Solar Power Notes


Updated – September 2017

This is a evolving series of notes prompted by a telemarketing call selling solar installation.

NZ Home Services

21-Jan-2016 “NZ Home Services” called offering a free on-site assessment for a solar power installation. The suggestion was that for sites with a power bill over $100 to install solar panels free of charge, to be paid off over 40 months. Being interested in solar power I said that I was not going to buy anything but would talk to the rep if they insisted 🙂

NZ Home Services have a web site HERE, but it doesn’t work for me (tried 2 browsers, displays nothing at all). The web site did work on a tablet but was not very informative. They have a short promotional Youtube video HERE.

What was offered

They had a number of system packages with the option to pay off over 40 weeks. The point was made that they only use Tier 1 solar panels (see notes below).

A 3KW system consisting of twelve 250 watt panels costing $13000 or $82/week for 40 months ($13280).

Considering  that I wasn’t going to buy a system, we didn’t talk details, but the rep was friendly and seemed keen to provide a good system.

If you are interested in solar power and longer term savings it’s probably not a bad deal. But the benefits will vary with your power use; how much and when.

Power Generated

The sales pitch suggests that the power generated averaged over the year will be the peak rating of the system x 5 hours per day. So a 3KW system will produce 15KWh per day x 365 d/y or 5475KWh. To buy 5475KWh would cost us just over $1000 at todays prices (exc. line charges).

Based on my experimental solar panel, my gut feeling is that “rating x 5 hours” may be slightly high for Wellington; but I don’t have any numbers to back that up.

Time of day, Generation and Use

If you work during the day when solar generation is at its peak, more power is sold back to the grid at a lower price; 7-8c/KWh. If you work from home as we do, our power use is more even throughout the day, but is not 3KW, so some would still go back to the grid. So you need to ask – Is the main benefit from reducing your power bill or selling excess power back to the grid.

If you sell most of the daytime generation back to the grid, the benefits will be less. Likely doubling the payback period.

Tier 1 Solar Panels and Warranty

I had to look this up, and found that it is not a reference to panel performance or quality. It is a rating system that defines the financial stability of the manufacturer. It could be argued that if the manufacturer has a history and is financially stable, then the product should be of good quality.

Most panels come with a 25 year output warranty and a 7 year or greater mechanical warranty. This suggests that the manufacturer is confident that the panel is capable of producing power for at least 25 years, although it may fall off the roof or blow away before then 🙂

The warranty is only as good as the operating life of the supplier and installer. If they are no longer operating, you are unlikely to be able to claim on the warranty.

Power Used and Payback

If you don’t use the power or feed back to the grid and don’t get paid a reasonable amount for the power, you are probably not making good use of the power available and won’t have a reasonable payback period.  Your load should be matched to your system or you need to store the excess for night-time use.

What About Batteries

Grid-Tie systems are popular because they don’t need batteries, which are very expensive and can be troublesome. A suitably sized battery system will allow you to go Off-Grid, eliminating line charges, but that requires a larger solar system and a large battery system. Most homes and life styles are not energy efficient enough to be able to comfortably go solar and off-grid.

Batteries are very expensive, require maintenance and will have to be replaced sooner than other parts of the system.

Recent research (September 2017) suggests that wet or flooded lead-acid batteries are still the best for life expectancy (with regular maintenance) and therefore more cost-effective.  You will need more capacity you expect to use because running the battery bank below 50% capacity will reduce it’s life.  150% capacity may be a good trade-off.

Our power use

As at January 2016 we are with Trustpower with a 15% prompt payment discount:

  • 24-4-15 to 25-5-15 = 781KWh = $194.90 paid (32 days, 17.58c/KWh, $1.94/day fixed)
  • 28-10-15 to 24-11-15 = 600KWh = $156.20 paid (28 days, 17.58c/KWh, $1.94/day fixed)

We work from home and have computers and a couple of monitors running 24/7 or for many hours a day. This accounts for a significant part of our power use.

As at September 2017 we are with Mercury Energy and have been for a year or so.

  • 18-07-17 to 07-08-17 = 541KWh = $145.27 paid (21 days, 16.31c/KWh, $1.8529/daily fixed)

For 2017 the total paid includes GST (tax) and a 0.15c/KWh Electricity Authority Levy (another tax).

To go off-grid

I estimate that without changing our power use significantly, we would need at least 6KW of solar panels and 1000Ah of battery at the 24V already operating.  Using more of the same 270W panels would mostly cover the roof and half of it would be less effective.

Equipment prices

Is $13000 a reasonable price for a 3KW system as at January 2016 ?

It’s difficult to say because there are so many options of panel and inverter. A complete system consists of a number of parts and installation. The panels themselves are not necessarily the most expensive part. Panel mountings are expensive as is a good inverter. Installation probably also accounts for a fair chunk.

Here are a few rough pricing guesses based on just buying the items from a local distributor:

  • solar panels – $4000
  • panel mountings – $1200
  • cabling and connectors – $1000
  • grid-tie inverter – $1500
  • installation – $2000

This totals $9700, which is about the $10000 mentioned on other suppliers sites for a 3KW system. I’d expect a regular installer to be able to get better pricing than this, but $10000 installed would probably be reasonable at early-2016 prices.

Apparently a 3KW system is typical and a 6KW system considered larger. Assuming 250W panels, 12 panels would be required for a 3KW system.

On-going Costs

Assuming you spend $13000 for a 3KW solar system, how long will it take to pay for itself and what will it cost to maintain over the 25 year life of the panels?

The panels will have to be cleaned maybe once a year. The panel mountings will have to be checked for corrosion and security every 5 to 10 years. The inverter will probably require service or replacement in 5-10 years. Cabling, if installed responsibly should last the full 25 years.

In 10 years it is likely that panels and inverters will be cheaper and more efficient, and that service labour will be more expensive.

Our Solar Experiment

Starting with one 190W 24V panel.

solar panel experimentOn a clear sunny day, early afternoon, one 190W panel produces up to about 160 watts using a PWM controller.

At typically 145W output and a grid power cost of 17.58c/KWh (+tax) the panel produces about 2.5c worth of power per hour. Assuming a good average day of output I estimate a payback period on the panel of 5-7 years. This does not include the cost of controller, cable, mountings or installation. So the actual payback may be more like 10 years.

This one panel was supplementing power to a couple of low power computers, including this web server. No grid-tie or inverters. It is all a 24V system.

Adding Panels

In 2017 I added two 270W panels and a 30A MPPT charge controller to the 24V system.  This was just hooked up to the existing system.  It’s interesting to note that the original PWM controller does all it can and the MPPT controller tops up from there, leading to the larger panels doing less of the work.  But in less than optimum conditions, the MPPT system takes over.

Without much battery capacity (still just 50Ah) we are not using the available power.

Power Factor, load quality and DC

Added September 2017

The large loads we have are electric cooking and heating, including hot water. The biggest loads are short duration so the overall energy used by these is not huge and hot water could be boosted with solar.  This leaves most of our energy used by mains powered appliances, most of which have simple switch-mode power supplies.  The are pretty bad for the mains, drawing most of their energy in short sharp bursts at the peak of the cycle – a story for another page.  Improving these power supplies to improve power factor and reduce distortion and noise on the mains is a good idea. Unfortunately, not practical for most people to achieve.

Running a DC system

If you have battery storage it would make sense to use the DC power directly.  This is not easily done in most homes.