"How many hours will a home battery run my house?" is the wrong first question, because "my house" is not a load — it is dozens of loads with different importance, timing, and startup behavior. A backup plan that starts from a product runtime tends to protect the wrong things. This guide starts from the other end: name the services the outage must preserve, attach an outage scenario to them, and only then size energy, power, recharge, and the approved way to connect it all. One small worked example runs through the whole method.

Two boundaries up front. Everything here produces a preliminary, educational requirement — the input you take to a qualified designer and your utility, not an installation design. And the parts that make backup dangerous — connecting a source to premises wiring, transfer and isolation, a stationary battery system, a generator — are Tier 3 work. This guide deliberately explains what to require and where to stop, not how to wire it.

Name the services the outage must preserve

Start with a critical load: the load the system is actually required to meet during a defined outage. That is how national-laboratory resilience planning frames it, and NREL's REopt tooling is built around exactly this idea — the critical load is what must be met during the outage you specify.

List the services, not a percentage. "Run 50% of the house" hides whether the surviving half includes the sump pump, the medical device, or the fridge holding medicine. For each service, record:

  • Consequence of failure — convenience, property (freeze, flood, spoilage), or life-safety. These are not interchangeable.
  • Energy it uses over the outage (average or interval, not nameplate).
  • Power — its continuous draw and any startup surge.
  • Duration and acceptable interruption — must it be seamless, or can it drop for seconds?
  • Recharge route and environment — how it gets topped up, and the season.

A percentage of the building load is fine for a rough first pass, but load-specific interval data is far more defensible when you can get it.

Worked essential-load worksheet

A deliberately small essential-load set for an outage day. All figures are at the AC load side (what the appliances consume); conversion to the battery comes in the next step. These are illustrative — not a typical-home figure:

Refrigeration  measured/duty-cycle energy  = 1.44 kWh   (≈ 60 W average over 24 h)
Lighting       40 W × 5 h                   = 0.20 kWh
Comms/control  20 W × 24 h                  = 0.48 kWh
                                    ──────────────────
Daily AC energy  E_day                      = 2.12 kWh/day

Sanity check: 2.12 kWh ÷ 24 h ≈ 88 W average, plausible for this small essential set. As with any cycling load, the refrigeration line uses measured or duty-cycle energy, not compressor nameplate × 24 h, which would badly overstate it. Add your real critical loads — heating, pumps, medical equipment — and the number climbs quickly. Build the figure with how to build a daily energy load profile.

Size energy for the outage scenario

Storage energy is the critical-load energy over the outage, translated to the battery through conversion efficiency, divided by the fraction of the battery you can actually use:

Nominal storage  E_nominal = (E_day × days) ÷ (η × u)

  E_day = 2.12 kWh/day     (AC, from the worksheet)
  days  = 2 days           (a 48-hour outage with no recharge)
  η     = 0.90             (storage-to-AC conversion efficiency)
  u     = 0.80             (usable fraction: depth-of-discharge + reserve)

  E_nominal = (2.12 × 2) ÷ (0.90 × 0.80) = 4.24 ÷ 0.72 ≈ 5.9 kWh nominal

Reverse check: 5.89 kWh × 0.80 × 0.90 = 4.24 kWh AC = two days of the worksheet. The ledger closes. The two divisors are separate losses and must not be double-counted — η is the round-trip/conversion loss, u is the inaccessible and reserved fraction that depends on chemistry, temperature, aging, and your reserve policy. At u = 0.70 the same outage needs about 6.7 kWh. This sizes energy only; it does not select chemistry or product or guarantee a runtime. The general method lives in how to design and size a small energy system.

Size power independently of energy

Stored energy tells you nothing about whether the system can deliver the load at a given instant. Power is a separate requirement: the source and delivery path must carry the simultaneous continuous load plus any startup surge. The classic trap is a motor start:

Coincident peak  P_peak = P_other_simultaneous + P_starting_load

  P_other_simultaneous = 200 W    (lights, comms, etc. already running)
  P_compressor_start   = 800 W    (measured/manufacturer starting demand — not a multiplier)

  P_peak = 200 + 800 = 1 000 W coincident peak

Daily energy could never reveal this 1 kW requirement. And even this is not the whole story: real inverter and battery sizing must also check VA (not just watts), surge duration, power factor, waveform suitability, and the battery/BMS current limit along the delivery path. Continuous and surge ratings are checked separately — see continuous power, peak power, and surge loads. Note too that some loads run in a reduced or prohibited mode during an outage; confirm each one's behavior rather than assuming it runs normally.

Define what happens when the grid disappears

A crucial reality check: ordinary residential solar alone generally does not power a home during a grid outage. The Department of Energy is explicit that grid-dependent PV shuts down when the grid is down, and that standalone operation requires a properly configured inverter and a stabilizing source — commonly storage. Reconnecting when the grid returns also needs the intended controls.

