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« Innovation Insights | Main | Smart Home - HEMS and haws »
Third year solar, 4400kWh, great harvest
Three years ago, just before the great Tohoku Earthquake and disaster, we connected our solar inverter to the grid. It is still noisy, but for power the system does perform. If all years are as good as this third year, when we sold 2702 kWh and consumed the rest, we will reach payback break-even after 11 years, not in 13 years as we initially expected, based on the installer's forecast of harvesting 3477 kWh a year. After that it is all profits for as long as the sun shines, the system works, and TEPCO buys the electricity.
On 2014-02-28, our inverter showeds 13567 kWh harvested, of which roughly
- 1/3 is own consumption, for which we do not need to pay on average 28 ¥/kWh, daytime rate with "8-Hour Night Service"
- 2/3 feed into the grid, most near peak time, reimbursed at the subsidized rate of 48 ¥/kWh
For reference: TEPCO electricity rates in English
So, how is life with solar?
Yokohama Green Power
Reviewing my prior post, there are few things to add. The Toshiba Feminity HEMS has been improved, now we can get power generation/consumption curves for day, month, year. Here a sunny Sunday.
click to enlarge
As we (and others I believe) requested, since 2013 summer we can download a table of our own generation and consumption data, making it easier to calculate our harvest data.
click to enlarge
The results confirm our 11 years solar payback calculation, including actual subsidized installation cost and feed-in tariff of 48 yen / kWh (guaranteed for 10 years) with daytime electricity cost of approx. 28 yen / kWh (night 8 plan). Rising utility rates are happening but it is not so much that it would reduce time to payback.
Would we do it again?
As in the past years, this question amounts to asking, "among technologies within our reach, which would we first install for economic and ecological benefits?" The simple answer: earliest payback first, that is usually energy-saving measures. Why do you think shops switch en masse from already energy-efficient fluorescent lights to LED? Payback in a 24h shop within 6 months. Even a bit shorter in a restaurant switching from halogen beams to LEDs.
With most low-hanging savings fruit in the basket, and new technologies shifting the economics, we go for technology that reduces our eco-footprint and still has reasonable reliability and payback. For lack of space, we did not install a cogeneration gas engine (14 years), nor a cogeneration fuel cell (17 years with subsidy). The fuel cell is also very expensive for home use, but large installations, e.g. for shops and data centers have both a decent payback and increase resilience in case of emergency.
Batteries: getting there
Looking for a few hundred watts of emergency power overnight or during a cloudy day, I found integrated battery and inverter solutions in the shops, quite expensive for the small power, but good to have in case of power outage. From mail-order components I can build my own at 1/2 to 1/3 the cost. For two commonly available battery technologies I estimated total cost of ownership and found the biggest factor is battery wear. The other components can last for decades. On a cost per kWh basis, assuming inverter efficiency of 0.8 it roughly comes to this:
- Deep cycle lead acid, 650 cycles at 50% capacity, about 160 ¥/kWh, good for 2 years if used daily, 5-7 years if only for backup.
- Lithium ion (cost 8x), 5000 cycles at 50% capacity, about 66 ¥/kWh, good for 13 years if used daily, longer if only for backup.
If Li-ion innovation already working in labs (one example) can be scaled and commercialized, the cost per kWh may go down below 22 ¥ which means grid parity here. For comparison, a 1.5 kW portable fueled inverter generator at current gasoline and oil prices runs at 100 ¥/kWh, which comes to 220 ¥/kWh if we include generator wear (depreciation over a service life of 3000h, mostly at 1/4 power). A hybrid car like a Prius idling the engine to power the 1.5kW inverter Toyota offers as an affordable option runs at near 80 ¥/kWh. I do not include depreciation because the main use of the car is besides the emergency.
Taking clues from Woodman's informative post on Backup Power and Bob Wilson's measurements, the Prius feeds the inverter from the (small) internal 12V battery. As that source goes down, it recharges from the traction battery, if that goes down, it autostarts the engine to recharge the batteries and keep the inverter going. If during the cold season we could use some the 87% waste heat, it would be quite the gain.
Any other technologies you know for resilience that are within our reach?
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Comments
Three years ago, just before the great Tohoku Earthquake and disaster, we connected our solar inverter to the grid. It is still noisy, but for power the system does perform. If all years are as good as this third year, when we sold 2702 kWh and consumed the rest, we will reach payback break-even after 11 years, not in 13 years as we initially expected, based on the installer's forecast of harvesting 3477 kWh a year. After that it is all profits for as long as the sun shines, the system works, and TEPCO buys the electricity.
On 2014-02-28, our inverter showeds 13567 kWh harvested, of which roughly
- 1/3 is own consumption, for which we do not need to pay on average 28 ¥/kWh, daytime rate with "8-Hour Night Service"
- 2/3 feed into the grid, most near peak time, reimbursed at the subsidized rate of 48 ¥/kWh
For reference: TEPCO electricity rates in English
So, how is life with solar?
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