Residential Battery Sizing: kWh by Backup Goal

Most home battery sizing advice mixes up two different engineering problems: keeping loads running during an outage, and storing daily solar surplus to use in the evening. A battery sized for one job often fails at the other. This guide gives a separate calculation for each, then shows how to reconcile them into one specification and check the power rating, efficiency and temperature limits that decide whether the system works in practice.

Last updated: November 2026

What you will learn:

• How to calculate backup kWh from a critical load audit versus a whole-home estimate
• Why self-consumption and time-of-use arbitrage use a different formula than backup
• The dual-check framework: kW limits what runs, kWh limits how long
• How DC-coupled and AC-coupled efficiency changes the nominal kWh you need
• Cold-climate and degradation margins, using published manufacturer figures
• California net billing and SGIP points that affect battery size
• A companion guide: residential battery sizing in kWh

Backup vs Self-Consumption: Two Different kWh Math Problems

Backup sizing answers “how long do the lights stay on when the grid goes down?” Self-consumption sizing answers “how much solar surplus can I store to avoid buying peak-rate electricity?” The formulas, margins and failure modes differ. Installers who run the wrong calculation end up with batteries that are either far too small for outage runtime or sit partly charged with nowhere to discharge.

Backup Sizing: How Long the Lights Stay On

Backup sizing starts with autonomy, the number of hours or days a battery must power loads without the grid:

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

Take an illustrative 10 kWh daily critical load and a 13.5 kWh usable battery. That gives 1.35 days, about 32 hours, before efficiency losses. The same battery against a whole-home average of 29.6 kWh a day (the EIA 2022 figure above) lasts 0.46 days, about 11 hours.

The critical load method is the responsible approach for single-battery systems. It makes the homeowner decide what matters in an outage: the refrigerator, internet and phone charging, a gas furnace blower, perhaps a well pump. EV charging, pool pumps, electric dryers and central air conditioning wait until the grid returns.

Outage length varies widely. Most outages are short, but storms, ice and wildfire shutoffs can stretch them to days. In high-risk regions, size for the number of hours the customer actually wants to ride through, and consider generator integration for multi-day events. For system architecture context, see our guide to on-grid vs off-grid vs hybrid solar, since backup is what turns an on-grid system into a hybrid one.

Self-Consumption and Time-of-Use Arbitrage: How Much Solar You Store

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

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

Without storage, much of the midday production is exported at whatever the export rate pays. With a battery, more of it is used on site. The value of the stored energy depends on the spread between what you pay to import and what you are paid to export.

In time-of-use markets the arbitrage window is the evening peak, when solar output falls and household demand rises. A battery sized for that does not need to cover full daily consumption. It needs to cover the evening gap and be fully recharged by the solar array the next day. For a commercial comparison, see commercial battery storage sizing, where demand charges change the economics.

Worked Example: Sizing to the Evening Gap

The numbers here are assumptions for illustration. Suppose a home draws an average of 1.2 kW between 4 PM and 9 PM, which is 6 kWh over five hours. Suppose its solar array has about 8 kWh of surplus on a typical winter day, after the home’s daytime use.

1. Evening gap: 1.2 kW × 5 h = 6 kWh that the battery should cover.
2. Recharge check: The winter surplus of 8 kWh is more than 6 kWh, so the array can refill the battery each day.
3. Efficiency: If the round-trip efficiency is 90%, putting 6 kWh of usable energy out takes about 6.7 kWh of energy in (6 ÷ 0.9), still under the 8 kWh surplus.
4. Result: A battery with about 6 kWh of usable energy fits this home. A 12 kWh battery would rarely fill in winter, and the extra capacity would earn nothing.

Run the same steps for summer and winter using the home’s own hourly data. If the winter surplus is smaller than the evening gap, size to the surplus and say so in the proposal.

Step-by-Step kWh Calculation for Backup Power

Backup sizing is the more demanding problem because it needs real load enumeration, surge accounting and autonomy planning. There are three stages: a critical load audit, a whole-home reality check, and a depth-of-discharge adjustment.

The Critical Load Audit (kW + kWh)

A critical load audit lists every device to keep running, records its running wattage, accounts for motor startup surge, and multiplies by expected runtime per day. The table is an illustration for a home with gas heat and a well pump. Replace each figure with nameplate or measured values.

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

But kWh is half the story. If the refrigerator, well pump and furnace fan all start within the same minute, the battery and inverter must deliver about 5.3 kW at peak even though the running load is only 2.2 kW. A battery whose continuous rating is below that cannot start this load profile. It has the energy but not the power. So every critical load audit must record both kWh (how long) and kW (whether the loads start).

