Storing solar power in batteries costs more than the grid

As Ireland does not seem to have a feed-in tariff for solar power at this time of writing, I have been curious as to what it would cost to store excess energy in batteries if I ever decided to get solar panels installed.

If the building is completely running off-grid, then using batteries to store power makes complete sense as the solar panels alone will not run high power appliances, not to mention provide no power at all after sunset.  However, when the grid is available, but does not offer a feed-in tariff, is it really cost effective to store the excess solar power in batteries?

Some may go further, wondering if it’s cost effective on a day/night tariff to store energy in batteries overnight from the grid and draw this supposedly cheaper power from the batteries during the day.  If one can get the batteries from free such as someone working for a scrapyard with a virtually endless supply of good batteries from write-off vehicles, then obviously this idea will work, assuming the battery charger and inverter are efficient enough to overcome the difference in day and night unit charges.

All rechargeable batteries have a finite number of discharge cycles, where the number of cycles varies according to the depth of discharge.  Based on recharging the battery during the day and using its power once the sun sets a typical set-up will consume one recharge/discharge cycle per day.  So to start with, it’s pretty clear one can work out the estimated lifespan based on the expected number of cycles.  1 cycle = day, so 1000 cycles works out at about 3 years.

Battery running cost

To work out how much energy a battery will provide over its lifespan, i.e. the total number of kilowatt hours, we first we need to work out roughly how many kilowatt hours the battery will provide in a single cycle and then multiple this by the estimated number of cycles the battery will provide before it needs to be replaced.  Once we have this figure, it is possible to work out the cost per kilowatt hour based on purchase cost of the battery.

• Energy (kWh) = (volts x Ah x DOD) / 1000

For example, if we have a typical 12V 110Ah leisure battery and cycle it with a 50% depth-of-discharge, this works out as follows:

• Energy kWh = (12 x 110 x 0.5) / 1000
• Energy kWh = 0.66kWh

Assuming that battery provides 500 cycles at 50% DOD, the total kWh we get out of that battery over its lifespan would be:

• Total kWh = 0.66kWh x 500
• Total kWh = 330kWh

So assuming all the input power was provided free of charge from the solar panels, that 330kWh of energy provided by the battery over its lifespan effectively cost the price of the battery.

So if let’s say the battery cost €100, then the cost per kilowatt hour works out as follows:

• Battery running cost =€100 / 330kWh
• Battery running cost = 30.3c/kWh

Most Irish electricity providers charge around 19c/kWh, excluding additional charges such as standing charge, PSO levy, etc.

It’s also worth noting that this running cost does not factor in losses such as inverter efficiency, battery temperature, recharging efficiency and so on.  So in this example, it will cost more to store excess solar energy in a typical leisure battery than just buying it of the grid when needed.

Specialised deep cycle batteries

Most users that plan running off-grid are going to choose batteries specifically designed for solar energy storage, which can handle far more cycles than leisure batteries typically used in caravans and motor-homes.

From a quick search, I’ll start with something near 1000Ah and found a 1150 SMG/S OPzV 2v Fiamm Gel battery specifically designed for solar energy storage.  So we need six of these in series to get 12v, which works out at 12v 1150Ah for six of these batteries.

Going by the technical specifications, that battery delivers 1500 cycles at 60% DOD or 5000 cycles at 20% with a constant temperature of 20C.

60% DOD scenario:

• Total energy kWh = (6 batteries x 2 volts x 1150 Ah x 60% DOD) / 1000
• Total energy kWh = 8.28kWh
• Lifespan total kWh = 8.28 x 1500 cycles
• Lifespan total kWh = 12420kWh

20% DOD scenario:

• Total energy kWh = (6 batteries x 2 volts x 1150 Ah x 20% DOD) / 1000
• Total energy kWh = 2.76kWh
• Lifespan total kWh = 2.76 x 5000 cycles
• Lifespan total kWh = 13800

Assuming it costs 19c/kWh to buy from the grid, these figures work out at €2360 for the 60% DOD and €2622 for the 20% DOD had we just used this energy from the grid instead of storing it.

This works out at €393 and €437 for the 60% and 20% DOD usage scenarios respectively, per battery.  The problem is, just try finding a Fiamm 1150 SMG/S OPzV 2v under either price to make the battery purchase worthwhile!

Lithium Iron Phosphate batteries

The above scenario assumes that the lead acid batteries are fully charged and only cycled to a certain depth of discharge.  In reality, it’s unlikely such batteries will be fully recharged except on the sunniest of days, so it’s likely those batteries will deliver far less kWh over its life span.

Lithium Iron Phosphate (LiFePO4) batteries have the advantage of not needing to be fully recharged and can also deliver far more cycles for a given depth of discharge than lead acid.  However, with their much higher up-front cost, are they cost effective compared to buying from the grid?

High capacity batteries seem to be tricky to find at this time of writing and the only one I came across is the Victron Energy 24V 180Ah.  Going by its technical specifications, it can deliver 2000 cycles at 80% DOD which seems brilliant compared to lead-acid batteries, so let’s work out what it can deliver over its lifespan.

• Energy kWh = (24v x 180Ah x 80% DOD) / 1000
• Energy kWh = 3.456kWh
• Lifespan total kWh = 3.456 x 2000 cycles
• Lifespan total kWh = 6912kWh

Assuming the grid price is 19c/kWh, the battery would need to cost under €1313.  Although I haven’t found actual pricing for this battery, I’m fairly sure it will cost many times this price considering how expensive the lower capacity 12v LiFePO4 batteries are.

Lithium Ion laptop battery pack

With eBay flooded with a wide range of low cost lithium ion battery packs, I was curious to see whether these would provide a reasonable storage cost if adapted for storing solar power.

From a quick check, I picked a Dell laptop battery rated at 7800mAh 11.1V, priced at €16.68 with free delivery.  It doesn’t mention its cycle life, but as Lithium Ion batteries provide 400 to 1200 cycles according to Wikipedia, I’ll pick 500 cycles as I don’t expect a high number of cycles for cheaply-made generic batteries and will assume the battery can provide 80% average DOD for these 500 cycles.  Obviously we’ll need many of these batteries to power a home, so I’ll pick 100, which gives a total up-front cost of €1668.

• Total energy kWh = (100 batteries x 11.1v x 7.8Ah x 80% DOD) / 1000
• Total energy kWh = 6.926kWh
• Lifespan total kWh = 6.926 x 500 cycles
• Lifespan total kWh = 3463kWh

Assuming the grid price is 19c/kWh, the total lifespan would have provided €658 of energy.  If these batteries manage to deliver 1200 cycles, we would be coming close to breaking even with them providing €1579 of energy of their lifespan.

Summary

From what I have determined, it’s pretty clear that most batteries cost more to store free energy (from solar, wind, etc.) than to buy that energy from the grid, at least in Ireland.  Of course, batteries still make total sense when going off-grid as solar panels don’t deliver energy overnight and the same with wind turbines when its calm outside.

Although there are improvements in battery technology such as Lithium Iron Phosphate (LiFePO4) batteries that deliver much greater number of cycles than lead-acid, they cost many times the price of an equivalent capacity lead-acid battery and still work out more expensive to store power than to simply buy it from the grid.

This also means that storing power from the grid overnight with a day/night tariff will actually cost considerably more than just paying for the energy during the more expensive daytime tariff.  The only exception will probably be for those who can get batteries for next to nothing, such as those working for a scrapyard.