Whole-Home Evening Backup: How Many kWh in 2026?

Short answer

For a typical whole-home evening backup load of 1,500 W for 5 hours, buy a 10 kWh home battery. The raw need is 7.5 kWh, but once you account for inverter losses, realistic discharge practice, and a small reserve, 10 kWh usable is the clean, low-regret answer for 2026.

The math

Start with the load, then work outward.

1) Raw energy need

Formula:

Energy (Wh) = Power (W) × Time (h)

Plug in the reference scenario:

1,500 W × 5 h = 7,500 Wh

So the home needs 7.5 kWh delivered to loads over the evening.

You can run the same math with the calculator behind these numbers.

2) Account for inverter losses

Most home battery systems feed AC loads through an inverter, and that conversion is not lossless. A practical planning assumption is around 90% inverter efficiency, which is in line with published inverter performance ranges from NREL and many manufacturer datasheets.

Formula:

Battery energy needed before inverter losses = Load energy ÷ inverter efficiency

Plug in the numbers:

7,500 Wh ÷ 0.90 = 8,333 Wh

So you now need about 8.33 kWh from the battery.

3) Account for depth of discharge

This step depends on the battery. In the real products below, several BYD models list total and usable capacity as the same number, meaning the manufacturer already states a usable figure. Pylontech lists lower usable than total capacity, so the discharge limit is already baked into the usable number there too.

For sizing a system from scratch, a conservative rule is:

Required nominal battery size = required usable energy ÷ allowed depth of discharge

If you assume 90% usable depth of discharge, which is common for LiFePO4 systems and consistent with the usable-vs-total pattern seen in many manufacturer specs, the math is:

8,333 Wh ÷ 0.90 = 9,259 Wh

That gets you to 9.26 kWh nominal.

If you are shopping from products that already publish usable kWh, you can skip this step and compare directly against the usable requirement.

4) Add a safety margin

Real homes are messy. Fridge compressors cycle. HVAC blowers spike. People turn on kettles and microwaves right when the battery is already busy. A 10% safety margin is a reasonable planning buffer.

Formula:

Final recommended size = adjusted battery need × safety margin

Plug in the numbers:

9,259 Wh × 1.10 = 10,185 Wh

Rounded:

≈ 10.2 kWh

That is why the practical recommendation is 10 kWh, not 7.5 kWh.

The full formula in one line

Battery size = (1,500 W × 5 h ÷ 0.90 inverter efficiency ÷ 0.90 DoD) × 1.10 margin

= 10.19 kWh

If you prefer to size from published usable capacity, use this simpler version:

Usable battery needed = (1,500 W × 5 h ÷ 0.90) × 1.10 = 9.17 kWh usable

That still pushes you into the 10 kWh class.

Real examples from our database

Below are five real products from full database, sized against this exact scenario: 7.5 kWh AC load need over 5 hours. Runtime is estimated as:

Runtime (hours) = usable capacity × 0.90 inverter efficiency ÷ 1.5 kW load

That keeps the comparison consistent with the math above.

Image	Product	Key spec	Runtime in this 1.5 kW scenario	Price
Image not yet available.	BYD Battery-Box Premium HVS 5.1	5.12 kWh usable, 7.68 kW continuous, LiFePO4	3.07 h	$4,200 MSRP
Image not yet available.	BYD Battery-Box Premium HVS 7.7	7.68 kWh usable, 7.68 kW continuous, LiFePO4	4.61 h	$5,800 MSRP
Image not yet available.	BYD Battery-Box Premium HVM 8.3	8.28 kWh usable, 9.2 kW continuous, LiFePO4	4.97 h	$6,300 MSRP
Image not yet available.	BYD Battery-Box Premium HVS 10.2	10.24 kWh usable, 7.68 kW continuous, LiFePO4	6.14 h	$7,200 MSRP
Image not yet available.	Pylontech US5000	4.32 kWh usable, 3.0 kW continuous, LiFePO4	2.59 h	$1,500 MSRP
Image not yet available.	Pylontech US3000C	3.2 kWh usable, 1.8 kW continuous, LiFePO4	1.92 h	$1,100 MSRP

Which of these actually fits the brief?

For this exact evening-backup target, BYD Battery-Box Premium HVS 10.2 is the cleanest match. Its 10.24 kWh usable capacity clears the calculated need, and its 7.68 kW continuous output leaves plenty of headroom for normal household peaks.

If you want the closest borderline option, BYD Battery-Box Premium HVM 8.3 gets very close on runtime at 4.97 hours in this scenario. That is near the target, but it leaves almost no reserve.

The BYD Battery-Box Premium HVS 7.7 is a reasonable pick only if your real average load is a bit under 1.5 kW, or if you are willing to shed some loads.

The smaller BYD Battery-Box Premium HVS 5.1, Pylontech US5000, and Pylontech US3000C are too small as single units for whole-home evening backup at this load level.

What goes wrong

1) Undersizing the battery

A battery sized to the raw 7.5 kWh number often misses the target in real use because AC conversion losses and normal reserve needs eat into runtime. That is why a 7.7 kWh class battery can look right on paper but still come up short.

2) Ignoring surge and continuous power limits

kWh tells you how long the battery can run; kW tells you what it can run at once. A battery can have enough energy for the evening and still trip out if a heat pump, well pump, or compressor startup exceeds inverter or battery output capability. IEEE and manufacturer inverter specs regularly distinguish continuous from surge output for this reason; check the actual equipment datasheet, not just the battery module.

3) Cold-weather capacity loss

Lithium batteries can lose available power and usable energy in low temperatures, and charging limits can tighten sharply near freezing. The U.S. Department of Energy notes that batteries are temperature-sensitive and system performance depends strongly on thermal conditions; if your battery lives in an unconditioned garage, treat the nameplate as best-case, not guaranteed-case. See DOE battery basics here.

4) Port and system mismatch

A battery module is not a complete backup system by itself. Communication protocol, inverter compatibility, nominal voltage, and backup gateway requirements can block an otherwise good purchase. This matters a lot with rack batteries and modular stacks: the battery may be fine, but your inverter may not support it.

When to step up a tier

Step up to the next-bigger model if any of these are true:

• Your average evening load is above 1.5 kW, even occasionally.
• You want to cover more than 5 hours without load shedding.
• You have a large motor load such as central AC, a well pump, or workshop tools.
• The battery will live in a cold garage or utility room.
• You want the system to age gracefully instead of feeling tight after a few years.
• You are planning to add loads later, such as an induction range, EV charging support, or electric water heating.

Applied to the products above, the 8.28 kWh class is borderline and the 10.24 kWh class is the safer buy. If your measured evening load is closer to 1.2 kW than 1.5 kW, then a 7.7 to 8.3 kWh unit becomes more realistic. But for a true “whole-home evening backup” framing, 10 kWh usable is the cleaner threshold.

A quick way to sanity-check your own house is to pull interval data from your utility portal or smart meter and average the load between roughly 5 p.m. and 10 p.m. for a few representative days. Then rerun the numbers in the calculator behind these numbers. If your actual average is 1,800 W instead of 1,500 W, your raw energy need jumps to 9.0 kWh, and the practical battery size moves well beyond 10 kWh.

How we picked the products above

We filtered our full database for batteries with published usable capacity, continuous power output, chemistry, warranty, and MSRP, then ranked candidates by how closely they match this exact 1.5 kW for 5 hours scenario after applying a 90% inverter efficiency assumption. We favored products that can meet both the energy target and the continuous power requirement, not just one of them. For the scoring framework behind our product coverage, see our scoring methodology.