Residential Battery Sizing: kWh for Backup vs Self-Consumption

Battery attachment rates on US residential solar proposals crossed 20% in 2024 and are climbing toward 35% in markets with time-of-use tariffs or unreliable grids. Homeowners are no longer asking whether to add storage — they are asking how much. The problem is that most sizing advice conflates two completely different engineering problems: backup power during outages and daily self-consumption of solar production. A battery sized for one purpose often fails at the other.

This guide solves that confusion with separate calculation methods for each use case, then shows how to reconcile them into a single specification. You will learn how to run a critical load audit, why kW power rating matters more than kWh capacity for large appliance startup, how DC-coupled versus AC-coupled architecture changes your nominal sizing by 3–8%, and why California’s NEM 3.0 rules punish installers who size for the old net metering paradigm. Every formula, table, and rule of thumb here is drawn from DOE, EIA, NREL, and manufacturer datasheets — not generic blog recycling.

What you’ll learn:

• How to calculate backup kWh needs using a critical load audit vs whole-home method
• Why self-consumption and time-of-use arbitrage use a different formula than backup sizing
• The dual-check framework: why kW (power) limits what runs before kWh (energy) limits how long
• How DC-coupled vs AC-coupled efficiency changes your nominal kWh requirement
• Cold-climate temperature derating and how to build it into your sizing
• California-specific SGIP and NEM 3.0 sizing traps that installers miss
• Real product runtimes: what 5 kWh, 9.3 kWh, and 13.5 kWh actually power in practice

Backup vs Self-Consumption: Two Different kWh Math Problems

The single biggest mistake in residential battery sizing is treating backup and self-consumption as the same calculation. They are not. Backup sizing answers the question: “How long do the lights stay on when the grid goes down?” Self-consumption sizing answers: “How much solar excess do I store to avoid buying peak-rate electricity?” The formulas, margins, and failure modes differ completely.

Understanding which problem you are solving changes every downstream decision — from nominal capacity to inverter pairing to warranty expectations. Installers who run the wrong calculation end up with batteries that are either catastrophically undersized for outage runtime or economically wasted because they sit partially charged with nowhere to discharge.

Backup Sizing: How Long the Lights Stay On

Backup battery sizing starts with autonomy: the number of hours or days a battery must power loads without grid support. The formula is straightforward:

Daily critical load (kWh) ÷ Usable battery capacity (kWh) = Days of autonomy

A typical US home with a 10 kWh daily critical load divided by a 13.5 kWh usable battery yields roughly 1.35 days of autonomy — about 32 hours of continuous runtime for refrigerator, lights, furnace fan, router, and well pump. That same 13.5 kWh battery against a whole-home average of 29 kWh/day collapses to 0.47 days, or about 10.8 hours before the battery is flat.

The critical load method is the only responsible approach for single-battery systems. It forces the homeowner to decide what actually matters during an outage: keeping the refrigerator cold, maintaining internet and phone charging, running a gas furnace blower, and perhaps pumping water. Everything else — EV charging, pool pumps, electric dryers, central air conditioning — gets deferred until grid restoration.

Average US outage duration is approximately 5.5 hours according to DOE and EIA reliability tracking [CITE]. A single Powerwall 3 at 13.5 kWh handles this comfortably for critical loads. But outage duration is not normally distributed. Hurricanes, ice storms, and wildfire PSPS events stretch outages across 24–72 hours. In high-risk regions, installers should size for 24–48 hours of critical load autonomy, which typically means 15–30 kWh of usable capacity or active generator integration.

For system architecture context, see our guide to on-grid vs off-grid vs hybrid solar. Backup capability is essentially what converts an on-grid system into a hybrid configuration.

Self-Consumption and TOU Arbitrage: How Much Solar You Store

Self-consumption sizing has nothing to do with outage duration. It is an economic optimization problem. The formula is:

Daily excess solar production (kWh) = PV generation − Immediate home consumption

Without storage, typical residential solar self-consumption rates run 25–40% [CITE]. The remaining 60–75% of production exports to the grid at retail or net metering rates. With a battery, self-consumption rises to 60–90% depending on array size, load profile, and battery capacity. The economic value of that stored energy depends entirely on the spread between your import rate (what you pay for grid electricity) and your export rate (what the utility pays for your surplus).

