What This Calculator Actually Solves
The calculator answers one narrow question: how many hours can a stated amount of sunset energy support a stated average load while preserving a chosen reserve? It does not predict tomorrow's solar production and it does not assume that the battery begins every evening fully charged.
The calculation is:
With the default example, 5,200 Wh is available at sunset, the average overnight load is 420 W and the reserve is 20%. The energy assigned to the night is therefore 4,160 Wh. Dividing 4,160 Wh by 420 W gives about 9.9 hours of estimated coverage.
That result is an energy estimate, not a guarantee that every connected appliance can operate. The inverter, BMS, battery and cabling must also support the required voltage, continuous current and short-duration surge current.
Input 1: Energy Available At Sunset
"Energy at sunset" should represent the battery energy you can realistically draw from at the moment meaningful solar charging has ended. This is different from the battery's advertised energy rating. A 10 kWh battery may enter the night with only 6 kWh remaining after daytime consumption, or it may be intentionally limited to a narrower state-of-charge window.
A battery monitor or inverter energy meter is usually a better source than simply multiplying nominal volts by nameplate amp-hours. If you must estimate from capacity, keep the assumptions visible. For example, a nominal 48 V, 100 Ah bank is roughly 4.8 kWh by simple multiplication, but the energy available at sunset still depends on actual state of charge, configured cutoffs and measurement accuracy.
Do not subtract the dawn reserve twice. The calculator already removes the reserve percentage from the entered sunset energy. If your "energy at sunset" number has already been reduced to a usable-to-cutoff value that includes the reserve, enter a 0% reserve or recalculate the starting value so each allowance appears only once.
Input 2: Average Overnight Load
The average load is the most common source of runtime error because household loads are rarely constant. A refrigerator may draw substantial power while the compressor is running and almost nothing between cycles. Pumps, fans, dehumidifiers and air conditioners behave similarly. Small network equipment, standby electronics and the inverter itself may run continuously.
For a better estimate, translate cycling loads into an overnight average. If a 120 W refrigerator runs about 35% of the time, its rough average contribution is 42 W. Add that to continuous loads such as a 12 W router, a 20 W security system and any inverter idle draw. Repeat this for all loads expected to operate during the night.
| Load type | Example method | Overnight contribution |
|---|---|---|
| Always-on device | Rated or measured watts | Use the measured average directly |
| Cycling appliance | Running watts × duty cycle | Use the resulting average watts |
| Short event | Watts × hours | Convert to Wh, then spread across the planned night if needed |
| Inverter standby | Measured idle watts | Add whenever the inverter stays on |
If you have an energy monitor, measuring total watt-hours used during several representative nights is even better. Divide the measured Wh by the number of hours to obtain an observed average power for those nights.
Input 3: Required Dawn Reserve
The reserve is the share of sunset energy you intend to leave unused by the overnight plan. A reserve can serve several purposes: keeping emergency energy available, avoiding an aggressive discharge target, allowing for forecast uncertainty, or preventing the system from reaching a configured low-state-of-charge limit before morning.
There is no single reserve percentage that is correct for every solar-battery system. The appropriate margin depends on battery chemistry, manufacturer limits, backup priorities, next-day charging expectations and the consequences of running out of stored energy. The calculator therefore asks you to choose the reserve explicitly rather than hiding a universal derating factor in the formula.
Reserve has a linear effect in this simple model. With 5,200 Wh at sunset, a 10% reserve leaves 4,680 Wh for overnight loads, while a 30% reserve leaves 3,640 Wh. At the same 420 W average load, those cases correspond to about 11.1 hours and 8.7 hours respectively.
Runtime Versus “Will It Last Until Sunrise?”
A runtime value becomes operationally useful only when compared with the length of the night you need to cover. If your critical period is 12 hours and the calculator returns 9.9 hours, the battery plan has an energy shortfall under those assumptions. You can close that gap by entering the night with more stored energy, reducing average load, accepting a smaller reserve where appropriate, or increasing storage capacity.
If the estimate is 13 hours for a 12-hour night, that does not mean the design has a comfortable margin. Small changes in actual load, battery state of charge or conversion efficiency can consume the remaining hour quickly. It is better to run several scenarios than to treat a single result as precise.
A useful three-case check is to compare:
- Expected night: measured typical sunset energy and typical overnight load.
- Heavy-load night: the same sunset energy with a higher average load.
- Weak-solar evening: lower sunset energy with the same or higher load.
If only the optimistic case reaches sunrise, the system is highly dependent on favorable conditions.
