Sizing for a 24–72 hour shutoff
Public-safety shutoffs in Boulder have lasted from about a day to three days or more. For essentials — refrigerator, furnace blower, lights, internet, medical devices — one 13.5 kWh battery with solar recharging it during the day often covers a multi-day event. Without solar, or in winter storms with snow on the panels, you’ll want more capacity or a stricter load list.
Our blackout runtime calculator includes a 72-hour preset for exactly this scenario.
Costs and what stacks
Powerwall 3: about $19,000 first, about $12,000 each additional. Enphase: about $17,000 for 10 kWh. SigEnergy: $15,500 for 9 kWh, +$3,500 per module. Stacking: Colorado’s 10% credit on equipment through tax year 2026, state sales-tax exemption on residential storage, the City of Boulder tax rebate (batteries eligible since April 2026). Xcel’s Renewable Battery Connect has reopened in 2026, but its current deadlines and amounts need confirming before we count it.
The 5–9 p.m. bonus
Outside of outages, a battery can cover your 5–9 p.m. weekday usage on Xcel’s time-of-use plan, when power costs about 2.7× the off-peak price. That doesn’t usually pay for a battery on its own, but it adds real value on top of backup.
FAQ
Questions we hear
Would a battery have kept my power on during the March 2026 shutoff?
A battery configured for backup keeps selected circuits running when Xcel de-energizes lines. How long depends on the battery size, your loads and whether solar can recharge it.
Can I get the Boulder tax rebate on a battery alone?
Batteries were added to the city’s solar and battery tax rebate in April 2026; check the city page for whether standalone batteries qualify and the grid-connection requirements.
Is a generator cheaper than a battery for Boulder shutoffs?
Usually cheaper upfront. A battery is quiet, automatic, needs no fuel, and works with solar to recharge. We compare them honestly in our battery-vs-generator guide.
Does Xcel control my battery if I join Renewable Battery Connect?
Participants let Xcel discharge part of the battery during a limited number of events per year in exchange for the incentive. Confirm current terms before enrolling.