Worked Example: Home Solar + Battery Sizing

Size the solar array

Annual household demand is 7,200 kWh/year. The existing solar calculator uses 950 kWh/kWp/year as an EngineerHub representative Baltic screening assumption; it is not an address-specific PVGIS or PVWatts result. For this worked example, an illustrative good-orientation factor of 0.95 and 5% additional shading are applied, giving:

A 100% annual-energy target therefore requires about 8.40 kWp. Using illustrative 440 W, 2.0 m² modules and a 12% layout allowance, that rounds up to 20 panels = 8.8 kWp, and the array uses about 44.8 m²—just within the stated 45 m² usable roof.

Important: 104.8% annual production does not mean the home is energy-independent. Production timing matters.

Check the seasonal mismatch

Using the same representative northern solar profile and a winter-heavy household demand profile, the array produces far more than demand in late spring and summer, but much less than demand in winter. December is roughly 38 kWh of PV against 743 kWh of household demand in this simplified monthly model; June is roughly 1,201 kWh of PV against 486 kWh of demand.

Each cell shows approximate PV production / household demand in kWh for the month. This is a screening profile, not an address-level forecast.

A household battery shifts energy by hours—not from June to December. It can improve daytime-to-evening matching but does not solve seasonal storage.

Size battery backup separately

For a 12-hour outage, suppose the home backs up a refrigerator, LED lighting, internet router, laptop and phone charging. Using the detailed appliance preset in the Battery Size & Backup Runtime Calculator, those loads require about 1.56 kWh before reserve. Adding a 15% energy margin gives 1.80 kWh at the load.

Using the Battery Size calculator’s EngineerHub screening assumptions of 90% allowable depth of discharge and 92% combined battery/inverter efficiency (replace these with actual product data for a real design):

The same load set needs only about 0.38 kW continuous after a 20% margin, but refrigerator startup raises the screening surge requirement to about 1.28 kW.

Lesson: a 10 kWh household battery would be much larger than needed for this narrow essential-load backup case. Whole-house backup is a different requirement and can be much larger.

Test a 10 kWh battery for solar shifting

Now ask a different question: can a 10 kWh battery improve use of the 7,545 kWh/year PV system? Use the following illustrative EngineerHub screening assumptions: 45% of PV is already self-consumed directly, 90% usable battery fraction, 90% round-trip efficiency and 250 useful cycling opportunities per year. These are not product guarantees or a site-specific dispatch forecast.

Before the battery, about 3,395 kWh/year of PV is used directly and about 4,150 kWh/year is surplus. A 10 kWh battery has 9 kWh usable stored energy per cycle. Across 250 useful cycles, the annual charging-throughput limit is 2,250 kWh. After round-trip losses, it delivers about 2,025 kWh/year of useful solar energy later in the day.

These values are deliberately annualized. Real self-consumption requires hourly or sub-hourly solar and load data; the screening result shows the direction and the energy limits.

Put a price on the shifted energy

For illustration only, assume electricity avoided in the evening is worth 0.25 currency/kWh and exported solar would otherwise earn 0.06 currency/kWh. With no grid charging, year-1 battery value is approximately:

At an installed battery cost of 6,000 currency, the simple payback is about 16.2 years before degradation, financing or other costs. That does not mean the battery is “bad”: backup resilience and tariff services can have value too. It means the solar-shifting economics alone are sensitive to the price spread and installed cost.

If another 5 kWh module costs 2,500 currency, the larger 15 kWh system captures more energy—but the incremental annual value is only about 186 currency/year under the same annualized assumptions, or roughly a 13.5-year incremental simple payback. This is the diminishing-marginal-value effect.