Runtime is one division: battery kilowatt-hours divided by the average load in kilowatts equals hours. Every battery sizing conversation is that division, plus three honest corrections. The battery holds less than its label says. The load averages less than its nameplates say. And the battery has power ceilings that can fail you even when the tank is nearly full.
The division
A tank and a drain. 16kWh divided by 0.4kW is 40 hours. 16kWh divided by 4kW is 4 hours. Same tank, wildly different answer, and the bottom number is the one you control.
The word carrying the weight is average. Almost nothing in a house runs flat out. A fridge draws its compressor watts for a while, then cycles off, so across an evening it averages far below the number on its label. Runtime cares about the average across the hours you are counting, and honest averages come in lower than adding up nameplates. When it matters, measure: a plug-in power meter on the fridge for a day beats any table on the internet.
Correction one: usable capacity is smaller than the label
The spec sheet for the EG4 WallMount 314Ah battery says nominal capacity 16kWh, and two lines down it says usable energy 12.86kWh at the recommended 80% depth of discharge. Both numbers are on the same page. The 16 is what the cells hold; the 12.86 is what EG4 recommends you actually cycle, because riding a lithium battery to empty every night shortens its life. Do runtime math on the usable number.
The 14.3kWh WallMount 280Ah battery makes the same point a different way. Its sheet states the full capacity and separately recommends a 20% state-of-charge cutoff. Apply that cutoff yourself and the everyday working capacity is about 11.4kWh. Same battery either way. The honest planning number is the smaller one.
There is a second small cut on top: the battery's round-trip efficiency is 95% on the 314Ah sheet, and the inverter takes its own few percent turning DC into AC. The napkin version: do the division on usable capacity, then round down.
Correction two: the battery has power ceilings too
Kilowatt-hours are the tank. Kilowatts are how fast you are allowed to pour. A battery has both, and the second one is where sizing mistakes hide.
The 314Ah battery's sheet: 200A maximum continuous discharge, which at its 51.2V nominal works out to the 10.2kW continuous discharge power printed on the same sheet, plus a peak of 12.8kW for 3 seconds. The 280Ah battery is more limited: 140A continuous, about 7.2kW.
Now put a real inverter next to that. The FlexBOSS21 is rated 12kW continuous output on battery alone. One 314Ah battery can only deliver 10.2kW continuously, so a single battery cannot feed that inverter at full battery-only output. EG4 says so directly: the FlexBOSS21 sheet lists a recommended minimum battery capacity of 600Ah per inverter. Two 314Ah units in parallel is 628Ah, which is exactly why a recent 44-panel whole-home design we ran carries two of them as one shared bank.
Surge stacks on top of this. The FlexBOSS21 can push 24kW for half a second to start motors, and the battery bank has its own peak rating. When the keep-running list has a well pump or a compressor on it, both sheets get checked, because the weaker ceiling is the one you meet.
The worked example
Say you want essentials backup: fridge, lights, fans, modem, phone chargers. Measured honestly, that averages around 0.4kW (a typical figure; your meter beats our estimate). One 16kWh wallmount:
12.86kWh usable divided by 0.4kW is about 32 hours. Round down for the inverter's cut and call it a solid day and a quarter with no sun at all.
Now the same house asks about whole-home with central AC. A typical central unit runs 3 to 5kW, so the evening average might sit near 4kW. 12.86 divided by 4 is about 3 hours. The battery did nothing wrong. That gap is the whole critical-loads conversation, and it is also why panels change the question: with PV refilling the tank every day, the battery only has to carry the night, and a 0.4kW night is a few kilowatt-hours against a 12.86kWh usable tank.
Write your own napkin in four lines: your measured or estimated average load, the usable kWh of the bank you are considering (from the spec sheet, never the nameplate), the division, and the power-ceiling check against your biggest motor. If the honest math says a smaller bank covers you, buy the smaller bank. We will tell you the same thing in a quote. Want the math run against your actual bill? Start the estimate.
Sources
- EG4 WallMount Indoor 314Ah Spec Sheet, v1.0.4
- EG4 WallMount Indoor 280Ah Battery Specifications Sheet, v1.1.7
- EG4 FlexBOSS21 Spec Sheet, v1.2.9