In time-of-use markets, the arbitrage window is typically 4–10 hours of evening peak rates — for example, 4 PM to 9 PM when solar generation drops but household demand rises as residents return home. A battery sized for TOU arbitrage does not need to cover full daily consumption. It needs to cover the evening peak gap, usually 5–15 kWh depending on the home’s evening load shape.

This is why a 5 kWh Enphase IQ Battery 5P can make financial sense in a pure self-consumption scenario even though it would be inadequate for whole-home backup. It stores enough midday solar to offset 4–6 hours of evening peak consumption, delivering daily savings without the cost of a larger unit.

For commercial-scale comparison, see our guide to commercial battery storage sizing. The same TOU arbitrage principles apply, but commercial peak demand charges create a different economic model.

Step-by-Step kWh Calculation for Backup Power

Backup sizing is the more technically demanding of the two problems because it involves real load enumeration, surge accounting, and autonomy planning. Self-consumption sizing can be approximated from smart meter data and solar production curves. Backup sizing requires you to know exactly what will be connected when the grid dies.

The process has three stages: critical load audit, whole-home reality check, and depth-of-discharge adjustment. Skipping any stage produces either an undersized battery that dies in four hours or an oversized battery that costs thousands of dollars more than necessary.

The Critical Load Audit (kW + kWh)

A critical load audit lists every device the homeowner wants to keep running during an outage, records its running wattage, accounts for motor startup surge, and multiplies by expected runtime hours per day. The result is a daily kWh requirement that directly determines battery capacity.

Here is a representative critical load audit for a 2,000 sq ft home with gas heat and a well pump:

This home needs 11.1 kWh per day for critical loads. For 24 hours of autonomy, the battery must provide at least 11.1 kWh of usable capacity. For 48 hours, it needs 22.2 kWh. For 72 hours, 33.3 kWh.

But kWh is only half the story. The peak simultaneous load matters too. If the refrigerator compressor (450 W surge), well pump (3,000 W surge), and furnace fan (1,600 W surge) all start within the same minute — which happens during automated thermostat cycles — the battery must deliver roughly 5.3 kW of peak power even though the continuous running load is only 2.2 kW. A 5 kWh battery with a 3.84 kW output rating cannot start this load profile. The battery has enough energy but not enough power.

This is why every critical load audit must record both kWh (energy) and kW (power). The kWh number sizes how long the battery lasts. The kW number sizes whether the battery can start the loads at all.

Whole-Home Backup Reality Check

Some homeowners insist on whole-home backup: they want every circuit active during an outage, including air conditioning, electric dryers, and EV chargers. This is technically possible but expensive. The starting point is average daily consumption.

The EIA reports average US residential consumption at 10,632 kWh/year, which equals 29 kWh/day [CITE]. Using the same 13.5 kWh Powerwall 3:

13.5 kWh ÷ 29 kWh/day = 0.47 days ≈ 10.8 hours

A single Powerwall 3 lasts just under 11 hours running an average US home at typical draw. In practice, whole-home backup without active load management is unrealistic on a single battery. Two Powerwalls (27 kWh) extend this to roughly 22 hours. Three Powerwalls (40.5 kWh) reach 33 hours — meaningful multi-day autonomy if the homeowner actively sheds non-critical loads.

A practical rule of thumb for installers: 1 kWh of usable battery capacity provides approximately 45–60 minutes of whole-home backup for an average US home. A 10 kWh battery gives 7.5–10 hours. A 20 kWh battery gives 15–20 hours. This rule breaks down for very large homes (over 40 kWh/day) or very efficient homes (under 15 kWh/day), but it is accurate enough for preliminary proposals.

The whole-home reality check serves a critical sales function: it stops homeowners from expecting a single $12,000 battery to run their 4-ton AC unit for three days. Set expectations with math before installation, or you will field angry support calls during the first outage.