How Inverter And Conversion Losses Fit Into The Estimate
The calculator does not provide a separate inverter-efficiency input. That is deliberate so the formula remains transparent, but it means you must decide where losses are represented. The cleanest approach is to use a battery-side sunset energy measurement and a battery-side average power measurement. Then the observed load already includes inverter and conversion losses.
If your load estimate is based only on AC appliance watts, battery demand may be higher because the inverter consumes additional power. Likewise, DC converters, battery heaters, control electronics and standby functions can draw energy that is not obvious from appliance labels. Add those loads to the average or reduce the sunset energy by a documented loss allowance—but do not do both for the same loss.
For systems with significant surge loads, runtime and inverter sizing must be checked separately. A battery may have enough watt-hours to run a device for many hours and still be unable to supply the device's startup surge without exceeding inverter or BMS limits.
Temperature, Battery Age And State-Of-Charge Accuracy
Available energy can differ from the nominal or displayed value because real batteries operate under changing conditions. Cold temperatures can reduce available discharge performance for many battery systems, while high temperatures can accelerate degradation. Battery age and cycle history can also reduce the energy that can be delivered before a low-voltage or state-of-charge cutoff is reached.
State-of-charge readings are estimates. A shunt-based monitor can drift if it is not synchronized correctly, and voltage-based state-of-charge estimates can be especially uncertain for chemistries with flat voltage curves. If the system repeatedly reaches low-voltage cutoff earlier than the calculator predicts, verify the load measurement, battery capacity, monitor calibration and configured protection thresholds before assuming the formula itself is wrong.
For planning, prefer measured delivered energy from recent use over old nameplate assumptions. A capacity test or several nights of logged battery data can reveal whether the bank still behaves close to its original rating.
A Worked Overnight Example
Suppose a home battery shows 7,500 Wh available at sunset. You want to preserve a 25% reserve. Your measured night-time load averages 500 W, including the inverter's own draw.
The usable overnight budget is 7,500 Wh × 0.75 = 5,625 Wh. Dividing by 500 W gives 11.25 hours of estimated runtime.
If the critical night is 12 hours, the expected energy shortfall is about 0.75 hour × 500 W = 375 Wh. That gives you a concrete target: either start the night with at least 375 Wh more usable energy, reduce the average overnight load by roughly 31 W across 12 hours, or revise the reserve only if doing so is consistent with the battery and backup strategy.
Now stress-test the same system after a poor solar day. If sunset energy falls to 6,000 Wh while load remains 500 W and reserve remains 25%, runtime drops to 9 hours. This comparison shows why the evening state of charge matters as much as the battery's nominal size.
Common Mistakes That Make Overnight Runtime Look Better Than Reality
- Using battery nameplate Wh as sunset energy. The battery may not be full when the sun goes down.
- Ignoring inverter idle consumption. A modest continuous draw can add hundreds of watt-hours over a long night.
- Using peak watts as the average. This can understate runtime dramatically for cycling loads; measure duty cycle instead.
- Using only appliance label watts. Labels may describe maximum or rated conditions rather than actual average use.
- Subtracting reserve or losses twice. Track exactly where each derating is applied.
- Assuming tomorrow's solar will arrive on schedule. Overnight coverage should be evaluated independently from the next day's recharge plan.
- Ignoring cutoff settings. A BMS or inverter may stop discharge before all nominal battery energy is accessible.
When To Use A Different Solar Battery Calculator
This page is specifically for the sunset-to-sunrise discharge window. Use a broader autonomy calculation when you need to model multiple days without adequate solar input. Use a recharge-time calculator when the question is how quickly tomorrow's PV production can refill the battery. Use an energy-loss calculator when you need to separate conversion losses from stored energy, and use a system sizing tool when you are choosing battery capacity for a target number of nights.
Keeping those questions separate prevents one calculator from pretending to model every part of a solar-storage system. Overnight runtime is the discharge side of the problem; daytime PV production and recharge capability are the next steps.
Final Checks Before Using The Estimate
Before using the number for equipment selection or backup planning, confirm four items: the sunset-energy figure comes from a credible measurement or conservative estimate; the overnight load represents realistic average demand; the reserve reflects your actual operating objective; and the connected equipment can support the required continuous and surge power.
Then compare the estimate with one or more observed nights. If the battery reaches dawn with substantially more or less energy than expected, update the inputs from the measured data. The calculator is most valuable as an iterative planning tool: measure, estimate, observe and refine.
For installation, wiring, protection, charging and battery-specific operating limits, follow the battery, inverter, BMS and solar-equipment manufacturer documentation and any applicable electrical requirements.