Whole-Home Backup Reality Check

Some homeowners want every circuit live during an outage, including air conditioning, dryers and EV chargers. This is possible but expensive. The EIA figure of 10,791 kWh a year works out to 29.6 kWh a day, so:

13.5 kWh ÷ 29.6 kWh/day = 0.46 days, about 11 hours

A useful rule of thumb follows from the same arithmetic: each 1 kWh of usable capacity covers about 49 minutes of an average home’s total consumption (60 minutes ÷ 1.23 kW average draw). It breaks down for very large or very efficient homes, but it is accurate enough for a first conversation.

The reality check does a sales job too. It stops a homeowner expecting a single battery to run a large air conditioner for three days. Set expectations with arithmetic before installation, not after the first outage.

Accounting for Depth of Discharge (DoD)

Not every kWh in a battery is available. Depth of discharge (DoD) is the share of nominal capacity the manufacturer allows for regular cycling.

Nominal capacity (kWh) × DoD = Usable kWh

Manufacturers differ in how they state capacity. Enphase’s IQ Battery 5P datasheet, for example, lists 5.0 kWh total capacity and 5.0 kWh usable capacity, and notes that the usable figure includes a 2% safety limit with a further 3% kept for battery electronics at night. Other makers state usable energy differently, so read the datasheet for the exact battery you quote. When comparing quotes, insist on usable kWh, not nominal. A “10 kWh” battery with 90% usable delivers 9 kWh of runtime.

The Dual-Check Framework: kW Power + kWh Energy

Battery marketing leads with kWh because it is easier to sell. Installers know that kW, the rate of power delivery, is the gatekeeper. A battery with plenty of kWh and too little kW cannot start motor loads. A battery with high kW and low kWh starts everything and dies in two hours. Both checks must pass on their own.

Why kW Rating Determines What Runs

Take a battery with 10 kWh of usable energy and 3 kW of continuous output. Ten kWh divided by a 1 kW load is 10 hours of runtime. It could also run a 2.5 kW heater for 4 hours (10 ÷ 2.5). But it cannot run a 5 kW compressor even for a minute. The energy is there; the power is not.

Motor loads are the usual failure point. Compressors, well pumps, sump pumps and pool pumps draw several times their running wattage for a second or two at startup. A battery sized only on kWh will calculate enough runtime and then trip on overload when the compressor starts. Check the surge rating against the largest motor on the critical load list.

Enphase publishes both numbers for the IQ Battery 5P: 3.84 kVA continuous and up to 7.68 kVA peak power, with a peak output current of 32 A for three seconds. That is about 0.77 kVA of continuous power per kWh of usable energy. Use the same ratio check, from the datasheet, on any battery you propose. For inverter pairing, see our hybrid inverter guide, because the inverter’s surge capability is often the limit before the battery’s.

Matching Inverter kW to Battery kWh

There is no universal kW-to-kWh ratio. Work backward from the load audit: the continuous rating must cover the running load of everything you want live at once, and the surge rating must cover the largest start. Then check runtime in kWh. A system with a high power-to-energy ratio suits whole-home backup with large motors. A lower ratio suits time-of-use shifting where the loads are modest.

Solar design software that supports energy-yield and financial modeling helps you test the energy side: it can run production against load profiles. Check separately that the battery and inverter pair meets the power and surge checks from the datasheets.

DC-Coupled vs AC-Coupled: The Sizing Penalty

Every conversion between DC (panels and battery) and AC (home loads) loses some energy as heat. The more conversion steps, the more nominal kWh you need to deliver the same usable AC kWh. An AC-coupled system that ignores this will deliver less runtime than the homeowner was promised.

Round-Trip Efficiency from a Datasheet

In a DC-coupled system, solar feeds the battery through a charge controller and a single hybrid inverter converts to AC. In an AC-coupled system, a grid-tied inverter makes AC from the panels, and the battery has its own inverter for charging and discharging. That adds conversion steps.

Manufacturers publish round-trip efficiency figures, and they depend on how it is measured. Enphase’s IQ Battery 5P datasheet lists an AC round-trip efficiency of 90% (AC to the battery and back to AC at 50% power rating) and a DC round-trip efficiency of 96%.

AC-coupled designs suit retrofits, because you can add a battery to an existing grid-tied solar system without replacing its inverter. DC-coupled designs tend to be more efficient for new installations. Which is right depends on the existing equipment and the customer’s goals.

