Solar power for my office

I’m getting a 10 square metre office built in the garden. Let’s solar power it! This page has notes.

Most bits are from Jaycar.

A 380 W pure sine wave inverter. The fan stays off under normal < 100 W draw use. No inverter noise. Not hot. Thumbs up!

Started with a single 80 W 12 V Chinese monocrystaline panel. Peaks at 5 A. Peak power at ~16 V. Comes with waterproof connectors.

Started with a PWM charge controller. Pick 20 A for future capacity. Developed a peak of around 60 W (12 V @ 5 A). Changing to a MPPT gives an extra ~30 % (16 V = +4 V @ 5 A).

20 A mains flex is fine. Not UV proof though.

Added a second panel.

Thinkpad R500 laptop with Ubuntu Precise takes 40 W @ 240 V with the screen on, 60 W peak. Charge controller said it was delivering 50 W so 10 W loss + error in the inverter. Power factor is nasty.

Single small spare car battery gives OK storage. Only really needed for bridging. Probably won’t last.

Mild cloud cover reduces output to ~1/3. Surprising.

Point the panel north, your latitude from the ground. I was surprised with how accurate this is at the start of Autum. Measured the altitude of the sun using my shadow and it matched well. Solar angle calculator says 70 degrees in summer, 46 in autumn, and 22 in winter, and winter is coming…

Going for best winter performance (31 degrees) gives 2.81 kWh/m2/day at worst, 4.54 in summer. Best summer gives 2.30 to 5.64. Best winter gives 22 % more energy at the most needed time.

(Panel is 1210 x 540 = 0.65 m2. 80 W and ~1 kW/m2 gives ~12 % efficient)

8 hours with 2 x 15 W lights, 50 W laptop, 10 W loss = 90 W * 8 hours = 0.72 kWh. 0.65 x 2 m2 of panels @ 12 % @ 2.81 hWh/m2/day = 0.44 hWh = 61 % of needed.

Azimuth of the sun changes by 90 degrees during the day.

Use a 230 V relay for automatic changeover. Draws < 1 W. Change in < 16 ms.


MP373512V/24V 30A MPPT Solar Charge Controller309
ZM930080W Recreational Solar Package Deal375
MP-3128Solar Charging Controller-43.9
ZM9097Powertech Monocrystalline Solar Panel - 80W359
MI5162380 Watt 12VDC to 230VAC Pure Sine Wave Inverter312
12 V car battery
WB15685 m 20 A mains flex24.5
Hobby box5
M4 x 126
M6 x 156
5 m aluminium channel60

Total is $1400. Too much.


Prototype on the back lawn

Chair happens to set the panel at the right angle

Box upside down showing charge controller and inverter


We have gas hot water and a heat pump. Consumption is 6500 kWh/year @ 20.44 c/kWh = $1335 / year. Peak is July at 22 kWh/day. Summer is 10 kWh/day.

The June 2009 Motukiekie Island ECEA study says 2.72 kWp cost $17 k. Batteries $12 k, inverters $13 k.

$17 k / 2.72 kWp = $6.25/Wh. A 80 W panel would be $500. Difference must be framing and installation.

Solarbuzz says the average price per Wp is $2.29 US for the module. Gives $7800 for the same system.

Guess and say we need a 3 kW inverter. $0.711 US/kW ~= $3000.

Auckland and Christchurch get 2050 sunshine h/year. 3.1 kWp ideally to cover it.

The iMiEV has a 16 kWh battery and can do 160 km. 20 km/day ~= 2kWh.

Michael Hope
Software Engineer