Accounting for Depth of Discharge (DoD)

Not every amp-hour in a battery is usable. Depth of discharge (DoD) defines the percentage of nominal capacity that the manufacturer permits for regular cycling. Exceeding DoD voids warranties and accelerates degradation.

The formula is simple:

Nominal capacity (kWh) × DoD = Usable kWh

Here is how three leading residential batteries compare:

Tesla and Enphase now specify usable capacity at 100% DoD because their lithium iron phosphate (LFP) chemistry tolerates full discharge without the catastrophic degradation that plagued older NMC batteries. LG’s RESU line, depending on model year, typically warrants 95% DoD. Always verify the specific model year — earlier LG chemistries were rated at 90% DoD, which changes sizing by a meaningful margin.

When comparing quotes, insist on usable kWh, not nominal kWh. A “10 kWh battery” with 90% DoD delivers only 9 kWh of actual runtime. Over a 20 kWh system, that 1 kWh gap equals 2–3 hours of refrigerator runtime — the difference between keeping food safe and throwing it out.

The Dual-Check Framework: kW Power + kWh Energy

Battery datasheets lead with kWh because it is the easier number to market. Homeowners understand “13.5 kWh” as “a lot of electricity.” Installers understand that kW — the rate of power delivery — is the gatekeeper. A battery with unlimited kWh and insufficient kW is useless for motor loads. A battery with high kW and low kWh starts everything but dies in two hours.

The dual-check framework requires both metrics to pass independently before a battery is correctly sized.

Why kW Rating Determines What Runs

kW measures instantaneous power delivery. A 3 kW continuous output means the battery can power 3,000 watts of load at any given moment. A 5 kW output means 5,000 watts. This is entirely separate from how long the battery can sustain that output.

Consider a 10 kWh battery with 3 kW continuous output. It can run ten 100-watt light bulbs for 33 hours (10,000 Wh ÷ 1,000 W = 10 hours… wait, 10 kWh ÷ 1 kW = 10 hours. Ten 100W bulbs = 1 kW, so 10 hours. But with 3 kW output, it can also run a 2.5 kW space heater for 4 hours. However, it cannot run a 5 kW HVAC compressor even for one minute. The energy is there; the power is not.

Motor loads are the critical failure point. HVAC compressors, well pumps, sump pumps, and pool pumps draw 2–3× their running wattage for 1–3 seconds during startup. A battery sized only on kWh will calculate sufficient runtime but will trip its overload protection the moment the compressor kicks on. Surge capacity ratings — typically 2–3× continuous for 3–10 seconds — must be verified against the largest motor in the critical load list.

For inverter pairing guidance, see our hybrid inverter guide. The inverter and battery must be co-designed because the inverter’s surge capability often becomes the limiting factor before the battery’s.

Matching Inverter kW to Battery kWh

There is no universal kW-to-kWh ratio, but industry practice follows a rough rule: 1 kW of inverter continuous output per 2–3 kWh of usable battery capacity produces a balanced system that neither starves the inverter nor leaves the battery underutilized.

A 5 kWh Enphase IQ Battery 5P paired with a 3.84 kW microinverter-based system works well for TOU arbitrage but struggles with whole-home backup. A 13.5 kWh Powerwall 3 with 11.5 kW output handles most residential startup surges without drama. LG’s RESU 10H at 5.0 kW sits in the middle — adequate for medium homes with managed loads, tight for whole-home with central AC.

solar design software helps model this accurately by running load profiles against inverter output curves. Instead of guessing whether a 5 kW inverter can handle a 4-ton AC startup, you import the actual load data and see the overload seconds per day. This matters because overload events — even brief ones — trigger protective shutdowns that homeowners experience as “the battery doesn’t work.”

DC-Coupled vs AC-Coupled: The 3–8% Sizing Penalty

Battery architecture affects sizing because every conversion step between DC (panels and battery) and AC (home loads) wastes energy as heat. The more conversion steps, the more nominal kWh you need to deliver the same usable AC kWh.