So separate four things that marketing tends to merge: annual solar production, the ability to island, instantaneous power, and stored energy. A grid-tied array can produce plenty of energy over a year and still be unavailable the moment you need it. Islanded backup is a distinct capability with distinct equipment, and the exact anti-islanding, grid-forming, transfer, and reconnection requirements are local and product-specific.

Model depletion, recharge, and recovery

An extended-outage claim is only as good as its recharge assumption. Do not assume a daily refill:

  • Solar varies with location, season, weather, orientation, and system condition — the same drivers DOE lists for the solar resource. A cloudy stretch can flatten harvest exactly when an outage runs long.
  • A generator depends on usable fuel on hand and safe operation.
  • The grid recharges only once it is restored.

Model at least a no-recharge case and a limited-recharge case, and ask what happens when the battery reaches its reserve: what sheds, what recovers first, and how long recovery takes. A single steady-load, beginning-of-life runtime number does not survive contact with a real outage full of cycling loads, surge, reserve, temperature, degradation, and uncertain recharge.

Approved isolation and transfer are non-negotiable boundaries

Any backup source connected to premises wiring needs an approved means of preventing dangerous backfeed and coordinating grid, backup, and load states. This is not a place for improvisation, and the failure mode is lethal.

Do not backfeed. In 2026 the U.S. Consumer Product Safety Commission warned consumers to stop using male-to-male "suicide" cords, which are used to backfeed a generator into a home through an outlet — an electrocution and fire hazard that can also energize utility lines and endanger line workers. Route every premises connection through approved transfer equipment installed by a qualified person. Improvised panel connections and male-to-male cords are a hard stop.

An automatic or manual transfer switch, an interlock, or an inverter designed for islanding is the mechanism that makes backup safe. Which one, and how it is installed, is code-governed engineering — see why protection is part of system design. Stop if transfer or isolation behavior is undocumented, or if the source or conductor state is ever uncertain.

Bring a system requirement to the authority and installer

Residential energy storage is evaluated and installed as a system, not a bag of individually rated parts. Current standards address interconnection, commissioning, operation and maintenance, electrochemical hazards, and dwelling-specific requirements — NFPA 855:2026 is one current example whose scope includes one- and two-family dwellings, and system-level certification schemes assess the assembled unit and its interacting parts. Which standards apply, and whether a given product is accepted, is decided locally; other regions use different standards and conformity systems entirely.

The practical consequence: take a requirement — named critical loads, outage scenarios, energy and power figures, recharge cases — to a qualified designer or installer and your utility, not a shopping list. Get qualified help for all ESS selection, siting, protection, installation, commissioning, and any post-incident assessment, and stop on any damaged, recalled, wet, swollen, hot, venting, or smoking equipment.

If you add a generator, treat it as its own hazard set: carbon monoxide, hot surfaces, fuel, and backfeed. Never run one indoors or near openings — in a home, garage, basement, crawlspace, or shed — follow its manual and local fire and carbon-monoxide guidance, and integrate it only through approved transfer.

Plan for the backup system to be unavailable

Backup capacity is not an emergency plan. Any electrical system can be depleted, faulted, out for maintenance, or simply outlasted by the outage — so a safety-critical outcome must never rest on a single optimistic assumption. If loss of a service could harm someone or cause serious property damage, it needs a non-electrical contingency:

  • Medical or life-safety loads — confirm each device's real duration, startup, and power-quality needs with its maker and care provider, and keep an independent fallback.
  • Freeze and flood prevention (heating, sump, well) — plan for the case where the system runs out mid-event.
  • Refrigerated medicine, communications, evacuation — verify against the actual requirement, not a runtime estimate.

Do not let a battery runtime, a single state-of-charge estimate, an optimistic solar yield, or assumed fuel stand in for a plan when a life-safety load is on the line.

Key takeaways

  • Start from named critical services and an outage scenario, not a product runtime or a percentage of the house.
  • Energy sizing: E_nominal = (E_day × days) ÷ (η × u); keep conversion loss and usable fraction separate and don't double-count them.
  • Power is a separate requirement — size for coincident continuous load plus startup surge, and check VA, surge duration, and BMS limits.
  • Grid-tied solar generally shuts down in an outage; islanding is a distinct capability, and recharge must be modeled, not assumed.
  • Approved transfer/isolation is mandatory — improvised backfeed with male-to-male cords is a lethal hard stop; treat ESS and generators as Tier 3 work.
  • Keep a non-electrical contingency for any life-safety or freeze/flood load; this is a preliminary requirement to take to a qualified installer and your utility.

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