A home battery is usually judged on two numbers: how many kilowatt hours it holds and how many kilowatts it can deliver. There is a third number that nobody puts on the brochure, and over a year it quietly subtracts a meaningful slice of what the system was supposed to save.
The Two Places Energy Disappears
The first is round-trip efficiency, which is the share of what you put in that you get back out. Home systems typically land somewhere near ninety percent. Put ten kilowatt hours in and about nine come out, with the missing tenth spent as heat in the inverter during two conversions and inside the cells themselves.
The second is idle consumption, and it is the one people are unprepared for. The inverter and its control electronics draw power simply to stay awake, whether or not anything is charging or discharging. Published figures for home-scale equipment generally fall in the tens of watts, often cited as roughly one to two percent of rated output. A unit sitting at forty watts of standby draw consumes close to a kilowatt hour a day doing nothing at all.
Over a year that is a few hundred kilowatt hours, which for many households is comparable to running a second refrigerator. It does not make a battery a bad purchase, but it does change the arithmetic, and it is usually absent from the payback estimate on a quote.
There is a third effect specific to cold climates that compounds the first two. Systems with active thermal management spend energy keeping the cells in range, and that spend is highest in exactly the weather where the stored energy is most valuable.
The practical framing is that a battery has a baseline cost of ownership measured in electricity rather than dollars, and it is paid whether the battery is working or idle. Sizing matters here too: two batteries have roughly twice the cell capacity but their shared electronics do not double the idle draw, so the per-kilowatt-hour overhead of a larger system is usually lower than a small one.
The Hardware Stack:
Tesla Powerwall 3: Publishes a round-trip efficiency figure in its datasheet, which is the number to compare rather than any marketing claim.
Emporia Vue: A whole-panel monitor that can show the battery's own consumption as a separate circuit, which is how idle draw becomes visible.
Enphase IQ Battery 5P: A modular system, relevant because adding capacity in blocks changes the ratio of fixed overhead to stored energy.
Vulnerability Score
Unbudgeted overhead: Standby draw and conversion losses are rarely included in a payback estimate, so the real saving is smaller than the one presented at the quote stage.
Winter compounding: Thermal management consumes stored energy in cold weather, which is when the reserve matters most and when solar is least able to replace it.
Oversizing penalty: A battery bought much larger than the household needs pays its fixed overhead every day regardless, which can outweigh the benefit of the extra capacity.
Measuring It in Your Own House
The figure that matters is not the datasheet's, it is yours. If you have a panel monitor, give the battery circuit its own channel and watch it through a quiet night with nothing charging or discharging. What you see is the real standby draw for your equipment at your temperature.
Use that number twice. First, add it to the running cost side of any payback estimate, because a few hundred kilowatt hours a year at your rate is a real figure. Second, use it when sizing: if the overhead is meaningful, a system matched to what the household actually uses will beat an oversized one, which is the opposite of the usual instinct when buying storage.
Written by Mason Vance
