup priorities, but worth naming out loud during the walkthrough so nothing gets backed up by accident and blows the budget.
How Solar Changes the Math
A battery with no solar behind it is really just an expensive UPS: it holds whatever charge it had when the grid went down, and once that’s gone, it’s gone until utility power returns. Pair it with solar and the math changes completely, because the battery recharges every sunny day the outage runs, not just once. That’s the difference between a battery that gets a household through one bad afternoon and one that gets them through a four-day spring blizzard or a wildfire-season power shutoff without ever fully draining.
That advantage shrinks in winter. Shorter days, a lower sun angle, and the occasional snow-covered panel mean solar recharge is real but noticeably smaller from November through February than it is in June. A system sized around summer solar production can come up short backing up the same essentials load during a cloudy January outage. We size around the weaker winter month, not the strongest summer one, specifically so that doesn’t come as a surprise later.
What a Real Battery Consultation Covers
This is the part that gets skipped when a system is sold off a generic package instead of an actual home visit. Before we spec anything, we walk through:
Your electrical panel - what’s already on it, how much room is left, and whether it needs upgrading to support a sub-panel or new circuits.
A load list - which specific circuits you want backed up, ranked by priority, not “everything” by default.
Your utility usage data - actual kWh consumption by time of day, when it’s available, instead of guessing at your household’s evening draw.
Your existing solar setup, if you have one - inverter compatibility matters, and adding a battery to an existing system is usually simpler than doing solar and storage from scratch, but only if the inverter can support it.
What outages have actually looked like at your address - a house that loses power twice a year for twenty minutes has different priorities than one that lost power for four days during a wildfire-season shutoff.
Two households with identical square footage and the same battery model can walk away with completely different wiring plans and completely different expectations for what stays on. That’s the point of the walkthrough. It’s not a sales formality, it’s the entire basis for whether the system does what you actually need it to do.
What This Looks Like Across Denver, Castle Rock, Colorado Springs & Pueblo
The essentials-versus-whole-home decision leans differently depending on where you are along the Front Range. In Denver and Castle Rock, most of the consultations we run land on essentials-only, because outages tend to be short, a few hours from a summer storm or wind event, and the priority is the fridge and the furnace fan rather than full comfort. In Colorado Springs, altitude and drier air keep AC loads lower than most homeowners expect, which sometimes makes whole-home backup achievable on fewer batteries than it would take at lower elevation. Pueblo runs the other direction: hotter, longer summers push central air into must-stay-on territory for a lot of households, which usually means budgeting for a multi-battery whole-home setup instead of a single essentials unit.
None of that is a hard rule. We’ve sized whole-home systems in Denver and essentials-only systems in Pueblo when that’s what actually matched the household. It’s a starting point for the conversation, not the conclusion.
Frequently Asked Questions
How many batteries do I actually need?
It depends on what you’re backing up, not square footage. An essentials-only setup - fridge, furnace fan, internet, a few outlets - is usually covered by a single battery. Add central air, an electric range, or EV charging to the backed-up circuits and you’re typically looking at two or more, since those loads alone can use up most of one battery’s continuous output.
Can I start with essentials-only and add whole-home backup later?
In most cases, yes, especially if the initial installation is planned with future expansion in mind. Adding a battery to an existing system is usually more straightforward than starting from scratch, but it depends on how much room is left on the inverter and panel. We flag this during the initial walkthrough whenever it’s likely to matter down the road.
Does a bigger battery always mean better backup?
Not necessarily. A bigger battery with the wrong continuous output rating can still struggle to start a central air compressor, and a battery sized for far more capacity than your actual essentials load just sits underused. Matching output and capacity to your specific loads matters more than maximizing either number on its own.
How does winter change what my battery can back up?
Solar recharge is real but smaller in winter, so a system that comfortably covers essentials through a summer outage on solar alone might lean more heavily on stored reserve capacity during a cloudy January outage. We size around the weaker winter production on purpose, so this doesn’t come as a surprise when it matters most.
Every battery consultation we run starts with your panel and your actual usage, not a default package. Impact Energy & Roofing installs and sizes battery backup systems across Denver, Castle Rock, Colorado Springs, and Pueblo - call now to schedule a walkthrough and get a system built around what your home actually needs, not a one-size-fits-all number.