Battery autonomy:
how long stored solar power will carry a load

Autonomy in plain terms

Autonomy is the number of hours a battery bank can supply a given load without any help from the sun or the grid. People often take the label capacity and divide by the load, then are surprised when the lights go out earlier. Three real-world factors stand between the label and the usable runtime.

The first is that you cannot drain a battery completely without shortening its life, so only part of the nameplate energy is usable. The second is that the inverter wastes some energy turning battery DC into household AC. The third is that the load itself is rarely constant, so the single figure you use should represent what the house will actually draw.

Because of those three gaps, a figure such as '10 kWh battery' is only the starting number. The more useful question is how many kilowatt-hours the house really receives from the bank, and that is what this calculation answers. It also gives you a way to compare two products honestly: a battery with a larger label but a shallower recommended discharge can deliver less than a smaller one that may be used more deeply.

The formula and its four inputs

It is easiest to read in two stages. The first three terms give the energy you can actually deliver to the house. Dividing by the load says how long that energy lasts.

Worked example: 10 kWh bank feeding a 1.2 kW load

Out of a 10 kWh label, only 7.2 kWh reaches the appliances, so the bank covers 72% of what a naive division would predict. The naive figure of 10 ÷ 1.2 would have promised 8.3 hours, a large and risky overstatement.

Work backwards when you are sizing. For 12 hours at 1.2 kW the house needs 14.4 kWh delivered, and the nominal bank must be 14.4 ÷ (0.8 × 0.9) = 20 kWh. Autonomy scales directly with bank size, so doubling the hours means doubling the battery you must buy.

Which input matters most

The load is usually the most controllable and the most sensitive. Halving the connected load doubles the hours; changing the efficiency by a few points barely registers. The table shows the same 10 kWh bank at different assumptions, one change at a time.

Notice that the discharge limit and the load act on the same equation in opposite directions. Tightening the usable fraction from 80% to 50% takes 6 hours down to 3.75, the same effect as raising the load from 1.2 kW to 1.92 kW. When a bank seems to be disappointing, check both: the appliances you added since the system was designed, and the settings the installer chose for the cutoff.

Series connections of parallel batteries do not change this arithmetic. What matters is the total energy in kWh, not the number of units or the voltage.

Corrections the formula does not show

• Standby draw of the inverter. Even with no appliance running, the inverter consumes a little continuously, which is effectively an extra load.
• Surge loads. Pumps, compressors and air conditioners start with a higher current than they run on, and the inverter must be able to supply that peak as well as the energy.
• Temperature. Cold and heat both reduce the capacity a battery can deliver, and cold in particular can cut it noticeably.
• Ageing. A bank loses capacity over its life, so a number computed for a new battery is optimistic after several years.
• Solar contribution. If the panels are producing during the outage, they cover part of the load and stretch the hours; the formula describes the worst case with no sun.

Because of these effects, plan a margin. Many installers keep a reserve and size to a runtime comfortably above the stated target.

As an example of the margin, a 10 kWh bank estimated at 6 hours may be planned as 5 hours of dependable runtime once standby draw and ageing are allowed for. That leaves headroom for a longer evening than you expected.

Choosing what to back up

The cheapest way to add hours is to shorten the list of loads. Separate essentials such as lights, fans, a router and the refrigerator from heavy discretionary loads. A modest essentials circuit might draw well under a kilowatt, and the same bank then lasts a long evening.

1. 1List each essential device with its wattage and typical hours.
2. 2Add the watts of what runs together to get the load figure in kW.
3. 3Apply the usable fraction and inverter efficiency from the product datasheets.
4. 4Divide and check the hours against the longest outage you realistically face.

A capacity-planning tool for renewable systems can help with the surrounding sizing questions, such as how much panel and battery you need over a day, and works well alongside the quick hours estimate here.

Finally, remember that the stated hours describe a continuous load at a constant level. Real homes draw less overnight and more in the evening, so an uneven profile can be handled by adding up energy over each hour instead of using one average. The total delivered energy still has to cover the sum, so the single-figure formula remains a good first check even when the load varies.