This is not a minor concern. An AC-coupled system that skips the 3–8% penalty during sizing will deliver measurably less runtime than the homeowner was promised. That gap shows up in negative reviews, not engineering spreadsheets.

Round-Trip Efficiency by Architecture

DC-coupled systems connect solar panels to a charge controller, then to the battery, then to a single hybrid inverter that converts DC to AC for the home. The battery charges and discharges in DC. There is one inversion step (DC to AC) when power leaves the battery for the home.

AC-coupled systems connect solar panels to a standard grid-tied inverter that outputs AC. The battery has its own inverter that converts AC back to DC for charging, then DC back to AC for discharging. There are two or three inversion steps depending on whether the solar energy flows directly to loads or through the battery first.

The efficiency difference is significant:

DC-coupled wins on efficiency because it avoids the extra AC-to-DC and DC-to-AC conversions. AC-coupled wins on retrofit flexibility: you can add a battery to an existing solar system without replacing the existing grid-tied inverter. This is why most retrofits are AC-coupled, and most new installations are moving toward DC-coupled hybrid inverters.

Adjusting Nominal kWh for AC-Coupled Losses

When designing an AC-coupled system, you must increase nominal battery capacity to compensate for conversion losses. The formula:

AC-coupled nominal target = DC-coupled usable target ÷ (AC round-trip efficiency ÷ DC round-trip efficiency)

Example: You need 13.5 kWh of usable AC energy. A DC-coupled system at 97% round-trip efficiency requires 13.5 kWh ÷ 0.97 = 13.9 kWh nominal. An AC-coupled system at 90% round-trip efficiency requires 13.5 kWh ÷ 0.90 = 15.0 kWh nominal.

The practical rule used by most installers: add 3–8% to your nominal kWh target for AC-coupled systems. If your load audit calls for 20 kWh of usable capacity and you are AC-coupled, specify 20.6–21.6 kWh nominal. If you are DC-coupled, 20.4–20.9 kWh nominal covers the same need.

These differences compound over time. A 3% efficiency penalty on a battery cycled 250 days per year equals 50–100 kWh of lost annual storage capacity. In a TOU arbitrage market with a $0.30/kWh peak rate, that is $15–30 per year — small, but real, and worth modeling accurately in financial proposals.

solar software simulation engines model this automatically by applying hourly efficiency curves to dispatch logic. The difference between a 94% and an 89% round-trip assumption changes payback periods by 6–12 months in some California TOU scenarios.

Temperature, Degradation, and Safety Margins

Batteries do not deliver their datasheet capacity in all conditions. Cold weather reduces available capacity. Calendar aging reduces it over ten years. Installers who size based on laboratory conditions at 25°C on day one deliver systems that underperform in winter and fade before the warranty expires.

Cold-Climate Temperature Derating

Lithium-ion battery capacity drops 10–25% when operating below 0°C [CITE]. The electrolyte becomes more viscous, ionic conductivity decreases, and the battery management system (BMS) may artificially cap discharge rates to protect cell chemistry. This is not a defect — it is physics.

Manufacturer operating ranges vary:

In Minnesota, Maine, or upstate New York, where winter temperatures regularly sit below −10°C for weeks, a 13.5 kWh battery may deliver only 11.5 kWh of usable energy. If the critical load audit called for 12 kWh/day, the battery fails on the coldest nights of the year — exactly when ice storms cause outages.

The practical fix: add 15–20% kWh margin in sustained sub-freezing climates. Size for the capacity you need at −10°C, not at 25°C. For a 15 kWh cold-climate requirement, specify 17.3–18.0 kWh of nominal capacity. Indoor installation (basement, garage) reduces this margin because the ambient temperature stays above freezing. Outdoor wall-mounted installation in cold climates maximizes it.

Some manufacturers offer active thermal management — resistance heaters inside the battery enclosure — that mitigate cold-weather losses at the cost of parasitic load. The Powerwall 3 includes this. The Enphase IQ Battery 5P does not. Account for heater draw in your critical load audit if the battery will be heating itself during an outage.

