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Estimate payback time on Electric Ireland's solar PV battery storage
#1
A good while back, I posted an article on my blog about how storing excess power in batteries would cost more than buying it from the grid (link).  This remains true for most lead acid batteries due to their limited cycle life even with shallow discharges.

With lithium ion prices dropping substantially in recent years, larger battery banks such as power walls have become more affordable.  The cycle life of modern lithium ion batteries are much greater, especially with LiFePO4 batteries capable of many thousands of charge cycles. 

After finding out Electric Ireland now sells a Solar PV Battery storage system (like a power wall) claiming a 20 year service life and a 10,000 charge cycle / 10 year guarantee, I became curious to see what the payback period would be like.

At this time of posting (22nd March 2021), their smallest 5kWh battery costs €6,700 inc. VAT including installation.  The battery system is made by Sonnen GmbH of Germany and uses LifePo4 cells.  It can be recharged using solar power and off peak power, such as to top-up during the winter or cloudy days.

As we will just be looking at the battery payback period, we’re going to ignore all solar savings and other electric running costs other than what the battery will be storing. 

Generally PV batteries get one charge cycle per day.  They store the excess solar PV energy during the day.  During the evening time or when there is a higher electrical load than what the PV array can supply, they release their stored energy to power electrical appliances.  During dull overcast and shorter winter days when there is not enough sunlight to fully recharge the battery, the battery's charge controller can top up the battery during the Night tariff where a night meter is installed. 

For our calculations, we will assume one 100% charge cycle on average per day.  In reality, it's likely to be over 100% during the longer sunny days where the battery will supplement intermittent heavy loads (washing machine, dish washer, etc.) and recharge again before the sun sets.  During winter months, it's likely to be less, such as where it's partially charged during the offpeak tariff and not enough PV input to fully top it off during the day.  

Scenario 1 – One full charge per day, 25% from Electric Ireland’s Home Electric+ Night Boost plan

Electric Ireland’s standard discount rates (direct debit + online billing) are as follows.

24 Hour Tariff (Traditional meter) – 18.81c inc. VAT (for reference)

Home Electric+ Day – 20.94c inc. VAT
Home Electric+ Night – 10.78c inc. VAT
Home Electric+ Night Boost – 5.82c inc. VAT

25% of 5kWh is 1.25kWh.
75% of 5kWh is 3.75kWh.

Savings per year:
1.25kWh x 365.25 = 456.56kWh stored from the Night Boost tariff
3.75kWh x 365.25 = 1369.69kWh stored from the solar panels

456.56 x (20.94 – 5.82) = €69.03 offset from the Night Boost tariff
1369.69 x 20.94 = €286.81 saved from the stored solar power

Total = €355.84 saving per year.

Payback period =  €6,700 / €355.84 = 18.8 years

Scenario 2 – One full charge per day, 25% from Bright Energy’s Night Meter plan

Bright Energy currently has Ireland’s cheapest standard (out of contract) unit rates.  As with Electric Ireland, we will use the Day/Night Meter rates to provide a cheap tariff to recharge from to cover the dull winter and cloudy days.

Standard Meter 24 Hour tariff – 16.146c inc. VAT (for reference)

Day/Night Meter Day unit – 18.217c inc. VAT
Day/Night Meter Night unit – 10.325c inc. VAT

Savings per year:
1.25kWh x 365.25 = 456.56kWh offset from the Night Boost tariff
3.75kWh x 365.25 = 1369.69kWh stored from the solar panels

456.56 x (18.217 – 10.325) = €36.03 offset from the Night rate tariff
1369.69 x 18.217 = €249.52 saved from the stored solar power

Total = €285.55 saving per year

Payback period = €6,700 / €285.55 = 23.5 years

Notes:

These scenarios do not take recharging and inverter energy efficiency into account.  A night meter also has a higher day unit rate (see above) and a higher standing charge in the case of Electric Ireland, so the payback period is likely to be a little longer.  There is a €600 grant available for qualifying homes that would knock about 2 years off the payback period based on the above savings.

With larger solar arrays, it would be possible to get additional charge cycles by scheduling heavy load appliances to operate around noon.  This way they run on the excess stored PV energy from the morning hours and allows the battery to store PV energy again for evening time use. 

There will be a feed-in tariff available from July 2021 that will allow excess energy to be sold back to the grid.  This will negate some of the savings that a battery would offer.  For example, if the feed-in tariff is 10c and the daytime unit rate is 21c, then the battery would only save 11c per kWh compared to selling the power to the grid and buying it back later.
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