Off-grid solar and battery sizing for Porirua
A 1 kW array in Porirua produces about 1,218 kWh a year — but that annual figure is not what sizes a system. June is, and in June the same array makes only 1.1 kWh a day, against 5.5 in January.
What do you need?
Battery bank
12.9 kWh
268 Ah at 48V
Solar array
3.81 kW
about 9 panels of 450W
Summer only
0.74 kW
if you only use it January-ish
Sizing by household
Year-round systems in Porirua, three days of autonomy, lithium.
| Use | Per day | Battery | Array | Panels |
|---|---|---|---|---|
| Weekend cabin LED lights, phone charging, a laptop, a small water pump |
1.0 kWh | 4.3 kWh | 1.27 kW | 3 |
| Basic off-grid living The above plus a small fridge and a few hours of television |
3.0 kWh | 12.9 kWh | 3.81 kW | 9 |
| Comfortable off-grid home Full-size fridge-freezer, washing machine, pumps, laptops |
6.0 kWh | 25.7 kWh | 7.62 kW | 17 |
| Everything except heating A normal household load, with heating and cooking on gas or wood |
10.0 kWh | 42.9 kWh | 12.69 kW | 29 |
Why June decides everything
Array needed to cover 3.0 kWh a day, month by month. The tallest bar is the one your system has to survive.
| Month | Yield per kW | Array needed | |
|---|---|---|---|
| January | 5.54 kWh/day | 0.74 kW | |
| February | 4.74 kWh/day | 0.86 kW | |
| March | 3.57 kWh/day | 1.14 kW | |
| April | 2.41 kWh/day | 1.69 kW | |
| May | 1.50 kWh/day | 2.72 kW | |
| June (worst) | 1.07 kWh/day | 3.81 kW | |
| July | 1.29 kWh/day | 3.17 kW | |
| August | 2.04 kWh/day | 2.00 kW | |
| September | 3.10 kWh/day | 1.31 kW | |
| October | 4.28 kWh/day | 0.95 kW | |
| November | 5.26 kWh/day | 0.78 kW | |
| December | 5.35 kWh/day | 0.76 kW |
What the numbers assume
- Battery capacity allows for inverter losses, round-trip efficiency, and a usable depth of discharge — 80% for lithium, 50% for lead-acid. Nameplate capacity is always larger than the energy you can actually use.
- Array size includes a 25% margin over the bare requirement. A system that exactly meets demand on an average day of June never recovers from a run of cloudy ones.
- Yield figures are measured, not modelled — PVGIS satellite and reanalysis data for 2005–2023, at the optimal angle for this location, with 14% system losses already deducted.
Other cities in New Zealand
Limitations
- No shading. Trees, terrain and buildings are not modelled and can dominate everything here.
- Nothing about heating. Electric heating or cooking multiplies these figures several times over. Off-grid systems almost always use gas or wood for heat.
- Batteries age. Capacity falls over years; installers commonly oversize to allow for it.
- This is an estimate, not a design. A real installation needs a survey, local electrical regulations, and someone accountable for the result.