10-Year Degradation Planning

All lithium-ion batteries degrade. Most manufacturers warranty 70% remaining capacity after 10 years or a specific cycle count. That means a battery purchased in 2026 will deliver roughly 30% less energy in 2036.

The year-10 usable capacity formula:

Year-10 usable = Nominal capacity × DoD × 0.70

Example: A 13.5 kWh Powerwall 3 with 100% DoD delivers 13.5 kWh today and approximately 9.5 kWh in year 10. If the homeowner sized for 12 kWh of critical load autonomy, the battery falls short by year 7 or 8 even if it meets the 70% warranty threshold.

Responsible installers size for 85–90% of initial capacity to cover degradation without requiring a battery replacement before year 12–15. If the homeowner needs 15 kWh of usable capacity for true multi-day autonomy, size at 16.7–17.6 kWh nominal. This adds cost upfront but avoids the “my battery doesn’t last like it used to” service call in year 8.

Degradation curves are not linear. Most batteries lose 2–3% in year one, then 1–2% per year through year 7, then accelerate as internal resistance builds. The 70% warranty floor is a legal guarantee, not a performance prediction. Real-world year-10 capacity is often 65–75% depending on cycling depth, temperature history, and BMS calibration.

Product Comparison: What 5 kWh, 9.3 kWh, and 13.5 kWh Actually Power

Datasheet numbers mean nothing without runtime context. A homeowner does not care about kWh — they care about whether the refrigerator stays cold, whether the lights work, and whether the furnace fan runs through the night. This section translates three common battery capacities into real-world runtime against a standard 10 kWh/day critical load.

Runtime against a 10 kWh/day critical load (refrigerator, lights, router, furnace fan, well pump):

The Enphase IQ Battery 5P is not designed for whole-home backup. Its 3.84 kW continuous output cannot run central air conditioning or large heat pumps, and its 5.0 kWh capacity dies quickly under whole-home load. Where it excels is modularity: homeowners can start with one 5P unit and add up to three more for 20 kWh total, all managed through the same Enphase app ecosystem. This pay-as-you-grow model reduces upfront cost for buyers who are uncertain about their storage needs.

The LG RESU 10H occupies the middle ground. At 9.3 kWh usable and 5.0 kW continuous, it handles medium homes with gas heat and partial backup requirements. It struggles with whole-home backup during summer AC season but covers critical loads for nearly a full day. LG’s warranty terms and distributor network make it a common choice for installers who value supply chain reliability over app ecosystem integration.

The Tesla Powerwall 3 is the current benchmark for whole-home backup. Its 11.5 kW continuous output and 20.5 kW surge capacity start 4-ton HVAC units without load shedding. Its 13.5 kWh usable capacity covers critical loads for over 30 hours or whole-home average draw for nearly 11 hours. Two Powerwalls (27 kWh) deliver genuine multi-day autonomy for efficient homes. The trade-off is installer dependency on Tesla’s ecosystem and availability constraints in some markets.

For homes with EV charging loads, see our solar EV charging integration guide. EVs are not part of standard critical load audits, but they represent the largest single load in many modern homes and must be explicitly excluded from backup circuits unless the battery system is massively oversized.

Costs, Incentives, and NEM 3.0 Implications

Battery economics changed dramatically between 2023 and 2026. Declining pack costs, evolving state incentives, and California’s NEM 3.0 transition have reshaped the value proposition. Installers who still size batteries using 2022 logic — maximize self-consumption, export the rest — are leaving money on the table or creating systems that cannot charge fully.

Installed Cost per kWh

US residential battery installed costs range from $700 to $1,300 per kWh according to EnergySage and NREL tracking data [CITE]. Tesla sits at the low end at roughly $700–$780/kWh installed when bundled with solar. Modular systems like Enphase and premium brands like Sonnen sit at the high end, sometimes exceeding $1,200/kWh for small single-unit installations.