Adjusting Nominal kWh for Efficiency Losses

Divide the usable AC energy you need by the round-trip efficiency to estimate the energy you must put in. For 13.5 kWh of usable AC energy:

• At 96% efficiency: 13.5 ÷ 0.96 = 14.1 kWh
• At 90% efficiency: 13.5 ÷ 0.90 = 15.0 kWh

In this example the lower efficiency needs about 7% more nominal capacity (15.0 versus 14.1). Use the datasheet efficiency of the exact product and architecture you are proposing, and say which measurement basis you used.

Temperature, Degradation, and Safety Margins

Batteries do not deliver datasheet capacity in every condition, and they fade over time. A system sized for 25°C on day one can underperform in winter and fall short before the warranty ends.

Cold-Climate Temperature Limits

Cold reduces what a lithium battery can deliver, and the management system may limit charge or discharge to protect the cells. Read the operating range on the datasheet. The Enphase IQ Battery 5P datasheet gives an ambient operating range of -20°C to 50°C for charging and -20°C to 55°C for discharging, with an optimum range of 0°C to 30°C. Outside the optimum range, plan for reduced performance.

The datasheet does not give a capacity-versus-temperature curve, so any margin you add is a design judgment, not a published figure. A reasonable practice is to add a margin for batteries installed outdoors in sustained sub-freezing climates, or to install them indoors where the ambient temperature stays above freezing. If the battery has a self-heating function, include its draw in the critical load audit, because it uses energy during the outage.

Degradation Planning

All lithium-ion batteries degrade. Warranty terms differ, so use the actual one. Enphase’s IQ Battery 5P datasheet lists a limited warranty of more than 60% capacity for up to 15 years or 6,000 cycles, whichever comes first. The formula for guaranteed capacity at the end of a warranty is:

Guaranteed usable at end of warranty = Usable capacity today × warranty retention

For a 5.0 kWh usable battery and a 60% retention floor, that is 3.0 kWh. A warranty floor is a legal guarantee, not a performance forecast, so ask the manufacturer for expected capacity-retention curves, and use them to set the homeowner’s expectations. If the homeowner’s critical load needs a fixed number of kWh in year 10, size the battery so that number is still available after degradation.

Reading a Battery Datasheet: What to Compare

Datasheet numbers mean nothing without runtime context. A homeowner cares whether the refrigerator stays cold and the furnace fan runs. This table translates generic battery sizes into runtime against two loads. It is arithmetic, not a product claim, and it ignores efficiency losses and power limits.

When you compare real products, build a short sheet for each: usable kWh, continuous kW or kVA, peak or surge power and its duration, round-trip efficiency and the basis, operating temperature range, warranty term and retained capacity, and whether it is AC- or DC-coupled. This guide quotes manufacturer specifications only where the manufacturer’s own datasheet was available. For an example of the full set of datasheet fields, the Enphase IQ Battery 5P lists 5.0 kWh usable, 3.84 kVA continuous, 7.68 kVA peak, lithium iron phosphate chemistry, a 15-year or 6,000-cycle limited warranty and a UL 9540A evaluation. Read the datasheet for any other battery directly.

A 5 kWh class battery suits time-of-use shifting for a small evening gap but is not built for whole-home backup with large motors. A larger battery with a high power rating suits heavier loads, at a higher cost. Match the product to the job and avoid defaulting to the largest option without running the numbers.

For homes with EV charging, see our solar EV charging integration guide. EVs are not part of a standard critical load audit, but they are often the largest single load, so exclude them from backup circuits unless the system is sized for them.

Costs, Incentives, and California Net Billing

Battery economics depend on local rates, incentives and the installer’s quote, and they change quickly. This section covers the points that most often affect sizing. Verify every figure against the current program documents.

Installed Cost per kWh

Installed cost per kWh varies widely by product, installer, electrical upgrades and region, and this guide does not quote a figure because published averages differ by source and date. Ask for quotes in dollars per usable kWh, and compare them on that basis, with equipment, labor, permitting and any electrical upgrades broken out. A homeowner paying for several kWh of capacity they will never use pays for it in full, so every kWh in a proposal should be justified by the load audit.

For payback modeling against real rate structures, use a tool that can run hourly dispatch, such as SurgePV’s generation and financial tool, and check the results against the customer’s actual tariff.

