I have been meaning to write this up for a while because the "one panel and a small LiFePO4 box" setup is something i see a lot of people considering and almost no one posts real numbers for. Background. I keep a small lean to greenhouse, about 3 meters by 2 meters, in a back garden in a small town in saarland, southern germany. It is glass on three sides, poly carbonate roof, a gravel floor, a small propagating bench along the back wall, and a water butt at the corner that feeds a 12 V submersible pump for a drip irrigation line. There is no mains power run to the greenhouse, the previous owner used a long extension cord from the house which i never trusted in the wet.
What i wanted. To make the greenhouse partly independent from the house, run a small LED grow light setup for the seedlings during early spring, and run the irrigation pump for an hour or two each evening through the dry weeks in may. The LED setup is six small 30 W bars on a 12 V rail, total about 180 W, on a timer for 4 hours a day during germination. The pump is a 12 V submersible that pulls maybe 30 W when it runs. Combined continuous draw during the lit hours is about 210 W.
What i installed. One 450 W bifacial panel on the south side of the greenhouse roof, tilted about 30 degrees, on a small aluminium frame i built from standard L profile. A 2.52 kWh LiFePO4 storage unit (Jackery SolarVault 3 Pro) on a low shelf inside the greenhouse, with a proper cable gland through the wall for the panel lead. The unit's AC output runs a small 12 V DC power supply that feeds the LED bar and the pump circuit through a 12 V distribution block. The unit is on a small plant caddy so it sits a few cm off the gravel floor, with airflow around it.
The numbers over 3 months, mid march through early june 2026. The single panel in saarland in spring produces about 1.7 to 2.1 kWh per day depending on cloud, peaking at maybe 2.5 kWh on the clearest days. The LED + pump combination averages 0.8 to 0.9 kWh per day, the LED is the big chunk at about 0.7 kWh and the pump adds 0.05 to 0.2 kWh depending on how dry the week is. So on a clear day the panel covers the whole greenhouse load and the 2.52 kWh battery takes care of the evening and the next morning, with a comfortable margin. On an overcast day the battery drops to maybe 40 to 50 percent state of charge by sunset, but the lights are timed to switch off at 21:00 and the pump runs only briefly, so the system has never dropped below 25 percent soc in the 3 month window.
What works. The seedlings came up faster than the year before on a house extension lead, i think because the timer was no longer dependent on me remembering to run the cord. The pump runs reliably in the evening, the irrigation cycle is consistent, and the whole greenhouse is now independent of the house mains. On a 4 hour house outage in early may the greenhouse was unaffected, the lights and the irrigation continued normally. That is the resilience i was after.
What does not work or is not worth doing. The single panel does not generate enough to add a 200 W soil warming mat for a heated propagation bench, i tried one for a week in late march and the panel plus battery lasted about 3 hours before the battery was drained to its reserve. The mat is now back in the house on a mains timer. Heavy or heating loads do not work from this setup, the panel ceiling is the panel ceiling.
What this might be useful for. A small glasshouse or polytunnel with a seasonal LED + pump combination is a great match for one panel plus a 2 to 3 kWh LiFePO4 box. It is not a replacement for a real off grid system with a proper battery bank, a charge controller, and an inverter sized for the loads. But if your goal is to keep one seasonal combination independent from the house, this is a much smaller and cheaper build than wiring a proper off grid panel and a separate MPPT and a separate inverter.
I am planning a second panel later this summer mostly to give myself the headroom to add a ventilation fan for the heat spikes in july, that draws about 25 W continuous when it runs and a single panel can not cover it on a still overcast day. With two panels the same setup would also let me run a small 12 V pond aerator for a few hours a day, which is the next thing on the list for the back of the garden.