BloombergNEF reports battery pack costs at approximately $108/kWh at the cell and pack level in 2025 [CITE]. The gap between $108/kWh manufacturing cost and $700+ installed cost is soft costs: labor, permitting, electrical upgrades, sales and marketing, installer margin, and interconnection fees. Soft costs represent 60–70% of total installed price in the US residential market, compared to roughly 40% in Germany and Australia where installation standardization is higher [CITE].

This cost structure matters for sizing decisions. A homeowner paying $1,000/kWh for 5 kWh of unnecessary capacity wastes $5,000 on soft costs that deliver no additional utility. Every kWh added to a proposal should be justified by the load audit, not by sales target pressure.

For payback modeling, use our generation and financial tool to run hourly dispatch simulations against real utility rate structures. Generic payback estimates based on “average California rates” mislead homeowners in specific utility territories where rate structures vary by 30% or more.

California SGIP and NEM 3.0 Sizing Traps

California is the largest US residential battery market, and it is also the most policy-complex. Two programs dominate sizing decisions: the Self-Generation Incentive Program (SGIP) and NEM 3.0 net metering.

SGIP offers rebates up to $1,000/kWh for qualifying battery systems through its Equity Resilience budget [CITE]. However, SGIP equity tiers cap battery capacity at 150% of solar inverter AC rating in some cases. An installer who specifies a 15 kWh battery with a 7.6 kW solar inverter may hit this cap and lose rebate eligibility. Always check the specific SGIP equity tier rules before finalizing battery capacity.

NEM 3.0, which took effect in April 2023 and continues to evolve, reduced solar export compensation to avoided cost rates — roughly 5–8 cents/kWh during most hours, compared to retail rates of 25–40 cents/kWh [CITE]. This eliminated the economic case for oversizing solar and exporting the surplus. It dramatically strengthened the case for batteries by making self-consumed solar worth 3–5× more than exported solar.

Under NEM 3.0, the correct battery sizing strategy is:

1. Size solar to cover annual consumption (no oversizing for export)
2. Size battery to capture daily solar excess and discharge during evening peak rates
3. Never size battery larger than what the solar array can realistically charge in a single day during winter

The oversizing trap is real. A homeowner with a 6 kW solar array producing 20 kWh on a good summer day cannot charge a 20 kWh battery from solar alone. The unused capacity sits empty, depreciating, while the homeowner paid full price for it. solar proposal software models NEM 3.0 payback by running month-by-month solar production against battery charge acceptance limits, preventing this exact error.

Five Mistakes That Waste Money on Oversized Batteries

Even experienced installers make predictable errors when battery sizing becomes a checkbox exercise rather than a load-driven calculation. These five mistakes account for the majority of post-installation complaints and negative online reviews.

1. Sizing for whole home without a critical load audit first.

Whole-home backup is expensive. A critical load audit often reveals that 60–70% of daily consumption is non-essential during an outage: pool pumps, EV chargers, electric dryers, second refrigerators. Running the audit first typically reduces the required battery capacity by 30–50%, saving $5,000–$10,000 in installed cost. Present the critical load backup number first, then offer whole-home as an upgrade with clear cost and runtime trade-offs.

2. Ignoring kW rating when sizing kWh alone.

A 10 kWh battery with 3 kW output cannot run a 5 kW HVAC startup surge even once. Installers who specify batteries based on energy capacity alone create systems that fail at the exact moment the homeowner needs them most. The dual-check framework — verify kW first, then kWh — prevents this. Every proposal should list both the continuous kW rating and the largest motor load the battery can start.

3. Forgetting the AC-coupled efficiency penalty.

AC-coupled retrofits are common, but the 3–8% efficiency shortfall is often omitted from runtime estimates. An installer who promises “12 hours of backup” based on DC-coupled math will deliver 11 hours in practice. That gap is perceptible to homeowners who timed their outage. Apply the AC-coupled correction factor to all runtime estimates and document it in the proposal.

