The Core Formula: Power Multiplied By Time
Daily electrical energy is calculated from the average power of a load and the time for which that power is drawn. If power is expressed in watts and time in hours, the result is watt-hours. Dividing by 1,000 converts watt-hours to kilowatt-hours.
Daily kWh = average W × operating hours ÷ 1,000
With the default example, a 180 W average load used for 9 hours consumes 1,620 Wh, or 1.62 kWh, in a day. The same load would use 11.34 kWh over seven identical days and 48.6 kWh over thirty identical days. Those longer-period values are useful for comparing charging requirements or utility energy, but they assume the daily pattern remains unchanged.
The calculation does not require battery voltage because it is describing the load's energy demand. Voltage becomes necessary when you later convert that energy requirement into battery amp-hours. Keeping those two steps separate helps prevent a common error: treating amp-hours from different-voltage systems as though they represented the same amount of stored energy.
Average Power Is More Useful Than A Nameplate Maximum
Many devices do not draw one fixed power level. A refrigerator cycles, a computer changes with workload, a pump starts and stops, and a radio or inverter may consume a small amount even while the main load is idle. For daily energy, the best input is the average power that represents the time period you are modeling.
If you have a watt meter or battery monitor, a measured average over a representative period is usually more informative than a peak reading. A brief startup surge still matters for whether the inverter, battery and BMS can supply the load, but it is a different design question from total energy consumed during the day.
For equipment with a predictable duty cycle, you can estimate average power before using the calculator. A 90 W device that runs 35% of the time has an approximate time-averaged load of 31.5 W. Over 24 hours, that corresponds to 756 Wh/day, or 0.756 kWh/day. This method is only as good as the duty-cycle assumption, so measured operating time is preferable when conditions vary widely.
Build A Multi-Device Daily Load Worksheet
A battery system normally serves more than one load. Calculate each device separately, then add the energy totals. This is clearer than adding nameplate wattages first because different devices can have very different operating times.
| Example load | Average power | Use per day | Energy per day |
|---|---|---|---|
| Router | 12 W | 24 h | 288 Wh |
| Lighting | 40 W | 5 h | 200 Wh |
| Laptop charging | 65 W | 3 h | 195 Wh |
| Total | — | — | 683 Wh/day |
In this illustrative worksheet, the three loads total 683 Wh/day, or 0.683 kWh/day. A real load list can include standby devices, communications equipment, refrigeration, pumps, entertainment, lighting and any other electrical demand that the battery must support.
If a device has several distinct modes, split it into separate rows. For example, a machine may consume 300 W for two hours and 40 W for another six hours. Treating those as two operating periods gives 600 Wh + 240 Wh = 840 Wh/day. This is often more accurate than inventing a single average without documenting how it was obtained.
From Daily Wh To Battery Amp-Hours
Once you know the load energy, you can make a first-pass conversion to battery capacity. The ideal amp-hour equivalent is energy in watt-hours divided by nominal battery voltage. For the 683 Wh/day example, an ideal 12.8 V equivalent is about 53.4 Ah.
That ideal figure is not yet a recommended battery size. Real designs may reserve part of the battery capacity, include inverter or converter losses, allow for aging, and maintain a margin for unusually heavy days. If a planning scenario assumed 90% conversion efficiency and an 80% usable battery window, the same 683 Wh load would require about 74.1 Ah at 12.8 V before adding any further contingency. The appropriate assumptions depend on the actual battery, inverter, BMS and operating conditions.
AC Loads, Inverters And Where You Measure Power
For an AC appliance powered from a battery through an inverter, measurement location matters. If a plug-in meter measures the appliance's AC input, that value describes what the appliance consumes on the AC side. The battery must usually supply more energy because the inverter is not perfectly efficient and may also have its own standby consumption.
If a battery monitor measures energy leaving the battery, those inverter losses and idle consumption may already be included. Adding another generic inverter-loss percentage to that measured DC energy would count the same loss twice. Write down where each measurement was taken and apply efficiency only across the part of the system that has not already been measured.
The same principle applies to DC converters and charge controllers. A transparent energy budget identifies each boundary instead of burying all losses inside one unexplained percentage.
Standby Loads Can Matter Over 24 Hours
Small continuous loads are easy to overlook because their wattage appears insignificant. Time changes the picture. A 5 W standby load that remains active all day consumes 120 Wh/day. Over thirty days, the same steady draw uses 3.6 kWh.
This is why routers, monitoring equipment, alarm systems, inverter idle draw and control electronics deserve their own rows in a battery budget. On a large home system the contribution may be modest; on a small portable or off-grid battery it can be a meaningful share of the available energy.
Daily Consumption Is Not The Same As Peak Power
Two systems can consume the same number of kWh per day while requiring very different hardware. One might use 100 W continuously; another might use a 1,200 W appliance for a short period. Their daily energy could be similar, yet the second system demands much higher instantaneous current and inverter capability.
Use this calculator to answer the energy question. Then verify the separate power question with the battery's continuous-discharge limit, BMS rating, inverter continuous and surge ratings, cable sizing, protective devices and connector limits. A battery can have enough stored energy for the day and still be unable to safely deliver a particular high-power load.
Using Daily Energy For Solar Or Backup Planning
A solar or backup design starts with the loads, not with an arbitrary battery size. Daily kWh tells you how much energy must be supplied over the modeled day. Battery autonomy determines how many days or hours of that demand you want stored, while solar production determines how quickly the energy can be replenished.
Do not assume that a 1 kWh solar array rating produces 1 kWh at the battery every day. Solar yield changes with irradiance, season, shading, orientation, controller behavior and system losses. Likewise, a battery's nominal kWh is not always fully usable. The daily-load result is therefore one input to a broader system model, not a complete solar sizing answer by itself.
How To Handle Uncertain Or Changing Loads
If usage varies, calculate more than one day rather than forcing a single precise-looking answer. A low, typical and high case can reveal whether the battery plan is sensitive to one uncertain assumption. You can also calculate weekday and weekend profiles separately or model seasonal changes in heating, cooling, refrigeration and lighting.
When the consequence of under-sizing is important, replace assumptions with measurements. A multi-day energy monitor can capture cycling behavior and user habits that a product label cannot. Record the dates, operating conditions and equipment configuration so the measurement can be interpreted later.
Common Daily-Energy Mistakes
- Multiplying peak watts by 24 hours: this can greatly overstate energy for intermittent equipment.
- Using amp-hours without voltage: 100 Ah at 12 V does not contain the same nominal energy as 100 Ah at 48 V.
- Ignoring standby consumption: a small continuous load can accumulate meaningful Wh over a full day.
- Applying efficiency twice: measured battery-side energy may already include inverter or conversion losses.
- Confusing energy with power capability: enough kWh does not prove that the battery can supply a high surge current.
- Assuming every day is identical: variable loads often require several scenarios or direct logging.
Final Check Before Using The Number
Write down what the result represents: the device, average power, hours per day, measurement location and any duty-cycle assumption. Then decide what you are using the number for. A utility-cost estimate may only need kWh, while a battery purchase requires additional voltage, usable-capacity and power-limit checks.
For safety-critical or permanent installations, use manufacturer documentation and applicable electrical requirements for the battery, charger, inverter, BMS, conductors and protection devices. This calculator is an energy-accounting tool; it does not certify equipment compatibility or installation safety.