Solar battery calculator: hour-by-hour self-consumption, savings & payback

How this battery calculator works

A home battery earns money in only one way: it moves solar electricity from the hours when you have too much to the hours when you have too little. To value it you need to know, hour by hour, how much surplus there is and how much demand follows it. Monthly averages hide exactly that, which is why rules of thumb and simple online calculators routinely overstate battery savings.

This calculator downloads a full year of hourly NASA POWER satellite weather for your location and simulates three things for all 8,760 hours:

1. Solar production with the same physics as our solar panel calculator: sun position, Hay–Davies sky model, cell temperature and inverter losses.
2. Household demand from a typical daily routine (working family, home office, retired couple, EV charged at night, or heat pump), shaped for weekdays and weekends and for the season, then scaled to your annual consumption. The heat-pump profile is driven by the actual hourly outdoor temperature of that year, so cold spells raise demand exactly when solar is weakest.
3. The battery, hour by hour: solar first supplies the house; surplus charges the battery up to its power limit and usable capacity; the remainder is exported. When demand exceeds solar, the battery discharges until it reaches the reserve level, and only then does the house import.

It then repeats the simulation for ten battery sizes from 0 to 30 kWh to find the capacity with the highest net present value.

The formulas

Worked example: 6 kWp + 10 kWh in Bucharest

The defaults describe a working family using 4,500 kWh a year with 6 kWp of south-facing panels at 32°. With 2024 weather the panels produce about 7,330 kWh (1,221 kWh/kWp). Without a battery the family uses only 24% of that directly, because most production happens while nobody is home, and covers 40% of its demand.

Add a 10 kWh battery (90% usable, 5 kW, 90% round-trip efficiency) and the battery delivers about 2,110 kWh a year, grid import falls from 2,714 to 688 kWh, and self-sufficiency rises to 85%. That is roughly 250 full cycles a year, so a 6,000-cycle LFP battery is limited by its 15-year calendar life, not by wear.

Money is the sobering part. Each shifted kWh saves the €0.25 import price but forfeits the €0.06 export credit, so the battery saves about €363 a year. At €4,500 installed it pays back in about 12 years; at a 5% discount rate its NPV is slightly negative. The sweep shows why: 5 kWh already captures €233 a year and 7.5 kWh €326, while going from 10 to 15 kWh adds only €24. In winter there is no surplus to fill the extra capacity.

How to read the results

• Self-sufficiency is the headline for energy independence; savings and payback are what decide whether to buy.
• Full cycles per year below about 150 means the battery is oversized for your solar surplus or your evening demand.
• Best battery size is the capacity with the highest NPV in the sweep, using a cost split between a fixed part (hybrid inverter, installation) and a per-kWh part. If no size has a positive NPV, the battery is a comfort or backup purchase, not an investment.
• The average summer day chart shows the battery filling by late morning and carrying the house through the evening and night. If it hits full before noon, a larger battery or more daytime loads would help.

Typical values

Southern Europe, the US Sun Belt and Australia reach the upper end of these ranges or above; northern Europe the lower end, because winter production is so small.

Tips to make a battery pay

• Size it to your evening, not your roof. Evening-plus-night consumption (typically 40–60% of daily use) is the most a battery can ever displace each day.
• Shift flexible loads first. Running the dishwasher, washing machine, water heater or EV charger at midday is free storage and often beats a second battery module.
• Look at the tariff. Time-of-use prices, dynamic tariffs or zero export payment widen the price gap and shorten payback. Enable the time-of-use option to test it.
• Prefer LFP chemistry. Lithium iron phosphate cells tolerate 4,000–8,000 cycles and 90–100% depth of discharge; lead-acid batteries lose roughly half their usable capacity to the 50% DoD limit and wear out after 500–1,500 cycles (try the lead-acid preset).
• Check what you are paid now. Under classic net metering a battery saves almost nothing; compare schemes in the net metering calculator.

Limitations

The dispatch is a simple self-consumption strategy: it does not charge from the grid, follow dynamic prices or keep capacity for forecast bad weather. Hourly resolution slightly overstates self-consumption, because short peaks from a kettle or oven exceed the battery’s power limit within an hour; real systems typically achieve a few percentage points less. Household profiles are typical shapes, not your own smart-meter data, so single days can differ substantially even when the annual totals are right. Battery degradation is modelled as a constant fade in annual savings, and costs exclude financing and possible replacement of the hybrid inverter.

Method & assumptions

Embed this calculator

Free for blogs, installers, schools and NGOs. Copy the code below; it updates itself automatically.