Charging batteries at night to use power by day: the numbers don't add up
A calculation using bargain-priced batteries and chargers plus DIY installation gave 18 cents per stored kWh, more than the peak rate. The sums just don't work.
English · Original discussion in Spanish · Published
Charging batteries on a night tariff to use power during the day doesn't pay off
The idea sounds perfect: plug in your storage batteries when electricity costs 0.05 €/kWh and draw from them when the peak rate costs 0.15 €/kWh, three times as much. A ten-cent spread per kilowatt-hour that, on paper, invites you to set up a small battery bank next to your electrical panel. The problem appears when you factor in cycle life, charging efficiency and the price of the inverter. That's when the savings deflate.
What it really costs to store a kilowatt-hour
According to a calculation done at the time — batteries and chargers bought at a good price, self-installed — the cost came to around **18 cents per stored kWh**. In other words, more expensive than simply paying the peak rate. The conclusion was clear: the scheme was missing "a little something" to break even, and everything depended on the gap between off-peak and peak rates widening.
On top of that there's efficiency. A lead-acid battery doesn't return everything it receives: **charging efficiency is around 75%**, so of every four cents of difference, one goes up in the air. And then there's the rest of the equipment: a 12/24 V to 220 V inverter, a regulator that cuts off when the batteries are full, cables, a charge meter. An extra cost that rarely makes it into the optimistic calculations.
Sizing: 30 kWh a day and 26 batteries
For a maximum consumption of **30,000 Wh a day**, the sizing balloons. Assuming a maximum 50% discharge to stretch battery life to around **1,150 cycles** — three years — you'd need 40 batteries of 250 Ah at 6 V. If you adjust for the hours when the batteries are actually used, the figure drops to 26 units, at a cost of **€4,118** for the batteries alone, plus a thousand euros for assorted accessories.
The alternative suggested in the discussion is the old-fashioned thermal storage heater: charging a high heat-capacity tank overnight and releasing the heat during the day. Less segarro, but the thermal kWh comes out at half the price.
The technological promise that never arrives
The recurring argument for not doing the math is to wait. Supercapacitors promise efficiency above 99% and can handle full discharges, but their price still doesn't match and they have serious safety issues and capacity loss above 85 ºC. Sodium-ion batteries are presented as the cheap future for stationary applications. And the electric car as a home energy bank.
Meanwhile, the technology moves slowly and the savings still don't appear. The open question is always the same: how many more years do we have to wait for storing electricity at home to stop being an expensive hobby?
This analysis is based on the calculations and experiences set out in the reference material. It does not constitute financial or legal advice.
The idea sounds perfect: plug in your storage batteries when electricity costs 0.05 €/kWh and draw from them when the peak rate costs 0.15 €/kWh, three times as much. A ten-cent spread per kilowatt-hour that, on paper, invites you to set up a small battery bank next to your electrical panel. The problem appears when you factor in cycle life, charging efficiency and the price of the inverter. That's when the savings deflate.
What it really costs to store a kilowatt-hour
According to a calculation done at the time — batteries and chargers bought at a good price, self-installed — the cost came to around **18 cents per stored kWh**. In other words, more expensive than simply paying the peak rate. The conclusion was clear: the scheme was missing "a little something" to break even, and everything depended on the gap between off-peak and peak rates widening.
On top of that there's efficiency. A lead-acid battery doesn't return everything it receives: **charging efficiency is around 75%**, so of every four cents of difference, one goes up in the air. And then there's the rest of the equipment: a 12/24 V to 220 V inverter, a regulator that cuts off when the batteries are full, cables, a charge meter. An extra cost that rarely makes it into the optimistic calculations.
Sizing: 30 kWh a day and 26 batteries
For a maximum consumption of **30,000 Wh a day**, the sizing balloons. Assuming a maximum 50% discharge to stretch battery life to around **1,150 cycles** — three years — you'd need 40 batteries of 250 Ah at 6 V. If you adjust for the hours when the batteries are actually used, the figure drops to 26 units, at a cost of **€4,118** for the batteries alone, plus a thousand euros for assorted accessories.
The alternative suggested in the discussion is the old-fashioned thermal storage heater: charging a high heat-capacity tank overnight and releasing the heat during the day. Less segarro, but the thermal kWh comes out at half the price.
The technological promise that never arrives
The recurring argument for not doing the math is to wait. Supercapacitors promise efficiency above 99% and can handle full discharges, but their price still doesn't match and they have serious safety issues and capacity loss above 85 ºC. Sodium-ion batteries are presented as the cheap future for stationary applications. And the electric car as a home energy bank.
Meanwhile, the technology moves slowly and the savings still don't appear. The open question is always the same: how many more years do we have to wait for storing electricity at home to stop being an expensive hobby?
This analysis is based on the calculations and experiences set out in the reference material. It does not constitute financial or legal advice.
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Summary of a discussion on Burbuja.info - Foro de economía, actualidad y política., translated from Spanish and reviewed before publication. Read the full discussion (29 replies).
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