Choosing the right battery backup system comes down to two questions: what do you want to power during an outage, and for how long? The answers determine the capacity (measured in kilowatt-hours, or kWh) and the output power rating (measured in kilowatts, or kW) you need. Getting those two numbers right — with a proper load analysis — is the foundation of a system that performs as expected when you actually need it.

Step One: Define Your Load

The first step is deciding which circuits or appliances the battery needs to power. This is called your “critical load” — the subset of your home’s electricity consumption that you can’t or don’t want to go without during an outage. For most homeowners, that list includes:

  • Refrigerator and freezer
  • Key lighting circuits
  • Internet router and modem
  • Medical or home-office equipment
  • Furnace blower motor (critical in Colorado winters)
  • A few general-purpose outlets

High-draw appliances — electric vehicle chargers, electric ranges, central air conditioning, electric dryers — consume so much power that including them requires significantly larger battery capacity. Many homeowners choose not to include them in the critical-load panel and run them from the grid when it’s available.

Step Two: Determine Duration

How many hours or days do you want the battery to sustain your critical loads? In the Denver metro, most utility outages caused by weather or grid events last a few hours to a day. A 10–15 kWh battery is typically enough to cover critical loads through an overnight outage or a short storm event. Longer coverage — two to three days without grid or solar recharge — requires more capacity, often 20 kWh or more depending on your specific load.

If you have solar panels, the calculation changes: your battery recharges during daylight hours, which can stretch coverage through extended outages as long as the sun is out. Colorado’s intense high-altitude sunlight makes this particularly effective on the Front Range — panels at 5,280 feet produce meaningfully more per rated watt than the same panels at sea level.

Battery Chemistry: What’s on the Market

Lithium iron phosphate (LFP) is the dominant chemistry in residential battery systems today. It handles temperature extremes better than earlier lithium-ion formulations, which matters in Colorado where garage and mechanical room temperatures can swing dramatically between summer and winter. LFP also has a longer usable cycle life — most systems are rated for 3,000–6,000 full charge-discharge cycles before significant capacity degradation. Lead-acid batteries (older backup technology) are cheaper upfront but have much shorter lifespans and lower usable capacity, making them less cost-effective over a 10–15 year horizon.

Whole-Home vs. Critical-Load Panel

Some battery systems back up the entire home through an automatic transfer switch — when the grid goes out, every circuit in the house runs from the battery. Others use a dedicated critical-load sub-panel that isolates only the circuits you’ve chosen, allowing a smaller (less expensive) battery to cover them effectively. Done’s electricians assess your panel configuration and help you decide which approach fits your budget and goals.

The right answer isn’t one-size-fits-all. A 3,000 square-foot home with a gas furnace and gas range has a very different critical load profile than an all-electric home. The only way to size a system accurately is to look at your actual energy use — ideally your utility bills showing monthly kWh consumption alongside a list of the appliances you want backed up.

Done handles the load analysis, system design, permitting, and installation. Visit our backup power page to get started, or explore financing options if upfront cost is a consideration. If you’re pairing storage with solar, our electrical installations team can coordinate both projects.