how to size home battery

P Peter Lu Energy Storage Product Manager, GreenMore A home energy storage battery stores electricity for use when you need it — at night, during a grid outage, or when the grid tariff is high. The system can be paired with rooftop solar or charged directly from the grid during off-peak hours. The right setup depends on your goal (backup, bill reduction, or both), your daily load profile, and whether you already have solar panels. Two reasons to add a home battery Most installations fall into one of two categories, and the choice shapes every downstream decision. Goal What the battery does Sizing driver Self-consumption (solar + storage) Stores excess solar generated during the day for evening use, reducing grid imports Evening load profile after the sun goes down; typically 5 kWh to 15 kWh for a single-family home Backup power (with or without solar) Keeps essential loads running during a grid outage; can be charged from solar or from the grid during off-peak hours Critical load wattage × required hours of autonomy; typically 5 kWh to 20 kWh Many systems combine both goals. A battery sized for evening self-consumption often provides a useful amount of backup as well, and a backup-sized battery can be set to charge from solar on normal days. The key is to decide which goal drives the minimum size, then check that the other goal is also reasonably met. Chemistry: why LiFePO4 is the default for home storage Lithium iron phosphate (LiFePO4, or LFP) has become the standard chemistry for home energy storage in most markets. The main reasons are thermal stability and cycle life. LFP cells tolerate higher temperatures and deeper discharge cycles than NMC (nickel manganese cobalt) cells, and they are less prone to thermal runaway under abuse conditions. The trade-off is lower energy density — an LFP pack of a given capacity is larger and heavier than an equivalent NMC pack. For a fixed installation in a garage, basement or utility room, that trade-off almost always favors LFP. NMC still appears in some space-constrained or portable products, and in higher-end residential units where weight and volume are tighter constraints. When comparing datasheets, look at the cycle life at the DoD you will actually use, not just the headline "cycles" number. Key specs that matter on the datasheet Spec Why it matters Usable capacity (kWh) This is the energy you can actually draw, after the BMS reserves. Do not compare products by nameplate capacity alone. Depth of discharge (DoD) The fraction of nameplate capacity the manufacturer allows you to use. Higher DoD means more usable energy from the same pack, but it usually shortens cycle life at that DoD. Cycle life at a given DoD Cycle curves should show how many cycles the pack delivers at different DoD and C-rate. A single "6,000 cycles" number without conditions is not comparable. Continuous and peak power (kW) Continuous power is what the battery can sustain; peak power is for short surges such as motor starting. Size continuous power against your typical load, peak against your largest single load. Round-trip efficiency The fraction of energy you get back after one charge-discharge cycle. Higher is better for self-consumption economics; for backup-only use it matters less. Operating temperature range LFP cells charge slowly or not at all below 0°C unless the pack has low-temperature charging protection. If you live in a cold climate, check this spec carefully. With solar vs without solar Adding a battery to an existing solar system is the most common use case. The battery stores excess solar that would otherwise be exported at a low feed-in tariff, and releases it when the household load exceeds solar generation. The result is higher self-consumption and lower grid imports. A battery without solar is a standalone backup or time-of-use arbitrage device. It charges from the grid during off-peak hours (or when tariffs are low) and discharges during peak hours or outages. The economics depend on the spread between off-peak and peak tariffs, or on the frequency and duration of outages in your area. In regions with unreliable grids, a without-solar battery is often the primary use case. How to size a home battery A simple starting point for backup sizing: Usable capacity (kWh) ≈ Critical load (kW) × Hours of autonomy ÷ DoD Inverter efficiency For self-consumption sizing, look at your evening load (typically 6 pm to 10 pm) and size the usable capacity to cover that window. In most single-family homes this is in the 5 kWh to 15 kWh range, but it depends on the number and type of loads. In both cases, the inverter or hybrid inverter power rating must cover your peak simultaneous load, not just the battery capacity. A 10 kWh battery paired with a 3 kW inverter can only deliver 3 kW at any moment, which may not be enough to start a well pump or an air conditioner. If you are specifying a system, see our residential range on the all-in-one home energy storage system page and the full residential lineup on the home energy storage category page. For project-specific questions, use the contact page. FAQ Can I use a home battery without solar panels? Yes. A standalone home battery charges from the grid during off-peak hours and discharges during peak hours or outages. The economics depend on your local tariff structure and the frequency of grid outages. How long will a home battery last? Cycle life depends on chemistry, DoD and C-rate. A typical LFP home battery delivers several thousand cycles at 80% DoD before capacity drops below 80% of initial. In daily cycling that translates to roughly 10 to 15 years of service, but the actual number depends on usage patterns and operating temperature. What size battery do I need for backup? List your critical loads (refrigerator, lights, internet, a few outlets) and estimate their combined wattage. Multiply by the number of hours you want backup. Then divide by the battery DoD and inverter efficiency to get the required nameplate capacity. For most homes, 5 kWh to 10 kWh covers essential loads for 4 to 8 hours. Is LiFePO4 safer than other lithium chemistries? LFP has higher thermal stability than NMC and NCA — the onset temperature for thermal runaway is higher and the energy released is lower. That said, no lithium battery is "zero risk." Proper BMS, enclosure, installation and certification all contribute to a safe system. Check for IEC 62619, UN 38.3 and relevant regional marks. P Written by Peter Lu — Energy Storage Product Manager, GreenMore Peter works with distributors, installers and system integrators to size residential and commercial storage around real load profiles. He focuses on LiFePO4 systems that match the inverter, grid rules and backup needs of each destination market.