I have a modest set of solar panels on an entirely ordinary house in suburban London. On average they generate about 3,800kWh per year. We also use about 3,800kWh of electricity each year. Obviously, we can't use all the power produced over summer and we need to buy power in winter. So here's my question: How big a battery would we need in order to be completely self-sufficient? Background …
It’s practical for someone with limited space for panels on a small room, but I ran these calculations by moving almost all loads to daytime, sizing the panel array to the (minimum daily usage + efficiency losses) * buffer factor for days long storms or equipment failure.
Start with the comparitively cheap panels if you have the space, move electrical loads to the daytime and design the house for thermal momentum, and size storage to the minimum inclusive efficiency losses times buffer. If you have the roof space the panels are the cheapest part and you should usually way, way over panel.
The most important thing is having thermal mass enough or living in a climate that allows your home to not need thermal input or extraction at night. Heat is expensive and exponentially moreso if you need to produce it from conventional storage.
It is possible that, not too long in the future, every home could also have a 1 MegaWatt-hour battery. They would be able to capture all the excess solar power generated in a year.
Braindead strategy, that most likely is discrete fossil fuel shilling, for purposes of making decision inpractical.
The cost of storage as a baselines is how much you can charge/discharge per day. Bonus for smaller (= cheaper) that can have more discharge/charge than its capacity per day. Plus the resilience/reserve capacity value which is a convenience factor. Resilience alternatives include fire places or gas generators (that are not expected to be used often) which tend to be cheap per kw. But noise, smell, variable costs, and startup effort are all inconveniences. Driving an EV to a public charger can be a similar inconvenience level to a generator for resilience value. If a 1mwh battery is used 10kwh/day it costs 100 times more per kwh than a 10kwh battery.
OP gives an example of 12kwh summer use (no AC?) which is very high for most people, but can include cooking and floodlights.
The braindead analysis parts are “because 100 days of 10kwh surpluses happen, I need 1mwh battery”. Actual battery storage requirements are the lowest theoretical winter solar production over 1-2 weeks, together with running pumps for heat (stored mostly in fall) distribution. A 10kwh/day maximum deficit for 1 week straight, with 60 day average deficit of 5kwh/day (without requiring additional heat input), means that any consideration for a large static battery should stop at 70kwh. This is sharply reduced with 1 or 2 EVs where summer surpluses are free fuel, and EV provides backcharging at 3kw whenever needed. 30kwh battery is plenty to charge an EV overnight (300km range for small car) before next day’s sunlight exceeds needs. Even less battery with 2nd lightly used EV, but 30kwh will be cheaper than un-needed EV.
Instead of relying on batteries for heat generation, which is where $100k 1mwh delusion proposition comes, heat generated from solar stored in under $1/kwh hot water and dirt storage. Outside of winter, this also provides completely unlimited showers and hot tub use, and a $10-20k heat pump and heating system (fossil fuel systems often cost the same) and insulation improvements is the the unquestionable non-distracting path.
It’s practical for someone with limited space for panels on a small room, but I ran these calculations by moving almost all loads to daytime, sizing the panel array to the (minimum daily usage + efficiency losses) * buffer factor for days long storms or equipment failure.
Start with the comparitively cheap panels if you have the space, move electrical loads to the daytime and design the house for thermal momentum, and size storage to the minimum inclusive efficiency losses times buffer. If you have the roof space the panels are the cheapest part and you should usually way, way over panel.
The most important thing is having thermal mass enough or living in a climate that allows your home to not need thermal input or extraction at night. Heat is expensive and exponentially moreso if you need to produce it from conventional storage.
Braindead strategy, that most likely is discrete fossil fuel shilling, for purposes of making decision inpractical.
The cost of storage as a baselines is how much you can charge/discharge per day. Bonus for smaller (= cheaper) that can have more discharge/charge than its capacity per day. Plus the resilience/reserve capacity value which is a convenience factor. Resilience alternatives include fire places or gas generators (that are not expected to be used often) which tend to be cheap per kw. But noise, smell, variable costs, and startup effort are all inconveniences. Driving an EV to a public charger can be a similar inconvenience level to a generator for resilience value. If a 1mwh battery is used 10kwh/day it costs 100 times more per kwh than a 10kwh battery.
OP gives an example of 12kwh summer use (no AC?) which is very high for most people, but can include cooking and floodlights.
The braindead analysis parts are “because 100 days of 10kwh surpluses happen, I need 1mwh battery”. Actual battery storage requirements are the lowest theoretical winter solar production over 1-2 weeks, together with running pumps for heat (stored mostly in fall) distribution. A 10kwh/day maximum deficit for 1 week straight, with 60 day average deficit of 5kwh/day (without requiring additional heat input), means that any consideration for a large static battery should stop at 70kwh. This is sharply reduced with 1 or 2 EVs where summer surpluses are free fuel, and EV provides backcharging at 3kw whenever needed. 30kwh battery is plenty to charge an EV overnight (300km range for small car) before next day’s sunlight exceeds needs. Even less battery with 2nd lightly used EV, but 30kwh will be cheaper than un-needed EV.
Instead of relying on batteries for heat generation, which is where $100k 1mwh delusion proposition comes, heat generated from solar stored in under $1/kwh hot water and dirt storage. Outside of winter, this also provides completely unlimited showers and hot tub use, and a $10-20k heat pump and heating system (fossil fuel systems often cost the same) and insulation improvements is the the unquestionable non-distracting path.
Do you like our hot sand?
https://www.euronews.com/green/2025/06/15/sand-batteries-could-be-key-breakthrough-in-storing-solar-and-wind-energy-year-round