California Net Billing (NEM 3.0) and SGIP

California’s Net Billing Tariff, widely called NEM 3.0, replaced the earlier retail-rate net metering for new applicants from April 2023 (CPUC Decision 22-12-056). Exports are valued hourly using the CPUC’s Avoided Cost Calculator, so export rates vary by hour and month and sit well below retail on average. Third-party guides put typical average export values in the single-digit cents per kWh, but your utility’s published schedule is the number to use.

The consequence for sizing is that self-consumed solar is worth more than exported solar, which strengthens the case for storage. A sound approach under this structure is:

1. Size solar to the household’s consumption instead of oversizing for export.
2. Size the battery to capture the daily solar surplus and discharge it in the evening.
3. Do not size the battery larger than the array can recharge on a winter day.

The oversizing trap is real. A battery that the solar array cannot fill sits partly empty, and the homeowner paid for it in full.

The Self-Generation Incentive Program (SGIP) offers incentives for storage. The CPUC’s SGIP page lists an Equity Resiliency budget at 1,000 in its “$/kWh” incentive column and describes it as available through 2025, so confirm the unit and the current budget status with the program administrator before promising any rebate. The page says eligible customers must meet additional criteria and enroll in a qualified demand response program.

Five Mistakes That Waste Money on Oversized Batteries

These five mistakes account for many post-installation complaints.

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

Whole-home backup is expensive. A critical load audit often shows that a large share of daily consumption is not essential during an outage, such as pool pumps, EV chargers and dryers. 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 battery with too little continuous or surge power cannot start a compressor, whatever its energy capacity. Every proposal should list the continuous rating and the largest motor load the battery can start.

3. Forgetting the AC-coupled efficiency penalty.

AC-coupled retrofits are common, but the extra conversion losses are often left out of runtime estimates. Apply the datasheet round-trip efficiency to every runtime estimate and document it in the proposal.

4. Oversizing beyond excess solar production.

Where exports are poorly paid, batteries must be rechargeable by the array. Calculate daily solar surplus, size the battery to capture most of it, then check that winter production can still charge it. If not, reduce the battery or enlarge the array.

5. Skipping temperature margin in cold climates.

A battery rated at 25°C can deliver less in a freezing outbuilding. Read the operating range on the datasheet, and either add margin or move the battery indoors.

Ready to test a design? Book a SurgePV demo to run a load profile against a solar and storage design, or read our AC vs DC coupled battery guide for the architecture trade-offs. For service lines that add storage to an installer’s offer, see adding battery storage services.

Frequently Asked Questions

How many kWh battery do I need for my house?

It depends on the job. For backup, divide daily critical-load kWh by usable battery kWh to get days of autonomy, and check the kW rating covers your largest motor start. For self-consumption, size to the evening gap your solar can refill. Whole-home backup for an average US home (29.6 kWh a day, EIA 2022) needs far more than critical loads.

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

kW is power, the rate a battery can deliver at any moment, and kWh is energy, the total it can store and deliver. A battery with plenty of kWh but too little kW cannot start a large compressor, so check the continuous and surge kW against your largest motor before you check that kWh covers the runtime you want.

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

For an average US home using about 29.6 kWh a day, a 13.5 kWh usable battery runs the whole house for roughly 11 hours before efficiency losses. That suits a short outage or a managed set of loads. A day or more of whole-home backup takes about double the capacity or active load shedding.

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

Usually yes, because the extra conversions lower round-trip efficiency. Enphase’s IQ Battery 5P datasheet lists 90% AC and 96% DC round-trip efficiency, so delivering 13.5 kWh of usable AC energy takes about 15.0 kWh at 90% versus 14.1 kWh at 96%, roughly 7% more. Use the datasheet figure for your product.

How does NEM 3.0 affect battery sizing in California?

NEM 3.0, California’s Net Billing Tariff, values exports hourly at far below retail on average, so self-consumed solar is worth more than exported solar. Size the battery to capture daily surplus and cover the evening peak, and do not size it larger than the array can recharge on a winter day.

How does cold weather affect battery sizing?

Cold can reduce the capacity and power a lithium battery delivers, so datasheet limits matter. Enphase lists a -20°C to 50°C charging range and an optimum of 0°C to 30°C for its IQ Battery 5P. Add margin for outdoor, sub-freezing installs, or place the battery indoors, and include any heater draw in the load audit.

How do I compare residential battery quotes?

Compare usable kWh, continuous and peak kW, round-trip efficiency and its measurement basis, operating temperature range, warranty term and retained capacity, and AC or DC coupling. Ask for price per usable kWh with equipment, labor and electrical upgrades broken out, so quotes can be compared like for like.