4. Oversizing beyond excess solar production.

In NEM 3.0 territories and high-export-tariff markets, batteries must be chargeable by the solar array. A battery that cannot fill from daily solar production is stranded capacity. The correct sequence is: calculate daily solar excess, size battery to capture 80–90% of that excess, then verify winter production can still charge the battery to at least 60% on average. If not, reduce battery size or increase solar array size.

5. Skipping temperature margin in cold climates.

A 13.5 kWh battery rated at 25°C becomes an 11.5 kWh battery at −10°C. Installers in Minnesota, Wisconsin, and upstate New York who size based on room-temperature datasheets deliver systems that fall short during winter ice storms — the exact conditions that cause multi-day outages. Add 15–20% capacity margin for cold climates, or install the battery indoors where temperature stays above freezing.

Frequently Asked Questions

How many kWh battery do I need for my house?

For critical load backup, most US homes need 10–15 kWh of usable capacity for 24–36 hours of refrigerator, lights, router, and furnace fan runtime. For whole-home backup, 20–30 kWh is the practical minimum to cover the average 29 kWh daily US consumption. For self-consumption and time-of-use arbitrage without backup needs, 5–10 kWh is often sufficient to store evening peak solar excess. Size kW (power output) first, then kWh (energy storage), and add 15–20% margin in cold climates.

What is the difference between kW and kWh in battery sizing?

kW measures power — the rate at which a battery can deliver electricity at any moment. kWh measures energy — the total amount of electricity a battery can store and deliver over time. A 13.5 kWh battery with a 5 kW continuous output can run a 1 kW load for 13.5 hours, but it cannot start a 6 kW HVAC compressor even if it has plenty of stored energy left. You must verify that the battery’s kW rating covers your largest continuous load plus startup surge before you confirm that kWh covers your desired runtime.

Is a 13.5 kWh Powerwall enough for whole-home backup?

A single Tesla Powerwall 3 (13.5 kWh usable, 11.5 kW continuous output) can back up an average US home for approximately 10.8 hours at typical 29 kWh/day draw, or longer if loads are actively managed. It is enough for whole-home backup during short outages under 12 hours, but not for multi-day autonomy. For meaningful whole-home backup across 24–48 hours, two Powerwalls (27 kWh) are the standard recommendation. The Powerwall’s 11.5 kW output handles most residential startup surges, which is why it works for whole-home backup where smaller batteries fail on power, not energy.

Do AC-coupled batteries need more kWh than DC-coupled?

Yes. AC-coupled systems incur an extra inversion step that reduces round-trip efficiency from 94–98% (DC-coupled) to 86–96% (AC-coupled). To deliver the same usable AC kWh, an AC-coupled battery needs 3–8% more nominal capacity than an equivalent DC-coupled system. For example, a DC-coupled target of 13.5 kWh usable requires roughly 14.2–14.7 kWh nominal if AC-coupled at 90–92% round-trip efficiency. Good solar software accounts for this automatically in hourly dispatch simulations.

How does NEM 3.0 affect battery sizing in California?

NEM 3.0 reduced California solar export compensation to roughly 5–8 cents/kWh during most hours, down from retail rate net metering. This makes exporting solar to the grid financially unattractive and shifts the value proposition toward self-consumption. Under NEM 3.0, batteries should be sized to capture excess daytime solar production and discharge it during evening peak rates (typically 4 PM–9 PM), not sized for export. Oversizing beyond what your solar array can charge daily wastes capacity. The SGIP equity resilience program offers up to $1,000/kWh in rebates, but caps battery capacity at 150% of solar inverter capacity in some tiers — a trap installers frequently miss.

How much does battery storage cost per kWh?

Installed residential battery storage costs $700–$1,300 per kWh in the US, according to EnergySage and NREL data. A Tesla Powerwall 3 runs toward the lower end at roughly $700–$780/kWh installed when purchased with solar. BloombergNEF reports battery pack costs at approximately $108/kWh at the manufacturer level in 2025, meaning soft costs — labor, permitting, margin, and integration — account for 60–70% of the total installed price. Modular systems like Enphase IQ Battery 5P cost more per kWh at small scale but allow incremental expansion without replacing the entire inverter stack.