SnapEnergyLab

Battery tools

Build my backup

Choose the loads you actually want running during an outage. The builder separates running load, average load and startup surge, then compares runtime across four battery sizes.

How long does a 10 kWh battery run a typical essentials backup?

The default selection — refrigerator, freezer, router, LED lighting and device charging — adds up to 410 W if everything ran at once, but only 163 W on average once cycling is accounted for, or 3.90 kWh over a full day. A 10 kWh battery delivers 8.28 kWh to AC loads after depth of discharge and inverter losses, which is 50 h 57 min of runtime. The worst-case startup surge is about 1,060 W (one motor starting while the other loads are already running), which is an inverter sizing question rather than a battery sizing one.

Choose the loads you want to keep running

  • Compressor cycles roughly a third of the time.

  • Cycles slightly more than a fridge; keep the lid shut.

  • Small continuous load, runs the whole outage.

  • Evening use only in most outages.

  • Screen size drives the draw.

  • Charger draw, not battery capacity.

  • Overnight only; humidifier heaters add 60–100 W.

  • Brief but high power — sizes the inverter, not the battery.

  • Large start surge; a common reason backup systems fail.

  • Dominates any backup load list it is added to.

  • Cycling depends heavily on outdoor temperature.

  • Low energy, high surge — sizing is a power question.

  • Keeps a gas heating system usable during an outage.

163 W

estimated average load

3.90 kWh per day if the outage lasts a full 24 hours

Estimate using typical assumptions
Running load
410 W
Average load
163 W
Peak / surge
1,060 W
Daily energy
3.90 kWh

Inverter sizing

Optional ratings. Leave blank to skip the power check; runtime is still calculated.

Power check

  • Continuous-power check off — enter an inverter continuous rating to run it.
  • Startup-surge check off — enter both a startup surge and an inverter surge rating to run it.

These checks compare power ratings only. They say nothing about how long the battery lasts, and a continuous rating does not by itself define what the inverter can supply for a few seconds. This calculator checks inverter ratings only; the battery and BMS must also be capable of supplying the required continuous and surge power.

Running load is not average load

Running load adds up every selected item at its full rated power at the same instant. No diversity or staggering is assumed, so it is a deliberately conservative figure for sizing the inverter. Average load discounts each item by how much of the outage it actually draws power — that is the number that sets runtime. Surge decides whether the inverter can start the load at all.

Runtime by battery size

Each figure applies 90% usable depth of discharge and 92% inverter efficiency to the nominal capacity, then divides the remaining energy by your average load. Capacities are generic sizes, not specific products. These are energy results only — they do not test whether the inverter can supply the running load or the startup surge.

5 kWh battery

25 h 29 min

4.14 kWh reaches your appliances out of 5 kWh nominal.

10 kWh battery

50 h 57 min

8.28 kWh reaches your appliances out of 10 kWh nominal.

13.5 kWh battery

68 h 47 min

11.2 kWh reaches your appliances out of 13.5 kWh nominal.

20 kWh battery

4 d 6 h

16.6 kWh reaches your appliances out of 20 kWh nominal.

Selected loads, as used in the calculation above.
LoadWatts × qtyDuty cycleAverage drawStart surge (each)
Refrigerator150 W × 135%53 W800 W
Freezer120 W × 140%48 W700 W
Wi-Fi router and modem15 W × 1100%15 W—
LED lighting (6 fixtures)60 W × 135%21 W—
Laptop and phone charging65 W × 140%26 W—

Method

  1. 10 kWh × 0.9 usable depth of discharge = 9 kWh available at DC
  2. 9 kWh × 0.92 inverter efficiency = 8.28 kWh delivered to AC loads
  3. 8.28 kWh ÷ 0.163 kW = 50.954 hours
  4. Running load = 410 W (everything at rated power at once, no diversity assumed)
  5. Worst-case surge = 1,060 W = running load − one unit's running draw + that unit's startup surge

Energy capacity and power capability are separate limits. The runtime matrix answers the energy question; the power check answers whether the inverter can start and sustain the load.

Assumptions

  • Usable depth of discharge 90% of nominal capacity.
  • Inverter/conversion efficiency 92% on the DC→AC path.
  • No reserve held back.
  • Load is treated as constant at the average value; real loads vary.
  • Temperature effects, ageing and self-consumption of the system are not modelled.

What can change the result?

  • Which loads you actually need to run, and how many of each
  • Duty cycle of each load — compressor cycling, lights used only in the evening, etc.
  • Startup surge of motor loads, which sizes the inverter rather than the battery
  • Inverter continuous and surge ratings, which determine whether loads can start at all
  • Battery usable depth of discharge, inverter efficiency and any reserve held back
  • Temperature, ageing and whether solar recharges the pack during the outage

How the calculation works

The builder runs every load through the same battery engine as the standalone runtime calculator. For each selected item it records the running power, the quantity, the duty cycle (how much of the outage it actually draws power) and any startup surge. Running load is the sum of nameplate power; average load is that sum weighted by duty cycle. Runtime is calculated by dividing the battery's usable AC energy by the average load.

Surge is treated separately: the worst realistic case is one single unit of one motor load starting while everything else is already running. The engine replaces that one unit's normal running draw with its startup surge, rather than stacking the surge on top of the running total or assuming that two identical motors start in the same instant. That gives the inverter's surge rating the correct load to prove against.

Two deductions stand between the battery label and your appliances. First, the battery management system reserves part of the pack: a system rated at 90% usable depth of discharge only lets you take 9 kWh out of a 10 kWh pack. Second, the inverter converts DC to AC at roughly 92% efficiency, so about another 0.7 kWh becomes heat. What is left — 8.28 kWh for the default selection — is the number that actually divides into your load.

Cold weather makes this worse. Lithium packs installed in an unheated garage can lose a noticeable share of usable capacity below freezing, which is precisely when winter outages happen.

Sizing for surge, not just for energy

Motor loads — well pumps, sump pumps, compressors, air conditioners — draw several times their running power for a fraction of a second at startup. That surge contributes almost nothing to energy consumption, so it barely moves runtime, but it decides whether the appliance starts at all. If your list includes a well pump, use the surge figure when checking the inverter and battery system's power capability, and use the average load when estimating the energy capacity needed for runtime. They are different constraints and they usually point to different products.

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