FAQ
TL;DR: A 1 kWp PV array in Poland yields ≈8 kWh on a clear summer day, lifting 100 L of water by 65 °C; “off-grid works, but only if voltage stays below 230 V” [Elektroda, gaz4, post #16345900]
Why it matters: Correct sizing prevents wasted watts, boiled tanks and blown electronics.
Quick Facts
• 100 L horizontal boiler needs ≈2.3 kWh to rise 50 °C [Elektroda, philips89, post #16334778]
• 1 kWp produces 6–8 kWh per sunny day in central Europe [Elektroda, gaz4, post #16345900]
• Used 2 kWp off-grid kit: ≈2 500 PLN (panels + MPPT) [Elektroda, anaba255, post #16346496]
• Safe DC string for heaters: 120–155 V to stay <230 V open-circuit [Elektroda, gaz4, post #16345900]
• Limescale forms rapidly above 60 °C—set thermostat ≤55 °C [WHO, 2017]
1. How much PV power heats a 100 L tank in summer?
Target 1.2–1.5 kWp. That array delivers 7–10 kWh on bright days—enough to raise 100 L by 50–70 °C while offsetting heat loss [Elektroda, gaz4, post #16345900]
2. What daily energy can I expect from 1 kWp in Poland?
Expect 6 kWh on average summer days and up to 8 kWh in peak sun (≈10 h) [Elektroda, gaz4, post #16345900] Annual yield is 900–1 100 kWh per kWp [PolPV Report, 2022].
3. Is direct DC-to-heater wiring efficient?
No. A 250 W panel wired straight to a 12 V heater may deliver only 0.4–0.6 kWh per day, losing 60 % versus inverter use [Elektroda, Editom, post #16336470]
4. What’s the over-voltage risk in off-grid strings?
If load disconnects, open-circuit voltage can exceed 300 V and destroy lamps or controllers. Keep Umpp 120–155 V and add fuses or MPPT to limit spikes [Elektroda, gaz4, post #16345900]
5. On-grid inverter or off-grid?
On-grid harvests every watt; 3 kWp with a quality inverter repays in ~6 years [Elektroda, Editom, post #16335463] Off-grid is cheaper up-front but wastes surplus energy and needs careful voltage control [Elektroda, Anonymous, post #16336665]
6. What does a basic 1 kW off-grid kit cost?
Five 195 W used panels (1 495 PLN) plus MPPT (590 PLN) and wiring (≈300 PLN) total about 2 400 PLN [Elektroda, anaba255, post #16346496]
7. Does panel tilt matter?
Yes. A 18° tilt boosts diffuse-light capture but raises cell temperature; each 10 °C rise cuts power ~5 % [Fraunhofer, 2021]. Match tilt to summer sun if water-heating is seasonal.
8. PV vs. solar-thermal: which is cheaper long-term?
Thermal collectors cost less per watt but need pumps, glycol and overheat protection. “I would never choose collectors” due to maintenance load [Elektroda, Anonymous, post #16336665] PV adds versatility—surplus powers appliances.
9. What happens on cloudy days?
Output drops to ≈1 kWh per kWp, raising 100 L only 8–9 °C [Elektroda, gaz4, post #16345900] Design backup (electric element or boiler) for comfort edge-cases.
10. Will higher temperatures shorten tank life?
Yes. Above 60 °C calcium carbonate precipitates fast, clogging coils and corroding welds; scale halves element life in hard-water zones [WHO, 2017]. Limit setpoint or flush annually.
11. How do I size a PV array for a small boiler?
- Compute energy: 4 200 J × L × ΔT / 3 600 000. 2. Divide by daily sun hours (e.g., 7 h). 3. Add 20 % for losses. Example: 100 L × 45 °C ≈5 kWh → 0.9 kWp array. [Elektroda, Anonymous, post #16337493]
12. Edge-case: what if both pumps fail in a drain-back solar-thermal?
Without circulation absorber stagnates; plate hits 180 °C, melting polystyrene DIY backing and boiling fluid—complete collector loss [Elektroda, idepopizze, post #16349355]
