Home battery sizing without guesswork

Decide what the battery must do

A battery sized to increase solar self-consumption is a different project from a battery intended to run essential loads during an outage. Define the loads, required duration and whether solar can operate while the grid is down. Many ordinary grid-tied inverters shut down during an outage unless the system has a compatible backup output and isolation arrangement.

Measure energy before buying capacity

Use a smart meter, inverter portal or plug-in meter to measure a representative day. For backup, identify refrigerators, freezer, water pump, heating controls, communications and lighting separately from discretionary loads such as cooking and space heating. Nameplate watts describe a possible operating point, not necessarily average daily energy.

Separate capacity from output power

Capacity in kWh determines approximate runtime. Inverter continuous power in kW and short surge capability determine which appliances can run together. Pumps and compressors can demand substantially more power while starting. A battery with enough energy can still shut down if the inverter or battery-management system reaches its current limit.

Work through a 15 kWh example

A 15 kWh battery at 90% usable depth and 92% inverter efficiency delivers approximately 12.42 kWh to AC loads. At a measured average load of 500 W, the simple estimate is about 24.8 hours. A 2 kW average load reduces that to roughly 6.2 hours. Real operation also includes standby consumption and varying efficiency.

Add reserve deliberately

Do not hide reserve inside an optimistic efficiency figure. Add explicit headroom for colder battery conditions, ageing, unexpected loads and the possibility of a longer outage. If backup is critical, compare a normal case and a conservative case and verify both energy capacity and power limits.

Solar changes the system, not just the arithmetic

A 7.81 kWp array may produce more than a household uses on a clear summer day and very little during dark winter weather. Battery sizing should use location-specific generation and the intended season. PVGIS can provide an independent production estimate, but backup operation still depends on inverter topology, grid isolation and equipment compatibility.

Installation boundary

Battery voltage, protection, earthing, transfer equipment, cable sizing and fire-safe placement are site-specific decisions. Use the calculator to prepare questions and compare quotations, then have the final design checked by a qualified installer.

An outage checklist is different from a savings checklist

For backup, list the circuits supported during an outage, output limit, transfer behaviour, reserve setting and the time until recharging is possible. For savings, collect time-stamped import/export, tariffs, battery losses and cycles. A product may serve both purposes, but retaining a large outage reserve reduces the energy available for daily tariff shifting.

For an illustrative 10 kWh battery, holding 40% instead of 20% reserve removes 2 kWh from daily cycling capacity. At 90% discharge efficiency, that is 1.8 kWh less delivered per full opportunity. It may be a sensible resilience trade-off, but it belongs in the savings model. Never count the same reserved energy as available for both backup and daily savings.

Sources and scope

European Commission JRC: PVGIS · EIA: measuring electricity. References explain units and technology; hypothetical examples use the site’s own model. Owner observations are not laboratory tests.