The result is an hourly comparison of the same solar setup with 0, 1, 2, 3, 4 and 5 kWh of nominal battery capacity. It is a planning model, not a product test or installation design.
Weather and generation inputs
Germany uses PVGIS 5.3 / SARAH3 output for the calendar year 2020: 8,784 hourly records per profile. The United States uses PVWatts V8 DC output from an NSRDB typical meteorological year: 8,760 hourly records. A typical year is not the weather for 2026.
Berlin, Hamburg, Munich, Salt Lake City and Los Angeles have south-facing vertical, south-facing 30°, east-facing vertical and west-facing vertical profiles. New York currently has a south-facing vertical profile only. Other orientations are disabled for that reference. There are 21 profiles, retrieved September 26, 2026.
Each source run uses 1 kW of nominal panels and zero configured system loss. PVGIS power output is treated as an array-power proxy; PVWatts uses its DC output field. The two models are not identical. This simplified harmonization has not been calibrated against a particular product or household. Do not compare small differences between countries as measured performance.
Losses and consumption
- Array power is scaled by your panel capacity, a fixed 10% system reduction and your additional shading percentage.
- Inverter efficiency is 96%; battery charge and discharge efficiency are 95% each.
- Usable battery energy is 90% of nominal capacity. Charge and discharge also respect the entered DC power limit.
- Three illustrative daily load shapes are normalized to your annual household use. They are not measured household data.
- The battery's entered continuous overhead is added to consumption for every hour. It is included in the net savings comparison.
PVGIS timestamps are converted to the reference city's local civil hour. PVWatts data uses local standard hour. No room-level shading, snow, tree geometry, weekend pattern or user-specific interval-meter data is modeled.
How each hour is balanced
- Use available solar to meet current consumption within the shared AC output limit.
- Store remaining solar within charging power and usable battery capacity.
- Discharge the battery to meet remaining consumption within the unused part of that same AC limit.
- Remaining generation is exported within the limit or curtailed. Export is assigned zero value.
- Carry the state of charge into the next hour. Each annual run starts empty.
The model represents DC storage sharing one inverter output. It does not represent a separate AC-coupled battery or a system that can power your home during an outage.
Economic definitions
Extra battery savings = savings with battery − savings without battery.
10-year added net = sum of 10 years of extra battery savings − full added battery cost.
Battery cost is nominal kWh × your cost per kWh, plus the fixed added cost. This is a linear estimate; real product quotes may have discrete sizes and different pricing. The calculator charges no fixed battery cost to the zero-battery case.
The 10-year model repeats the reference weather and load shape. Solar output decreases by 0.5% per year; usable battery capacity decreases by 2% per year. These are explicit model assumptions, not manufacturer warranty predictions. Tariff growth, export compensation, discounting, repair and replacement costs are zero. Subsidies and tax benefits are not automatically deducted.
Whole-system net also subtracts the solar setup cost in new-system mode. Retrofit mode excludes past solar costs. Undiscounted 10-year net is not net present value, a financing return or guaranteed payback.
Simple payback divides upfront cost by first-year annual savings, holding that year constant. The battery-only version divides added battery cost by added battery savings. It omits later degradation, replacement and discounting; values beyond ten years are labelled as such rather than treated as lifespan forecasts.
How the suggested size is chosen
Zero storage is always eligible. If no battery improves 10-year added net above zero, the result suggests no battery. Otherwise, it chooses the smallest tested capacity within the larger of 20 currency units or 5% of the best added net, while keeping added net non-negative. The table shows all tested sizes so you can see the trade-off.
The −20% / +20% weather scenario changes available solar energy only, using the suggested battery size. It is a sensitivity test, not a probability range.
Use a different method when
You need time-of-use tariffs, demand charges, paid exports, custom coordinates, detailed shade, a separate AC battery, emergency backup or precise equipment selection. Your actual bills, interval-meter data and product documentation are better inputs for a site-specific assessment.
Data sources and version
Calculator model: 1.0.0. Research source review: September 26, 2026. Individual downloads include the inputs, source period and model version.