Actually your solution may be a USB rechargeable power bank. It is possible to put the ESP8266 into sleep mode which will reduce consumption. Most power is used in transmission so making the logging interval as high as possible may help.
eBay has a 100Ah powerbank for under £10 - I'd take capacity with a pinch of salt but, if the SV is drawing less than 200mA you might get a month between charges.
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